Journal of African Earth Sciences 113 (2016) 165e180
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Journal of African Earth Sciences
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Morphology and development of pahoehoe flow-lobe tumuli and
associated features from a monogenetic basaltic volcanic field,
Bahariya Depression, Western Desert, Egypt
Ezz El Din Abdel Hakim Khalaf*, Mohamed Saleh Hammed
Cairo University-Faculty of Science-Geology Department, Giza, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 June 2014
Received in revised form
6 October 2015
Accepted 13 October 2015
Available online 28 October 2015
The dimensions, landforms, and structural characteristics of pahoehoe flow-lobe tumuli from Bahariya
Depression are collectively reported here for the first time. The flow-lobe tumuli documented here
characterize hummocky flow surfaces. These tumuli are characterized by low, dome-like mounds, lavainflation clefts, and squeeze ups. Flow-lobe tumuli are of various shapes and sizes, which are affected by
the mechanism of inflation because they formed in response to the increase of pressure within the flow
when the flow's crust becomes thicker. The tumuli often appear isolated or in small groups in the middle
sectors of the lava flows, whereas in the distal sectors they form large concentration, suggesting the
presence of complex lava tubes inside of the flow.
Tumuli exhibited by El Bahariya lava flows are between 3.0 and 50 m in length and up to 5.0 m in
height with lenticular geometry in aerial view. The flow emplacement of flow-lobe tumuli is controlled
by variations in local characteristics such as nature of the substrate, flow orientation, slope, interferrence
with other lobes, and rate of lava supply. Their presence generally towards the terminal ends of flow
fields suggests that they seldom form over the clogged portions of distributary tubes or pathways. Thus,
localized inflations that formed over blockages in major lava tubes result in formation of flow-lobe
tumuli. The three-tiered (crust-core-basal zone) internal structure of the flow-lobe tumuli, resembling
the typical distribution of vesicles in P-type lobes, confirms emplacement by the mechanism of inflation.
All the available data show that the morphology and emplacement mechanism of the studied flow-lobe
tumuli may be analogous to similar features preserved within topographically confined areas of the
Hawaiian and Deccan hummocky lava flows. Considering the age of the studied volcanic fields (~22 Ma)
it is most probable that the structures described here may be amongst the oldest recognized examples of
lava inflation.
© 2015 Published by Elsevier Ltd.
Keywords:
Volcanic fields
Tumuli morphotypes
Baharyia depression
Dilation fractures
1. Introduction
Continental flow fields have been attracted the view of volcanologists for a long time. Untill recently, the development of great
lengths of these flows was interpreted as being due to low viscosity,
high effusion rates, and a rapid emplacement (e.g., Walker, 1967;
Shaw and Swanson, 1970; Walker, 1973). Recent researches carried out on active and historic lava flows from Hawaii have changed
this traditional view of the process (e.g., Walker, 1989, 1991;
Wilmoth and Walker, 1993; Cashman et al., 1998; Hon et al.,
* Corresponding author. Tel.: þ202 01014652107; fax: þ202 35676866.
E-mail address: [email protected] (E.E.D.A.H. Khalaf).
http://dx.doi.org/10.1016/j.jafrearsci.2015.10.010
1464-343X/© 2015 Published by Elsevier Ltd.
1994; Keszthelyi, 1994, 1995; Trusdell, 1995; Keszthelyi and
Denlinger, 1996; Self et al., 1998; Calvari, 2004; Riker et al., 2009).
Particularly, Hon et al. (1994) proposed an emplacement mechanism for moderate volume effusions (0.1e10 km3) known as
“inflation”. This process initiates with a thin lava sheet, not
exceeding 0.5 m in thickness, termed “sheet flow” (Ballard et al.,
1979) that becomes thicker through the entire flow in response
to the continuous addition of lava under an external chilled crust
that wraps the flow. As inflation progresses, the upper surface of
the flow lifts, and the separation between individual flow lobes
vanishes, forming a molten core of interconnected pathways within
the flow (Hon et al., 1994; Kauahikaua et al., 1998; Self et al., 1998;
Anderson et al., 1999; Schaefer and Kattenhorn, 2004; Anderson
et al., 2012; Hoblitt et al., 2012). Inflation broadly affects the
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E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
entire flow because of this hydraulic connection. The result is a flat
to hummocky flow surface bounded by steep, rifted margins
(Anderson et al., 2005, 2012; Hoblitt et al., 2012).
Hummocky flow surfaces are characterized by the presence of
tumuli-low, dome-like mounds, commonly 1e5 m high, but occasionally exceeding 10 m in height (e.g., Wentworth and Macdonald,
1953; Walker, 1991; Chitwood, 1994). Most tumuli are crudely circular to elliptical in map view with deep axial cracks (e.g., Walker,
1991) and form in response to magmatic overpressure within the
flow as the flow's crust thickens (Walker, 1991; Hon et al., 1994;
Peterson et al., 1994; Anderson et al., 1999, 2012). Those areas
that inflate the most form tumuli, while the lows between tumuli
experience significantly less, or even no, inflation. In practice, all
low mounds that define the surface of hummocky flows, and which
formed by inflation, are called tumuli. These aspects are linked to
the control of intrinsic and extrinsic factors of volcanic activity such
as duration and effusion rates of the eruption, viscosity of the lava,
and topography of the emplacement area. It has been demonstrated
that surface textures and crustal thickness of tumuli may help
quantify eruption duration (Mattisson and Hoskuldsson, 2005;
Nemeth, 2010).
In some instances, inflation is focused over preferred pathways,
such as incipient tubes, within a flow to form a discontinuous series
of elongate tumuli (Kauahikaua et al., 1998; Glaze et al., 2005). Such
hoehoe lava on a low-slope
chains of tumuli can also form when pa
surface fails to spread out, for instance by lateral topographic
confinement (Glaze et al., 2005). In this case, the geometry of the
flow alone focuses inflation within the flow's narrow width, so that
tumuli appear to be aligned. While the formation of a series of
tumuli over a well-established lava tube occurs relatively rarely
(Walker, 1991; Anderson et al., 2012), such occurrences have been
documented (Kauahikaua et al., 1998; Duncan et al., 2004). Lava
tubes offer an efficient thermal delivery to the lava because of their
lower cooling gradient (z0.5 C/km) (Greeley, 1987; Pinkerton and
Wilson, 1994; Keszthely, 1995; Sakimoto et al., 1997; Cashman et al.,
1998; Keszthelyi and Self, 1998; Sakimoto and Zuber, 1998). Where
tumuli form over such lava tubes, they tend to be more elongate,
sometimes with a sinuosity that matches that of the underlying
lava tube (Keszthelyi and Pieri, 1993; Hon et al., 1994; Cashman and
Kauahikaua, 1997; Self et al., 1998).
Although published works has been done on the stratigraphy of
the Bahariya Depression (El Sharkawi et al., 1987; El Aref et al.,
1999; Helba et al., 2001; El Aref et al., 2006; Salama et al., 2014),
the physical volcanology of the basaltic lavas remains largely unknown. We focus here on the development of the tumuli and its
associated features that occurred in response to fluctuations in
discharge. So, the goal of this paper is first to report on and describe
the morphological characteristics and the emplacement mechanism of tumuli lava flow accompanying lava tube within the
Bahariya flows. The young age of the flow fields and the semi-arid
to arid climate of the Bahariya Depression have preserved excellent
exposures for the study of lava surface features and flow features,
unique for this area.
