Sand Geology Lab
Chapter 5
Measuring Grain Size
• Grain size is a direct
result of the amount of
energy available to
transport the sediment in
the environment.
• Example:
High energy environment:
– all one size like sand dunes
– NO silt or clay
Low energy environment:
– Lots of silt and clay (mud)
– Example: protected bay or
estuary
Grain size can also tell
about the distance from
source of sediments.
Environment Examples
MUD
Low Energy: no waves to
transport silt & clay out to sea
Measuring Grain Size
• Sand grain size is
measured using a grain
size card.
• The size should be
described as a range
from smallest to largest.
–
–
–
–
–
vc = very coarse
c = coarse
m = medium
f = fine
vf = very fine
• Example:
Medium to very fine
pebble
Degree of Sorting
Record the degree of sorting for each sample.
Sorting is a qualitative description of the range of grain sizes
found in the environment.
Sorting gives information about the consistency of an energy
source. For example, poorly sorted sediments have a
fluctuating energy source.
Degree of Roundness
Terrigenous Sand (abiotic) – Non
Living Sediments
Examples of
terrigenous sand
Obsidian
Garnet
Olivine or Peridot
Granite Derived Sand
• Associated with continental crust in
temperate (30 - 60) and polar (> 60 )
climate zones.
• Quartz is the most common sediment.
Feldspar minerals break down into clays.
• Minerals such as micas (glittery), garnet
(ruby red), and magnetite (black &
magnetic) are also common in granite
derived sands.
• Use photo cards and handouts to assist in
identifying these materials.
Granite Derived Sand
Quartz – colorless, translucent, glassy.
Feldspar – pink, white, opaque, striated;
feldspar tends to chemically weather to
clay minerals. Most common mineral.
Mica – black, coppery, flat, shiny.
Quartz Sand Grains
Frosted from sand dunes.
One of the most
common minerals on
Earth
Mica & Magnetite
Mica flakes:
flat, shiny,
black, gold
to coppery
in color.
Magnetite – dull, rusty,
dense, magnetic, rodshaped mineral.
Volcanic Derived Sand
• Associated with active volcanism at
hotspots and some volcanic island arcs.
Can be either tropical or temperate
climate.
• Basalt volcanism –
– Rocks – cinders (red), basalt (black),
obsidian (volcanic glass)
– Minerals – olivine (green to rusty green)
Volcanic Derived Sand
Active volcanic
eruptions.
More basalt = more
recent eruption
Pitted, dull,
black
sediments
More Volcanic Derived Sands
Olivine – green, glassy, somewhat translucent
volcanic mineral. Associated with basalt lava.
Volcanic Derived Beaches
Cinder beaches (older)
are red due to rust
Combination of Granite & Volcanic
Sand
• Could represent:
– Continental volcanism that is
inland from the coast.
– Remains of an extinct volcano
– Volcanic arc system due to
subduction
Beach Slope
• Beach slope is how steep the beach is
(i.e. do you have to walk down a berm
to the water or is it flat?)
• Coarse sediments (pebbles, gravel) =
steep beach slope
• Fine sediments (fine sands, mud) =
gentle beach slope
Other Abiotic Sediments
Biogenic Sediments – from Living
Organisms
Barnacles
Barnacle fragments
have ridges and are
flat.
Suggest rocky
shore beach.
Can be purple/white
in color.
Sea Urchin Spines
Also suggests a rocky
shore beach. Can be
striped or purple.
Shape is elongated or
circular in crosssection.
Gastropods &
Operculums
Gastropods
are snails.
Can be any
color.
Operculums
are protective
coverings for
snails.
Cat’s Eye
Operculum is found
in many Hawaiian
sands.
Coral
Coral is only found in the
tropics & in areas where
there is little continental
sediments.
Coral can be white, pink, or
beige.
Coral can be angular,
rounded, and porous.
Sponge Spicules
Spicule
Spicules are glassy
structures that act as support
(skeleton) for certain types of
sponges.
Foraminifera-Plankton Shells
Tiny shell-like
sediments that can be
found anywhere.
Operculum
Foraminifera
Ooids – Hydrogenous Sediment
Implications: Only can form in
shallow, quiet, seas with
water temperature greater
than 80 degrees F. No
terrigenous input. Very rare.
Ooids are made of
calcium carbonate
that precipitated
out of the water
and coat shell
fragments.
How to Make Inferences
Sand Sample #1
• Well sorted, coarse
sand size
• Rounded to well
rounded
• 100% Biogenic
– 25 % Coral frags
(golden)
– 10% shell frags (white)
– 65% cat’s eye (round
disks)
How to Make Inferences
Sand Sample #2
• Mod. sorted, vf pebble to
med. sand
• Angular to sub-rounded
• 30% Terrigenous
– All rock fragments
(granite?)
• 70% Biogenic
– 50% Snail shell frag
– 18% Sea Urchin parts
(spines & skeleton)
– 2% operculums (snail
doors)
How to Make Inferences
Sand Sample #1
• Tropical climate due
to coral.
• High energy due to
well sorted and
rounded grains.
• Possible rocky beach
due to large
population of snail
shells.
Sand Sample #2
• Continental beach due
to granite.
• Rocky beach due to sea
urchins.
• Inconsistent energy due
to large range of grain
sizes.
• Close to terrigenous
source.
• Temperate climate (no
coral).