Geology of the Hawaiian Islands
Class 12
19 February 2004
A
B
C
D
F
97
88
77
66
51
94
87
73
64
29
92
86
73
60
91
85
mean
69
85
60
84
57
Exam Scores
Any Questions?
Mean = 71
Median = 82/77
82
Sedimentary Rocks
„
„
Rocks made of fragments of other rocks
Rocks precipitated from a solution
„
Example: Limestone
Sedimentary Stages in the Rock Cycle
Sediment
Material that has been moved by geologic
processes such as wind, gravity, streams,
ice, ocean currents, and waves
„ May be carried as solid fragments or in
solution to be precipitated later
„
Sedimentary rocks
Compacted sediment
„ Not very voluminous in Hawaii
„ Overall very important components
of Earth's crust
„
Sedimentary Rock Classification
Level 1: Detrital vs Chemical
Detrital sediments formed of material
eroded and transported before being
deposited
„ Chemical sediments precipitated
directly from solution
„
„
1
Fragment size
Sedimentary Rock Classification
Level 2: Size
Size of fragments > 2mm =
conglomerate
„ 2 to 1/16 mm = sandstone
„ 1/16mm to 1/256 mm = siltstone
„ < 1/256 mm= claystone or shale
„
„
„
„
„
Quartz
„
The dominant mineral found in
sedimentary rocks in most continental
areas
Quartz
„
Quartz
A sedimentary rock cannot have quartz
unless the source terrane has quartz
„ Almost no source rocks in Hawaii with
quartz,
„ So quartz is very rare in Hawaiian
sedimentary rocks
„
The dominant mineral found in
sedimentary rocks in most continental
areas -- WHY?
Quartz
The dominant mineral found in
sedimentary rocks in most continental
areas -- WHY?
„ Most stable mineral at the Earth’s
surface
„
Clastic Sediment
Most of the sediment formed on and
near the Hawaiian islands
„ What are the clasts ?
„ Volcanic, because the sediment comes
from the erosion of volcanic rocks
„
Partly a function of original size of the
fragments as they were eroded from the
source rocks
Much of the size difference in sedimentary
rocks occurs during the transportation of the
fragments
Fragments may be abraded or broken, so
that they generally become smaller when
they have been transported long distances
Clastic Sediment
Temporarily accumulates in the valleys
and bays, but eventually most of the
sediment is washed into the ocean
„ Because the islands have very steep
slopes, most of the sediment washed
into the ocean is carried away from the
islands into the deep ocean basin
„
2
Local Sedimentary Rocks
Accumulate around the coast lines on
or near beaches and in the shallow
water offshore
„ Most are made up of chemical
precipitates
„ Start out as coral or as shells secreted
by marine animals
„
Most important mineral type in local
sedimentary rocks
„ Forms broken down pieces of
shells and coral
„
Transport affects sediment in several ways
Sorting: measure of the variation in the range
of grain sizes in a clastic rock or sediment
„
WellWell-sorted sediments have been subjected to
prolonged water or wind action.
„
PoorlyPoorly-sorted sediments are either not farfarremoved from their source or deposited by
glaciers.