2. Terminology and methodology
Three major groups of tumuli involving lava-coated tumuli,
shallow-to moderate-slope tumuli, and flow-lobe tumuli have been
distinguished based on the morphology (Walker, 1991; Nemeth
et al., 2008; Nemeth, 2010). Duraiswami et al. (2001) concluded
that any distinction of tumuli based on slope is unsuitable usage
because of the ancient and eroded nature of the volcanics like El
Bahariya volcanics. In the latter, flow-lobe tumuli could be
distinctly recognized accordingly.
A geological mapping of the studied area was performed
through multispectral satellite images. Analyses of these images
defined the eruptive vents, the length and areal extension of the
tumuli, and verified the influence of different external controls to
the flow advance such as slopes and ground obstacles. The length
(tl), width (tw), and height (th) of the tumuli were measured using
a metal carpenter'stape. Crustal width (cw), which is the sum of the
widths of various crustal slabs minus that of the clefts, has also
been measured. Besides these, tumuli orientation and their position within flows are also recorded. The morphological aspects of
the flow-lobe tumuli are described in Table 1.
3. Geological setting
The Bahariya Depression (1800 km2) is located between 27480
and 28 300 N latitude and 28 350 and 29100 E longitude (Fig. 1). It
has a large oval shape with its major axis running northeast that
was naturally excavated in the Western Desert at about 320 km, SW
of Cairo. Its greatest length, northeast to southwest, is about 94 km;
and its greatest width measured at right angles to its length is about
42 km. Its capital town El Bauiti is situated 350 km south west of
Cairo (Fig. 1). The average depth, from the general desert plateau
level to the floor of the excavation, is less than 100 m. It is enclosed
from all sides by plateau of Eocene carbonates with, and locally
without Upper Cretaceous rocks. Its floor and surrounding scarps
are mostly made up of Cenomanian clastic deposits (Fig. 2)
(Soliman et al., 1970; Dominick, 1985; El Bassyouny, 1994; Sadek,
2010). The study areas involving the volcanic rocks are the southern reach of the northeastern plateau of this depression. These
areas are typical karst terrains dominated by conehills with cockpits and discrete depressions of which, El Hefhuf, El Gedida,
Ghorabi and El Harra are the most pronounced. These depressions
which host high grade ironstones are excavated Cenomanian clastics. Whereas, El Gedida is a closed depression within the plateau,
the other three depressions are opened to the main Bahariya
depression. However, each of these depressions is characterized by
a central elevated hills or inselberg (e.g. Ghorabi inselberg and Lion
hills in El Gedida mine area), being partially or completely surrounded by annular or semicircular valleys.
The Bahariya Oasis, situated on the StableeUnstable Shelf contact, is highly deformed (Said, 1962). Structurally, the distinct
wrench deformation affects this Oasis (Sehim, 1993, 2000;
Moustafa et al., 2003). A series of double-plunging anticlines and
synclines being arranged in an echelon pattern along NE-dextral
wrench faults (Fig. 2A, Ghorabi & El Harra faults) are reported
(Sehim, 1993; Iron Exploration Project IEP, 1993e1997). The wrench
deformation prevailed in Late Cretaceous and occasionally reactivated during Late Eocene (Sehim, 1993; Moustafa et al., 2003).
Ghorabi, Dumbell, El Ghaziya, El Gedida, and El Harra domes and
anticlines are the most pronounced examples of the wrenchrelated folding.
Several volcanic episodes took place during the Phanerozoic in
north Egypt (Said, 1962). Most of them occurred in Mesozoic and
Cenozoic times, particulary in the Neogene. The age of Cenozoic
volcanism extended from Late Eocene to Middle Miocene (40-15
Ma: Steen, 1982; Meneisy, 1990). These volcanics are part of the
Afro-Arabia Large Igneous Province (Bryan and Ernst, 2008). In
Bahariya Depression, these volcanics flowed over Cretaceous and
Tertiary sedimentary rocks (Fig. 2). Their eruptive rocks occur in the
form of fluvio-lacustrine compound pahoehoe lava flows and volcanic cones in NNEeSSW and NEeSW alignments through fissures
in areas presently surrounded by the populated zones (Figs. 2 and
3A). Frequently, flat topography of the flow surface is interrupted
by lava landforms, which are associated to the lava emplacement
mechanism. These morphological features mainly comprise tumuli,
lava rises, tumuli channel, and tumuli ridges. The lava flows and its
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
167
Table 1
Dimensions and morphological descriptions of the tumuli measured during the present study.
Ser.No. Tumulus Location
no.
Dimensions (m)
tl
tw
cw
Morphological characteristics
th
tl/
tw
2.0
1.5
2.2 1.0 Elongate tumulus with asymmetry in dip amount. Squeeze-up give rise to small outflows.
0.79 1.0 Bun-shaped tumulus with prominent axial clefts are partially filled with squeeze-ups and glassy
fragments. Traces of ropes are visible on the surface.
1.0 1.0 Asymmetrical polygonal tumulus with axial-and less clefts. Clefts are completely filled with squeeze-ups.
Surface of tumulus shows relict festoons of folds.
1.36 1.0 Large tumuli with large axial clefts and a couple of lesser clefts characterize it. Numerous squeeze-ups
occupy the axial clefts.
0.90 1.0 Slender tumulus with prominent axial clefts that are filled with numerous squeeze-ups.
1.68 1.0 Elongate tumulus with distinct asymmetry in dip amounts. Squeeze-ups give rise to small outflows.
1.26 1.0 Elongate moderate-sized lobate tumulus with lesser inflation and abundant squeeze-ups of variable size.
4.4 1.0 Moderate-sized, ridge-like tumuli, elongated in shape, with crustal slabs having folded surface. In section,
these tumuli exhibit vesicular zonation. Large vesicular are elongated and stretched in places where
crustal surface dip.
7.5 1.0 Elongated ridge-like tumuli with numerous inflation clefts and carbonate-rich vesicules. Axial cleft is
invariably filled with thick squeeze-ups while the lesser clefts are partially filled.
1.03 1.0 Flow-lobe tumuli with partly eroded upper surface. Crust shows dip of about 40 in one direction.
Numerous inflation clefts crosscut give a star-like appearance.
0.71 1.0 Flow-lobe tumuli with rope upper surface. Radial inflation clefts are occupied by squeeze-ups.
0.69 1.0 Flow-lobe tumuli with well preserved crust. Surface tumuli shows complex orientation.Axial deep cleft is
common.
2.72 1.0 Elongated flow-lobe tumuli with numerous inflation clefts. Conspicuous horizontal clefts are present
Thick bulbous squeeze-ups occupy the clefts.Grooves and striations are associated with these tumuli.
1.4 1.0 Asymmetric elongated flow-lobe tumuli with numerous clefts that are occupied by thick squeeze-ups. The
crust is vesicular and vesicle banding is common.
1.0 1.0 Equent tumulus with a prominent oblique cleft that is filled completely by a single squeeze-up.
4.76 1.0 Large whaleback flow-lobe tumuli with prominent axial clefts. Surface of tumuli is festooned with folds
and rope that show complex orientation. Tumuli crust is columnar jointed interior and highly vesicular,
glassy, micro-relief-rich outer carapace.
2.55 1.0 Elongated channelized flow-lobe tumuli with more relatively shallow clefts and rare surface breakout.
2.93 1.0 Truncated flow-lobe tumuli with lesser inflation and no prominent squeeze-ups.
2.11 1.0 Moderate flow-lobe tumuli with inflated features. Tumuli crust is jointed sheets and lava balls with three
distinct three zones consisting of well-defined columnar jointed (upper), vertical surface (middle) and
banded planar fracture (basal).
1.40 1.0 Flow-lobe tumuli with completely eroded upper surface. Axial cleft is present and is occupied by
numerous squeeze-ups.
1
2
M1
M2
Mandisha
Mandisha
5.5
2.75
2.5
3.5
2.5
3.5
3
M3
Mandisha
3.75
3.60
3.60 1.05
4
M4
Mandisha
7.5
5.5
5.5
2.05
5
6
7
8
M5
M6
M7
E1
Mandisha 2.5
Mandisha 7.4
Mandisha 4.90
Engliz
20
2.76
4.40
3.90
4.5
2.76
4.40
3.90
4.5
5.0
1.73
1.65
2.0
9
E2
Engliz
2.0
2.0
1.5
10
A1
Agoz
2.85
2.75
2.75 1.45
11
12
A2
A3
Agoz
Agoz
2.50
3.75
3.5
5.45
3.5 0.99
5.45 2.0
13
Ma1
Mayesra
15
5.5
5.5
2.5
14
Ma2
Mayesra
11.5
8.2
8.2
1.7
15
16
Ma3
H1
Mayesra
Hefhufe
0.85 0.85 0.85 0.5
50
10.5 10.5 1.50
17
18
19
H2
H3
Mar1
Hefhufe
Hefhufe
Marssos
24
6.75
15.50
9.40
2.30
7.35
9.40 0.75
2.30 0.85
7.37 2.37
20
Mar2
Marssos
11.65
8.35
8.35 2.65
15
cw/
tw
substrate has been deeply undermined by a drainage network
which partially exposed the contact between the basalt and the
underlying rocks. The substrate is constituted by interspersed beds
of red sandstones, siltstonesand claystone related to Tertiary units
of the El Bahariya basin (Fig. 2). These outcropping lava flows has
been divided into volcanic fields according to their geographical
characteristics. The main volcanic fields are: Gabal Mandisha, Gabal
El Engleez, Al-Agoz Hill, Gabal Mayesra, Gabal El-Hefhuf, Gabal El
Marssos, and in the plateau is El Tibniya, but minor exposures also
occur in the El-Bahr area, Nagb Siwa (Quzzeih) (Fig. 3A). Within the
depression, the volcanic outcrops display field relations only with
the Upper Cretaceous rocks of El Bahariya, El Heiz, and El Hefhuf
Formations. Most of these volcanic occurrences are bounded by the
north and south wrench faults (Fig. 3A). These intra-continental
volcanics emitted from a ponded zone have erupted mainly
basaltic compositions, but also include evolved variants such as
hawaiites, mugerites, and sodic basalts (El Kaluobi, 1974; Abdel Aal,
1981; Abdel Monem and Heikal, 1981; Youssef, 1982; Wassif, 1983;
Medani, 1995), represented by sub-alkalic to alkali volcanics and
typically monogenetic with an intraplate geochemical signature.
These volcanics have been dated between 22 and 16 Ma based on K/
Ar method (Meneisy and El-Kaluobi, 1975). The architecture of
these pahoehoe lava, up to 10 m thick, is compound, with inflated
segments. Their eruptions flowed through open-channels created
by multiple inter-fingering flows and developed fans at its terminus
after encountering topographic high (sensu, Hamilton et al., 2013).
The origin of these Neogene volcanics is associated with decompression partial melting of metasomatized sub-continental
lithospheric mantle remmants during crustal thinning (Khalaf,
2012; Moufti et al., 2013). However, it is important to note that
there is debate around the possible involvement of a mantle plume
in magma generation (Baker et al., 1998; Moufti et al., 2013),
although the general orientation and location of volcanism indicate
that it is at least structurally related to the Red Sea Rift (Moufti et al.,
2013).
4. Tumuli morphology
The compound pahoehoe flows around Bahariya Depression
belong to the Qatrani Formation (Fig. 2B). These flows show a
hummocky architecture in its lower horizons and sheet flow toe
longated flow-lobes and vesicles-rich tumuli in its upper surface,
displaying ropy festoons. Flow lobes are seen to have numerous
inflation clefts in vertical sections (Fig. 4A). The vesicles from the
inflated lobe themselves do not define any preferred orientation.
4.1. Mandisha volcanic field (MVF)
The Mandisha lava field (3 km2) which appears as ovaloid hill is
located 5 km to the northeast of Bawiti capital and about 3 km to
the south of Gebel El-Hefhul (Fig. 3B). The surface of the flow is
broad low-slope fan complex, which sub-sequently formed in the
later eruption stage (Fig. 3B). The surface morphology, 2.0 kmacross and 5.0 km in length, is rubbly pahoehoe type with scoriaceous flow at the top. These lava flows have thin, rubbly bases and
tops, and are generally poorly vesicular along with typical
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E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
Fig. 1. Location map of the Bahariya Oasis, Western Desert, Egypt.
columnar joints.
The central zone of the Mandisha volcanic field is composed of a
fissure fed multiple vent scoria and lava spatter cones (Fig. 4B).
About 2.0 km away from this central zone, equant flow-lobe tumuli
(M1eD4) are common forming a random network of surface irregularities a few metres high with whale-back shaped features
(Fig. 4C). These tumuli are formed at the front of the fan complex
(Fig. 3B), where the main feeder tube, enters the fan complex. The
tumuli are between 2.75 and 7.5 m in length and up to 3 m in height
with semi-circular in plan view (Figs. 4CeD). Many of these
observed tumuli exhibit axial and radial fractures that appeared to
have largely evolved from early formed cooling joints (Figs. 4C and
E; Peck and Minakami, 1968; Hon et al., 1994; Rossi and
Gudmundsson, 1996; Schaefer and Kattenhorn, 2004). Tumuli
have an average tl/lw ratio ~1.59 and the ratio of crust width to
tumulus width (i.e. cw/tw) equals unity (Table 1). The surfaces of
these tumuli are smooth, although a majority of the tumuli surfaces
are festooned by folds and ropes (Fig. 4C). In addition, the tumulus
“M5” appears as slender in shape with large axial clefts which
extend deep into the tumulus core (Fig. 4F). In the flow fields
around the Bawitti area, lava fields are generally characterized by
asymmetric steep-flanked elongate tumuli (M6 and M7) in
commonly truncated shape (Fig. 4G). Its length/width ratio averages 1.47. The fine surface relief of most of the tumuli is completely
removed by wind erosion, but the interior structure of the jointing
pattern is exposed. Individual columns are decimetre wide and
arrange perpendicular to the surface while the joints in the core are
vertical to subvertical (Fig. 5A).
4.2. El Engleez volcanic field (EVF)
Gabal El Engleez is located east of Gabal Mandisha (Fig. 3B). It
has lava flows formed by rubbly pahoehoe with flow-top composed
of lava fragments, mixed with titled thin (cm-scale) dm-wide slabs.
Elongate, ridge-like tumuli appear near Gabal El Engliz foot
(Fig. 5B). These tumuli are topped by a relatively well developed
axial cleft (Fig. 5C) that exposed textures indicative of inflation
(Hon et al., 1994; Anderson et al., 1999). The axial cleft/or crack, up
to 2 m in width and 1.7 m in depth, widened and deepened as
inflation progressed. The flanks of these tumuli expressed no significant inflation rifts as is seen on the sides of inflated sheet flows
(Hon et al., 1994; Hoblitt et al., 2012). However, long cracks within
or vertical offset formed sub-parallel to the axial clefts along the
lower flanks of the tumuli in response to local breakouts. Ridge-like
tumuli exhibited by the “EVF” (E1 and E2) are between 5 and 20 m
in length and up to 2 m in height with lenticular geometry (Fig. 5D).
Their surface exhibits a well-marked distribution pattern of vesicular zones and massive blocky lava. According to Valentine et al.
(2006), the development of these structures takes place near flow
margins, which appear as “squeeze-up ridges”. In addition, they
were generated during crystallization of the lava core, in which
pulses of lava would have risen as diapirs.