Roundness and sphericity
Transport affects sediment in several ways
Sorting
WellWell-sorted
Calcium Carbonate (CaCO3)
PoorlyPoorly-sorted
Roundness : measure of how rounded the
corners are
Sphericity:
Sphericity: measure of how much it is like a
sphere
Sorting, roundness, and sphericity all increase
with amount of transport
WellWellsorted
Angular
Sand
PoorlyPoorlysorted
Rounded
Sand
Types of chemical sedimentary rocks
Limestone
CaCO3
Chert
SiO2
Salt
NaCl,
NaCl, KCl,
KCl, K2SO4
Gypsum
CaSO4 • 2H2O
Coal
altered organic debris
3
Common Sedimentary Environments
Relative vs Absolute Age
Age of the Islands
Relative: Rock “A” is older (or
younger) than rock “B”
„ Absolute = knowing the age of a rock
in years
„ Usually geologists first establish
relative ages then try to get absolute
age dates
„
Principle of Superposition
Principle of Superposition
Youngest rocks
Sedimentary rocks are deposited in a
layerlayer-cake fashion:
Although this is really obvious, it was
not stated until 1669
„ This principle generally applies to
volcanic rocks as well as sedimentary
rocks
„
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one
above and younger than the one
below
Oldest rocks
Principle of Superposition
Principle of Original
Horizontality
„
Layers of sediment are deposited in a
nearly horizontal position
4
Principle of CrossCross-cutting
Relationships
Principle of Original Horizontality
Note that original horizontality is not
strictly applicable to volcanic rocks -why?
„ Because they are often deposited on
slopes
„
Layers of rock are said to be
conformable when they are
found to have been deposited
essentially without
interruption
„
Something (such as a dike or fault)
that cuts across a layer must be
younger than the layer
Unconformity
Results from interruption of deposition
„ Represents a long period of time during
which there either was no deposition, or
earlier deposited material was eroded
away
„
Unconformity
Often not easy to recognize if the
layers are all parallel
„ Much easier to recognize when there is
a period of folding of the rocks before a
period of erosion and renewed
deposition
„
5
The Great Unconformity of the Grand Canyon
Angular unconformity, Grand Canyon
Summary of Geologic Events in a
small area
6
„
„
„
Correlation
Within sedimentary layers there are
often the remains of small animals
(fossils)
fossils)
„ Fossils are quite useful for correlating
between two sections that are not
laterally continuous
„
These methods work well in small areas
where we can see the relationships
between rock layers.
What happens when we want to tell the
relative ages of the strata on Oahu with
respect to strata on Maui?
We have to figure out some way to
correlate the layers of interest.
Correlation
„
Fossils have evolved through time, so
when we find a fossil of the same type in
two different areas, we are pretty sure
that the rocks are about the same age
7
„
Hawaii
Why?
„ Fossils helpful in sedimentary rocks,
but usually no fossils in volcanic rocks
„ Sometimes small amounts of sediment
between layers of volcanic rock (such
layers might have fossils), but most
rocks in Hawaii do not have fossils
Magnetic time scale
Magnetostratigraphy
This technique is not very useful in
Technique that works best in volcanic
rocks
„ Time scale based on polarity reversal of
Earth's magnetic field
„ Major problem is that Earth's magnetic
field has been constant for the past
700,000 yrs (no reversals), so this
doesn't work for very young rocks
„
0-700,000 -- Normal
700,000 - 2.5 my -- Reversed
„ > 2.5 my -- Normal
„ Ko`olau lavas mostly reversed in
polarity, so they must be older than
700,000 yrs, but younger than 2.5 my
„ Lavas on Kaua`i and in Wai`anae
Range are positive, so they must be
older than 2.5 my
„
„
Radiometric Dating
Radioactive
Decay of
Rubidium to
Strontium
Use of radioactive decay to determine
the age of a rock
„ Key principle: Half Life = time required
for 1/2 of the nuclei in a sample to
decay
„
Radioactive
Decay of
Uranium 238
to
Lead 206
Radiometric Dating
In addition to Uranium - Lead series,
geologists also use RubidumRubidum-Strontium,
C14 and Potassium - Argon for dating
„ CarbonCarbon-14 used for very young rocks.
„
Radiometric Dating
Shows that the earth is much older than
people had previously suspected
„ Earth formed about 4.6 billion years
ago
„
8
The Geologic time scale
Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
„ Built using a combination of
stratigraphic relationships, crosscrosscutting relationships, and absolute
(isotopic) ages
„
For Tuesday
Questions?
Hydrologic Cycle and
Stream Erosion
„ Please read Chapters 8+9 in Text
„
9