4.3. Al-Agoz volcanic field (AAVF)
Al-Agoz volcano is small subdued mesa with 500 m in diameter
and 10 m height above the surrounding relief (Fig. 3B). El-Agoz field
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
169
Fig. 2. (A) The geological map of the study area (based on El Akkad and Issawi, 1963). (B) Stratigraphic section of Bahariya Oasis (based on Said, 1962; El Akkad and Issawi, 1963).
is an isolated hill located to the east of Gebel Mandisha and occupies an area of about 400 km2. It is separated from Gabal Mandisha by the NW striking planar extensional fault which cuts the
sediments and basaltic rocks keeping the sediments underlying the
basaltic rocks as horizontal strata (Fig. 3B).
Flow-lobe tumuli are found along the western half of “AAVF”
(A1, A2, and A3, Table 1). They are generally asymmetric, being
higher and steeper on one side with radial inflation clefts (Figs. 5E
and F). Around the El-Agoz Hill center, the volcanic field is generally
characterized by steep-flanked unsymmetrical tumuli with deep
clefts (Table 1, Fig. 5G). The flow-lobe tumuli, up to 3.80 and 5.45 m
in length and width, respectively, are covered by a network of
cracks including axial (up to 0.35 m in width) and several or
branches cracks/or clefts. This latter structure was not simply linear
feature but bumpy along its trend, especially where it adjusted to
bends in the tumuli, and in some places was divided into subparallel en echelon segments (Fig. 5E). Average length/width ratio
of flow-lobe tumuli approaches unity (~0.99, Table 1) which
reflectes the main role of the inflation that occupies the top of the
flow in the final stage and may have formed during and after the
general thickening of the flow (Draiswami et al., 2001).
4.4. El Mayesra Volcanic Field (EMAVF)
Gebel El Mayesra is an arcuate and circular elongated lava flows
and lava ridges, located about 1.5 km to the northeast of Gebel
Mandisha and occupies an area of about 2.8 km2 (Fig. 3C). These
lava flows form a complex network of flow lobes and associated
surface morphological features such as skylights and tumuli. The
morphotype of the flow lobes presents as a lava zone made of non
uniform stacks of tabular and curved boulders, usually less than a
meter in width and length and a few cm in thickness. Zones of
pahoehoe exhibit small areas of ponded lava (a few m in diameter),
and lava channels forming elongated flat structure. Near source,
truncated flow lobes -coated tumuli are a few meters across and
less than 5 m high (Ma1and Ma2, Table 1) with numerous inflation
clefts (Fig. 6A). Many of these tumuli are asymmetrically cracked,
and their flanks are covered by micro-pahoehoe glassy, moderately
vesicular lava tongues with dm-wide lobes that reach a few meters
in length. Equant tumulus (0.85 0.85 m) (Ma3) with a prominent
oblique cleft is associated with these tumuli (Fig. 6A). Many of these
tumuli display several pulsed dilation fractures (Fig. 6B). The latter
fractures reach up to 13 cm in length and attain 0.5 m in width.
Such large tumuli are common to form individual hills in the
otherwise flat lava fields. These tumuli are usually associated with a
few tens of metres-long channelized lava lobes with high lava
marks (up to 0.5 m) (Fig. 6C). These channelized lobes vary from 10
to about 8.0 m in width and reach up to 1.0 m in height. These lava
lobes are interpreted to be open lava channels similar to those
described from lava fields with long lava flows from Australia
(Stephenson and Griffini, 1976; Joyce and Sutalo, 1996; Stephenson
et al., 1996, 1998), Argentina (Nemeth et al., 2008), and Iceland
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E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
Fig. 3. Areal distribution of the volcanic fields in the selected areas. (A) Areal distribution of volcanic occurrence of Bahariya Depression on Google map, Western Desert, Egypt. (B)
Google Earth image showing late-stage fan shape of Mandisha volcano. Note tumuli occurrence in NW-part of Mandisha volcano. Note the mesa topography of Gabal Al Agoz
volcanic field (AVF). (C) Google Earth image showing circular elongated shape of Gabal El Mayesra volcanic fields. Note the occurrence of numerous tumuli in the eastern half of El
Mayesra volcano. (C) Google Earth image showing lensoidal shape of El Hefhufe volcanic field overlying Cretaceous sediments of Bahariya Formation. Note the presence of NWtrending volcanic cones. Note the pahoehoe lobes and radial fingers in these volcanic fields (white arrows).
(Rossi and Gudmundsson, 1996).
4.5. El Hefhufe volcanic field (EHVF)
El Hefhufe area lies in the southeastern part of Bawiti to the
south of Gabal Mandisha. The volcanic rocks covers an elliptical
area of about 7.5 km2, and occurs in the circular forms and volcanic
cones (Fig. 3D) with NW oriented long axis at the northern part of
the ENE-WSW oriented exposure of El Hefhufe area. The lava flows
are compound type occupying an almost 2 km-long and 10 kmwide and comprise many stacked lava lobes. This morphotype is
made up of a thin lava crust, very dense to moderately vesicular and
large cm size vesicles being either spherical or irregular in shape.
In the vicinity of the lava plateau (Fig. 3D), irregularly shaped en
echelon sinuous flow-lobe tumuli of a few metres in diameter up to
50 m in length and 2e3 m in height as well as ~10 m in width are
common (Fig. 6D). Their inner textures consist of a massive, dense,
commonly columnar jointed interior that is covered by a highly
vesicular, glassy, micro-relief-rich outer carapace with strong curvature (Fig. 6E). Such large tumuli are common and form individual
hills in the otherwise homogeneous and flat lava fields. The surface
crust of these tumuli has scoriaceous clinker. A few km away from
the central lava shield and fissure cones, lava flows form more
channelized distribution pattern and broad symmetrically cracked
tumuli with common more relatively shallow clefts (Fig. 6F). The
size of the tumuli in this proximal section varies from a few to many
of meters across (Fig. 6F). Although tumuli are commonly lenticular
in plan view, elongated lava rise tens of meters long are more
common in distal areas. The main clefts of the tumuli in distal areas
are parallel to the lava flow direction where lava flow tubes have
been preserved. The distribution pattern and textural characteristics of these distal tumuli are identical to flow-lobe tumuli that
have been described from Iceland (Rossi and Gudmundsson, 1996)
and the Deccan, India (Duraiswami et al., 2002; Duraiswami et al.,
2004; Bonder et al., 2004). The transition between tumuli to
more distal lava surface rises, or whaleback structures, are also
similar to other lava fields in Iceland, India or Columbia River flow
fields (Rossi and Gudmundsson, 1996; Thordarson and Self, 1998;
Bonder et al., 2004; Matsson and Hoskuldsson, 2005; Nemeth
et al., 2008). Rare surface breakouts occur (Fig. 6G), along with
these channelized tumuli and localized spreading lava ridge
structure. Near these channelized tumuli, small truncated-shaped
tumuli has been observed with marginal inflation clefts and no
prominent squeeze-ups (Fig. 6H). Their size varies from a few
meters to 10 m across, however, their slope is generally low. Many
of these tumuli are asymmetrically cracked, and their flanks are
covered by moderately vesicular lava tongues with dm-wide lobes
that reach a few meters in length.
4.6. El Marssos volcanic field (EMVF)
Gabal El Marssos is located to the east of the El Bahariya-Farafra
road, 4 km from the Heiz village. It is “crescent-shaped” lava flows
which attain ~70 m in thickness. Near the northern parts of the
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
171
Fig. 4. Typical lava morphotypes of Mandisha volcano. (A)Vertical section of pahoehoe flows comprising hummocky lower flow lobes exposed along Gabal Mandisha at Bahariya
Depression. (B) Vent area view exhibits pyroclastic cones with two nested craters as well as older products surrounding the lava flow field. Note the appearance of tumuli in the
front of photo. (C) Polygonal outline of tumuli network of positive surface crust with whale back features. Note the inflation clefts occupied by squeeze-ups. The tumulus surface
showing festoons of fold. (D) Equant tumulus with numerous inflation clefts. (E) Sketch showing detailed description of tumuli in Fig. 4C. (F) Slender tumulus with axial inflation
cleft occupied by numerous squeeze-ups. (G) Elongate steep-flanked asymmetric truncated lobate tumuli. Note the presence of squeeze-ups.
Gabal El Marssos, underlying reddish pyroclastic mound and
overlying flow lobe tumuli have been identified (Fig. 7A). A reddish
mound resulting from the oxidation of volcanic Fe-bearing minerals characterizes the flow basal zones. This pyroclastic mound
(PM, ~1 m thick) has rounded surface and comprises scoria lapilli,
blocks, and bombs with mainly submillimeter vesicles exhibiting
cauliflower texture (Fig. 7B). The flow-lobe tumuli are 10 m wide
and more than 20 m thick. The crust of these lobes is characterized
by numerous inflation clefts (up to 0.3 m wide) involving squeezeups (Fig. 7A). The blocks of massive lava that cap these tumuli are
composed of sheeting jointing and lava balls/or pillows (Fig. 7A).
Larger lava balls/or pillows, 10e30 cm in diameter, are composed of
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Fig. 5. Typical lava morphotypes of Mandisha and Engleez volcano.(A) Close up view showing columnar jointing in the upper tumuli surface. (B) Panoramic view showing elongate
ridge-like tumuli close to the source area of Gabal El Engleez. ©Sketch of ridge-like tumulus showing a prominent axial cleft, breakouts, and ropy surfaces. (D) Gentle-flanked
asymmetric ridge-like tumuli with relatively deep clefts and numerous squeeze-ups. (E) Sketch of flow-lobe tumuli with inflation clefts arranged in radial pattern. (F) Close up
view showing flow-lobe tumuli with steep slope and asymmetric dip. Note radial inflation clefts. (G) Steep-flanked asymmetric tumuli with relatively deep clefts.
plagioclase, augite, and carbonate-rich vesicles displaying subophitic to intergranular textures. Lockwood and Hazlett (2014)
have observed pillow lavas forming in shallow fresh water (less
than 2 m). Very similar features (flow channels, tumuli, lava toes,
inflated flows, scoria) also occur in submarine environments
(Lockwood and Hazlett, 2014, pp. 362e371). The presence of such
pillows reflects the major role of surface water in the formation of
this structure, suggesting lacustrine or fluvio-lacustrine
environment for these volcanics and associated sediments (Martin
and Nemeth, 2004; Khalaf et al., 2015). Many authors elucidated
that the volcanics and the underlying sediments involving siliciclastic and ironstone sequences were formed in a lagoon or a
lacustrine environment during Oligo-Miocene time in El Bahariya
area (El Akkad and Issawi, 1963; El Sharkawi et al., 1987; El Aref and
Lotfy, 1989; El Aref et al., 1999; Helba et al., 2001).
Based on the internal structure, three definite zones have been
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
173
Fig. 6. Typical lava morphotypes of Gabal El Agoz, ElMayesra and El Hefhufe volcano. (A) Truncated flow-lobe tumuli with numerous inflation clefts occupied by thick squeeze-ups.
Note the appearance of equant tumulus in the back of photo. (B) Close-up view of pulsed inflated lava lobe comprising b1 to b3 bulges on the lobe surface. (C) Channelized lava flow
lobes associated with flow lobe tumuli in the distal volcanic field in Gabal El Maysera. Note the high lava marks indicated by the arrow in the margin of the channel. (D) Panorama
view showing en echelon sinuous flow-lobe tumuli with steep dip in one direction. (E) Flow-lobe tumuli showing columnar joints with folded outer carapace. (F) Channelized flow
lobe tumuli with shallow clefts (arrows) in distal lava field in Gabal El Hefhufe. (G) Surface breakout associated with flow-lobe tumuli. Note the darker color than the surrounding
areas. (H) Asymmetric truncated-shaped tumuli with lesser inflation clefts and no prominent squeeze-ups. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
recognized within these tumuli: an upper zone (UZ) exhibiting
volcanic balls with well-defined columnar joints, a middle zone
(MZ) composing of vertical dense fractures, and massive reddish
basal zone (BZ) characterized by inflection clefts and carbonate
rich-vesicles. These features correspond to structures that result
from segregation of residual liquids and gas bubbles that ascend
from inner and lower sectors of the flow (Bernardi et al., 2015).
According to Goff (1996), the development of these structures takes
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E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
Fig. 7. Typical lava morphotypesof ElMarssos volcano. (A) Close up view showing pyroclastic mounds (PM) underlie flow lobe tumuli. Note three distinct zones consisting of planar
surface in the basal zone (BZ) followed by vertical planar fractures in the middle zone (MZ) and ended with columnar joints with volcanic balls-rich carapace in the upper zone (UZ).
(B) Close up view of pyroclastic mound showing cauliflower morphotype of dark fragments with typical vesicular texture embedded in reddish fine-grained glassy matrix. Note the
stretching of some fragments and vesicles, due to deformation. (C) Squeeze-up occupying the axial cleft of tumuli. (D) Close-up view of tumuli peripheries showing triple-junction
clefts occupied by thick squeeze-ups. (E) Close up view showing grooves and striations that are exposed along tumuli walls. (F) Cross-sections of tumuli observed near el Mayesera.
(i) tumulus M7 exhibits prominent squeeze-ups and an outflow. (ii) tumulus Ma2 displaying vesicle banding, squeeze-up and toes at the base. (G) Crossed Nicols photomicrograph
showing glomeroporphyric clots of plagioclase (plag) and clinopyroxene (cpx) displaying intersertal and intergranular textures. Note the presence of vesicles in the groundmass. (H)
Cross Nicole photomicrograph showing zoned plagioclase phenocryst occupying the basal lobe.
place between the cessation of flow movement and the columnar
joint propagation into deep zones of the flow. In addition, these
vesicle structures were generated during crystallization of the lava
core, in which a differentiated residuum enriched in incompatible
elements and volatiles, would have risen as diapirs through a secondary vesiculation process (Goff, 1996; Rogan et al., 1996).
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
5. Associated features
The studied flow-lobe tumuli are characterized by inflation
clefts and squeeze-ups of varying size (up to 0.7 m in length) and
shapes (Figs. 4e7; Walker, 1991; Whitehead and Stephenson, 1998).
Around flanks of some tumuli (M3 and M4), squeeze-ups formed in
response to local thin (<10 cm) breakouts, attached to the tumuli
walls (Fig. 7C). Many squeeze-ups are enriched in plagioclase
phenocrysts, as compared with the tumuli (Draiswami et al., 2001).
There was no apparent vertical asymmetry in the growth history of
the two flanks of the tumulus anywhere along its length (Anderson
et al., 2012), nor did the axial crack contain lava squeeze-ups, as is
often seen on other tumuli (Walker, 1991; Duraiswami et al., 2001;
Anderson et al., 2012). Numerous thick squeeze-ups partially
occupy the clefts and are attached to the cleft peripheries as shown
in Mandisha tumuli (Fig. 7D). In these tumuli, three cutting clefts of
triple junction that frequently follow planes of weakness have been
observed (Fig. 7D). The walls of these clefts present striations and
small-scale folds (Ma1, Fig. 7E) as the result of the flow friction with
the ground. As a result, these network of cracks were not simply a
single linear features but jogged sharply along its trend that appear
to have largely evolved from early formed cooling joints and expose
textures indicative of inflation (Swanson, 1973; Hon et al., 1994;
Rossi and Gudmundsson, 1996; Anderson et al., 1999; Schaefer
and Kattenhorn, 2004).
6. Internal structure and petrography
Systematic measurements were carried out on tumuli M7, Ma1,
and Mar2 (Table 1) exposed in Mandisha, Mayesra, and Marssos
area. The internal structures of these flow-lobe tumuli reveal
interesting features (Fig. 7F). Tumulus M7 (Fig. 7Fi) is lobate with
two prominent lava inflation clefts. The axial cleft is large and extends deep into the tumulus core. A small squeeze-up partially
occupies the cleft and is attached to one of the cleft wall. The
marginal cleft is subhorizontal and is occupied by a thick squeezeup. Along with the axial and marginal clefts, an outflow is seen on
the smooth surface of this tumulus. Tumulus Ma2 (Fig. 7Fii) is
similar in morphology and internal structure to M7. The basal zone
is characterized by several levels of pipe vesicles and is bounded on
the lower side by a thick glassy rinds and a number of pahoehoe
toes which are of variable dimensions and have smooth surface.
The core is devoid of any vesicles. Joints propagating from the crust
penetrate the upper parts of the core. The crust displays at least fine
prominent vesicle bands. A single, large squeeze-up completely
occupies the axial cleft. In tumuli Mar1, upper, middle, and basal
zones are recognized based on fabric and composition (Fig. 7A). The
basal zone is a thin layer (<50 cm in thickness), and generally
shows vesicles with elliptical geometry and large sizes than those
of the upper zone. This lower vesicular sector is characterized by
the presence of vesicle pipes (Fig. 7AeB). These features result from
the upward escape of gases from the base of the flow and comprise
curvilinear structures with cylindrical geometry that do not exhibit
magmatic fills. The pipes reach up to 10 cm in length and show
circular cross-sections and are filled by carbonate. Wilmoth and
Walker (1993) defined those vesicle pipes-bearing lavas as P-type
pahoehoe flows, and noted that these flows are commonly developed only on flat ground. This zone is composed of glomeroporphyritic zoned plagioclase and augite aggregates with
intersertal and intergranular textures (Figs. 7GeH). The middle
zone is greyish black in color, cryptocrystalline, fine-grained, and
olivine-bearing. In this zone, prominent axial inflation clefts and
numerous squeeze-ups are widespread with less, or no vesicles
(Fig. 7A). The uppermost zone (UZ, in Mar1) is composed of
plagioclase phenocrysts set in vesicles-rich matrix. The flow roof of
175
this zone is characterized by volcanic balls/or pillow and is
frequently intercepted by columnar joints (Fig. 7A). Cross sections
of columns exhibit polygonal geometries and are well registered to
the upper sector of the flow cores, below which columns disappear.
The upper vesicular crust is partitioned by several vertical and
horizontal cracks that frequently follow planes of weakness, mainly
determined by vesicle alignments. Several chilled squeeze-ups
occupy this zone along marginal inflation clefts. The upper zone
represents the cooling crust formed during the flow emplacement
in which gas bubbles coming from the massive central zone, are
retained. Frequently, this layer shows gradational changes in
diameter and density of vesicles, where bubbles sizes increase with
depth while their density decrease (Bernardi et al., 2015).
These three zones (Fig. 7A) were described in Hawaii and Deccan volcanics (Walker, 1971; Walker, 1993; Walker, 1991;
Duraiswami et al., 2001). The latter author suggested that the
lower and middle zones of the flow-lobe tumuli had formed in the
initial stage whereas the upper zone was erupted during the final
stage of tumulus growth, as the result of a major breakout from the
tube system during high discharge rate. The presence of the
petrographic and textural variations in these zones (Fig. 7A) are
attributed to complex cooling history in which their constituents
were formed at fast to slow cooling rate from upper to lower zone,
respectively (Duraiswami et al., 2001). Viscosity controls many
parameters during emplacement of silicate melts such as transport
dynamic, eruptive style and speed with which the physicochemical
processes occur such as degassing and crystallization, among
others (Giordano et al., 2008). Therefore, it is interpreted that the
emplacement mechanism that acted during the effusion of these
lavas was controlled in part by this property. Tumuli described in El
Bahariya basaltic flows correspond to “lobe by lobe emplacement”,
as proposed by Deshmukh (1988).
7. Discussion
7.1. Development of an inflated lava tube and tumuli
The El Bahariya lava flow exhibits a transition between two
emplacement modes. The proximal zones present sheet flow
characteristics with flat roofs uniform thickness and an accentuated
radial scattering (Hon et al., 1994), while the distal section presents
hummocky flow characteristics with inflation structures. The ability of topography to confine a flow must be closely tied to the
discharge. While all flows will be confined by sufficiently high
topography, flows fed by progressively lower discharge can be
confined by correspondingly lower topography, even down to the
centimetric scale (Hon et al., 1994; Rossi and Gudmundsson, 1996;
Hamilton et al., 2013). Based on morphological features, we interpret that during the initial stages of the emplacement the lava
effusion rates were high and sustained overtime. This aspect added
to an almost flat substrate, led to the formation of sheet flow that
grew and moved throughout the development and coalescence of
lobes at the proximal lava flow front. A particular feature that evidences this process can be observed in most of the edges of the
proximal and middle portions of the sheet flow whereby these
sectors are characterized by the presence of numerous lava lobes
which fronts show “pahoehoe fingers” in a radial arrangement
(Fig. 3). This advance mode is the “lobe by lobe emplacement” (Hon
et al., 1994; Self et al., 1997) and promoted radial spreading with
generalized and uniform inflation across the entire lava body.
Initial inflation of the flow was minimal, and the flow failed to
resurface itself as it usually does during flow emplacement through
pahoehoe lobe extension (Peterson et al., 1994). The reduction of
the slope angle could have caused a decrease in forward speed,
allowing the confined flow to expand and occupy depressions and
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E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
Fig. 8. Cartoon showing development of lava flows and tumuli in the studied area. A. Lava flow spread and thin crust develops. Shortly after emplacment; well-developed tube not
yet formedand inflation is minimal. B. Pressure in the liquid part domes up the thickening crust and tumuli has begun to form. C. Some tumuli push up, crack open and squeeze out
bulges of liquid. In others, the lava drain back and top subside. Pressurization of tube has resulted in breakout from tumulus flank. D. Partial solidification of the flowcore at
emplacement final stages. The lava began tomove through narrowspathswithin core (lava tubes). If a section of a tube were blocked, then lava accumulates leading to the located
swelling on the sheet flowsurface resulting in tumuli and lava rises.
abandoned channels (Fig. 8, Bernardi et al., 2015). The inflation
process began after the stagnation of the flow on this site. At distal
site, the inflation process initially operated under high rates of
supply regime, leading to lava tunnels or tubes that continued
feeding terminal lobes, due to movement of lava lobes down slope
controlled by the topography (Duraiswami et al., 2001). Inflation
was possible only while the tube was completely full and lava was
in contact with the tube roof (Hon et al., 1994. Significant flow
thickening via inflation and overplating occurs during this advance.
On the other hand, when a flow is fed at an eruption rate well below
the long-term average, or when a flow near or above the average
rate is subdivided into smaller branches, the lava may stall on the
hummocky surface and make little forward progress. Thus, flow
advance and lava tube formation occurred more rapidly than is
typical for low discharge flows at Bahariya area and did not involve
significant lateral spreading. The inflation that occurred subsequently was focused along the axis of the incipient lava tube, which,
when it began to form, was correspondingly thinner than the total
thickness of the flow and probably had with a wide, elliptical cross
section (Kauahikaua et al., 1998). Conduction of heat through the
roof, floor, and sides of the tube caused crustal growth and thickening, forming P-type pahoehoe (Mattsson and Hoskuldsson,
2005), evidenced by vesicle banding (Figs. 7A/B, Wilmoth and
Walker, 1993). The subsequent corresponding pressure increase
within the tube, due to decreasing cross-sectional area without a
decrease in flow rate, forced the tube roof to arch up (Fig. 8; Hon
et al., 1994; Kauahikaua et al., 1998). Peterson et al. (1994)
describe another process by which an arched tube roof can formdaccretion of lava onto levees during channelized flow. If the
channel is sufficiently narrow these levees can grow together,
forming an arched roof over the channel. As the time passed, the
continuous decrease in the lava supply from the source and pressure increase within the tube, the lava body began to solidify and its
core began to crystallize, limiting the migration of lava to narrow
preferential pathways (lava tubes). This type of tube structure grew
and moved throughout the development and coalescence of lobes
at the proximal lava flow front. The lava input led to places to higher
internal lava and gas pressures resulting in perched or ponded the
overlying flow upper crust forming lava rises (Fig. 6C) or uplifted
inflation and tube structures that masked the morphology of the
sheet flow surface initially formed (Figs. 4C and 7D).
The inflation structure field that occupies the top of the flow in
the final stage may have formed during and after the general
thickening of the flow. It is inferred that this abrupt change in the
thickness of lava lobes at its terminus could be due to three aspects:
1) trapping of heat inside the tube causes crustal growth, 2) income,
confinement and accumulation of the flow inside an earlier channel, and 3) progress of the inflation mechanism which increased the
average thickness of the flow in this sector (Kauahikaua et al., 1998;
Bernadi et al., 2015). During the latter, flow inflation and lateral
spreading slow and stop and a well-developed lava tube forms
quickly. Pulsed inflation was described by many authors (e.g.
Anderson et al., 1999; Duraiswami, 2009; Hoblitt et al., 2012),
which is observed in El Mayesra from the Bahariya Depression
(Fig. 6B). The development of this structures takes place as the
result of variation in lava supply, forming pulses or bulges (Fig. 6B)
during the propagation of the lava flows (Duraiswami, 2009;
Hoblitt et al., 2012). Each pulse has enough pressure, which extrudes a limited volume of lava through the inflation cleft, producing local humps bounded by deep grooves (Fig. 7E, Hoblitt et al.,
2012) during the lava cooling (Duraiswami, 2009). Although the
inflation process by formation and coalescence of lobes comprises a
slow emplacement mechanism, it involves the lava tubes formation, which allows an almost isothermal delivery of lava. It is
interpreted that this latter aspect as well as the lower viscosity
determined the great areal and longitudinal development achieved
by the El Bahariya flow (Bernardi et al., 2015).
Inflation is possible only while the tube is completely full and
lava is in contact with the tube roof. It was determined that during
the inflation process the continued growth of the vesicular upper
crust occurs and the blistering observed in this layer is mainly due
to fluctuations in internal pressure or in the supply of liquid lava to
the core (Hon et al., 1994; Cashman and Kauahikaua, 1997). The
occurrence of vesicular structures (sheets and cylinders) is limited
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
to the time when the inflation of the flow lobe is finalizing and the
core has begun to crystallize (c.f.Hoblitt et al., 2012). The empirical
formula of Hon et al., 1994, t ¼ 164.8 H2, where t is time in hours
and H is crustal thickness in meters, yields a total duration of ~7
days to form ~1 m-thick crust. This period should be considered a
minimum value which takes into account the loss of thickness of
the upper vesicular crust as result of erosion. According to
Thordarson and Self (1998), this cooling model is not an appropriate procedure of an inflated lobe because cooling rates are
affected by many external and intrinsic factors. However, currently
this technique is the only quantitative method of estimating the
duration of active emplacement of ancient lava flows. Keszthelyi
(2012) modified the formula of Hon et al. (1994) to better account
for emplacement and environmental conditions.
7.2. Emplacement mechanisms
The pahoehoe flows of the Bahariya Depression akin to subaerial
monogentic hummocky lava flows. These pahoehoe fields
emplaced on low-slopes (b ~ 2 ) commonly thicken endogenously
via flow inflation and are usually broad because flow advancement
is generally accompanied by considerable flow widening (Walker,
1991; Hon et al., 1994; Peterson et al., 1994; Cashman et al., 1994;
Cashman and Kauahikaua, 1997; Kauahikaua et al., 1998; Self
et al., 1998; Anderson et al., 2005, 2012; Walker, 2009; Hoblitt
et al., 2012). Tumuli described in El Bahariya flows correspond to
a “flow lobe tumuli” type, as proposed by Walker (1991) and Rossi
and Gudmundsson (1996) who classify flow-lobe tumuli by height/
width and width/length aspect ratios (Table 1). These authors noted
that for their formation, low rates of lava supply from the source are
necessary. This would allow a better development of the outer
chilled crust to withstand the incoming lava without breaking for a
longer period (Bernardi et al., 2015). Therefore, this type of tumuli is
common in the middle and distal portions of a lava field, where
flow rates are lower, viscosity is high, and the supply pressures
increase towards the flow front. The tumuli show a variety of
shapes (Table 1) attributable to variations in site characteristics
such as nature and slope of substrate, interference due to other flow
lobes, and rate of lava supply. Occasionally, flow surface of these
tumuli deforms in response to compressive forces, displaying
folding of the visco-elastic crust which has cooled between 800 and
1070 C (Figs. 4C and 6E, Fink and Fletcher, 1978; Hoblitt et al.,
2012). This deformation is further supported by stretched subelliptical clasts and vesicles (Fig. 7B).
The distal regions of hummocky flows display evidence for flow
inflation and were presumably linked by lava tubes (Keszthelyi and
Pieri, 1993; Walker, 2000; Chatterjee et al., 2001; Zimbelman and
Johnston, 2002; Duraiswami, 2003; Crumpler et al., 2007). Where
tumuli form over such lava tubes (Fig. 8D), they tend to be more
elongate, sometimes with a sinuosity that matches that of the
underlying lava tube (Keszthelyi and Pieri, 1993; Hon et al., 1994;
Cashman and Kauahikaua, 1997; Self et al., 1998; Keszthelyi et al.,
1999; Kilburn, 2000). The alignment of tumuli in the Bahariya
lava flows (Figs. 3 and 6D) and the presence of elongated open lava
channels which show flat roofs and a greater areal extent
(Figs. 6CeF) indicates the presence of such a tube system (Walker,
1991; Manga, 1996; Self et al., 1998; Calvari and Pinkerton, 1998).
Moreover, the tumuli form large concentrations (Figs. 5B and 6D)
which are aligned with each other, suggesting the presence of lava
tubes inside of the flow (Bernardi et al., 2015). Such chains of tumuli
can form when pahoehoe lava on a low-slope surface fails to spread
out, for instance by lateral topographic confinement (Helz et al.,
1995; Glaze et al., 2005). Its presence also shows conclusively
that tumuli can form over major lava tube systems, a process
questioned in the past (e.g., Walker, 1991). Because growth of the
177
tumuli over the tube are more rapid in some areas than in others,
the height of the tumuli vary along its length so that elongate, darkcolored whaleback structures are formed (Figs. 4e8). The height
variations of tumuli (from 0.5 to 5 m, Table 1) may have been
related to local variations in other factors, such as tube width, tube
slope, or flow thickness (Bernardi et al., 2015). Flow-lobe tumuli are
mostly interpreted to have been supplied with magma from tubes
that originate in overflow from and/or through a flank fissure
connected with the lava lake of shield volcanaes (Ollier, 1964;
Greeley and Hyde, 1972; Rossi and Gudmundsson, 1996; Calvari
and Pinkerton, 1998; Duraiswami et al., 2001; Mattsson and
Hoskuldsson, 2005) similar to the scenario described from Crater
Basalt and El Baharyia volcanic fields. The terminal parts of pathways within such pahoehoe flows are often prone to blockages. The
latter result in localized inflation of flow lobes, and the formation of
flow-lobe tumuli overlying lava tube at the sluggish flow. Tumuli
aligned with a lava tube, like that which we describe here, have not
been described before from Bahariya area. While the formation of a
series of tumuli over a well-established lava tube occurs relatively
rarely (Rowland and Walker, 1990; Walker, 1991; Anderson et al.,
2012), such occurrences have been documented (Kauahikaua
et al., 1998; Duncan et al., 2004). Kauahikaua et al. (1998)
describe “a train of inflating, elongate tumuli” that developed
o
lava tube on the gently sloping coastal plain during
over a Pu'u ‘O’
1996e1997. These tumuli were a source of breakouts during pulses
of lava through the tube following eruptive pauses. Glaze et al.
(2005) describe a chain of large tumuli on the 1843 flow from
Mauna Loa (Hawai'i). Chitwood (1994) describes inflated lava fields
in central and southeast Oregon, USA, and indicates that long narrow tumuli can form over lava tubes.
Tumuli are domed structures that demonstrate the inflation
process because they formed in response to the increase of pressure
that acts uniformly over the flow core due to the continuous
incoming of liquid lava (Duraiswami et al., 2001; Bernardi et al.,
2015). Many of the observed tumuli exhibit axial and radial fractures with V-shaped cross sections (Figs. 4E, 5F and 7D) formed as
the result of distention strength caused during the lifting of the
outer crust, termed lava-inflation clefts (McMillan et al., 1989;
Walker, 1991; Gudmundsson, 1995). These fractures are always
partially filled by aeolian sediments and therefore it was impossible
to determine the crack extension and its morphology in depth.
Often, tumuli are associated with other structures of similar origin
termed lava rises which show flat roofs and a greater areal extent
(Walker, 1991; Fig. 6C). These landforms have irregular shapes in
aerial view and are up to 2 m in height above the surrounding relief
(Fig. 3). Hoblitt et al. (2012) interpreted the lava rise structure as
inflation rifts as is seen on the sides of inflated sheet flows (Hon
et al., 1994; Hoblitt et al., 2012). Inflation is either continuous or
discontinuous process, resulting in discontinuous inflation called
“pulsed inflation” (Self et al., 1998). The latter is observed as cyclic
numerous bands (Fig. 6B) in El Mayesra Volcanic Field as the result
of variations in lava discharge (Hoblitt et al., 2012). The tumuli
contain lava squeeze-ups (Fig. 8) which mark the sites of leakage
from the tube system (Duncan et al., 2004), as is often seen on other
tumuli (Walker, 1991; Misra, 2002; Duraiswami et al., 2001;
Anderson et al., 2012). Squeeze-up features that may cause a
pressure drop inside lava tubes (due to degassing, Hon et al., 1994;
Self et al., 1998) occur when the magmatic pressure exceeds the
strength of the confining crust and lava escapes (c.f.Rossi, 1997).
Morphology and low length/width ratio (av. tl/tw ¼ 1.07) of
most flow-lobe tumuli suggests in situ inflation without elongation
(Fig. 9). However, a significant down slope elongation during
inflation (as indicated by higher tl/tw ratios ¼ 3.65 in case of tumuli
number (M1, E1, E2, Ma1, H1, H2, H3, Mar1, Table 1) may be
responsible for elongated tumuli (Fig. 9). Flow-lobe tumuli from the
178
E.E.D.A.H. Khalaf, M.S. Hammed / Journal of African Earth Sciences 113 (2016) 165e180
suggest that the eruption began with intense initial fire fountain
activity comprising pyroclastic deposits of Hawaiian/Strombolian
characters. Following this initial phase, the eruption became effusive emplacing inflated compound pahoehoe lava flows and tumuli
formation. These volcanics flowed over Cretaceous and Tertiary
sedimentary rocks. The tumuli often appear isolated or in small
groups in the middle sectors of the lava flows, whereas in the distal
sectors they form large concentration, suggesting the presence of
lava tubes inside of the flow. The studied tumuli are characterized
by inflation clefts and squeeze-ups of varying size and shapes. The
effusion rate, the shapes, and dimensions coupled with the degree
of slope of the intra-lobe space and topographic irregularities (old
volcanic reliefs or substrate elevations) guide the trajectory,
morphology, and emplacement of the lava tumuli.
The studied lava flow can be subdivided into two main sectors
according to the emplacement mode. During the emplacement of
proximal sheet flow, radial dispersion was the predominant process
that resulted from an advance mechanism given by a continuous
formation and coalescence of lava lobes and its explosive rocks at
the lava flow front, enabled by a sustained regime of high effusion
rates and a gentle slope relief. The distal sector shows an irregularsmall scale “hummocky” relief constituted by a field of inflation
structures. At this site, the inflation process initially operated under
high rates of supply regime, leading to a tabular lava body. As the
time passed, the decrease in the lava supply led to the development
of lava tunnels that continued feeding terminal lobes. Finally, when
lava arrived to a blocked section or reached high points of tubes, it
accumulated there, leading to localized inflation of different parts
of flow surface. Although the inflation process by formation and
coalescence of lobes comprises a slow emplacement mechanism, it
involves the lava tubes formation, which allows an almost
isothermal delivery of lava.
Fig. 9. Tumulus width versus crust width in meter for tumuli from Hawaiian, Deccan,
and Bahariya volcanic field (Modified after Walker, 1991; Duraiswami et al., 2001).
present study are characterized by an axial or radial-like system of
lava inflation clefts and formed without any lateral shortening or
lateral compression (Phadke and Sukhtankar, 1971; Walker, 1991;
Bondre et al., 2000a, 2000b), which is supported by the fact that
the ratio of tumulus crust to tumulus width (cw/tw) equals unity
(Table 1). All the available data elucidate that there are similarities
involving geometry, sizes and aspect ratios (Fig. 9) for Deccan,
Hawaiian, and Bahariya volcanic fields. Moreover, the similarity in
vesicle distribution patterns in Hawaii P-type pahoehoe lobes and
flow-lobe tumuli (Figs. 7A, B, and F) indicates a similar emplacement mechanism viz., inflation, cooling and degassing history. Evidence of inflation has been documented in the Recent Hawaiian
flows (Hon et al., 1994) and for flows belonging to the ¼ 15 Ma Roza
Member, Wanapum Formation (Tolan, 2009), Columbia River Basalt
Group (Thordarson and Self, 1998). However, the bulk of the
Bahariya took place between 22 and 16 Ma in close proximity to the
CretaceouseTertiary boundary (Cisowski, 1990) with the duration
of eruption not exceeding a few million years (Duncun and Pyle,
1988). Therefore, considering the age of the Bahariya it is most
probable that the structures described here may be amongst the
oldest recognized examples of lava inflation.
8. Summary and conclusion
The morphology, physical characters, and emplacement mode of
flow-lobe tumuli and associated features are studied for the first
time in the Bahariya Depression. Detailed studies of this Depression
Acknowledgments
We thank Dr.T.Heikel for his reading the first version of the
manuscript. Comments by reviewer Dr. J.P. Lockwood Volcano,
HAWAII helped to clarify many of the points made inthe paper. An
anonymous reviewer improved the quality of the paper and is
gratefully acknowledged.
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