BACKGROUND:
Erosion is the removal of weathered material on the earth’s surface by
to the action of wind, ice, heat, water, and humans. The agents of
weathering break apart parent rocks into smaller fragments. These fragments
may become sand if sufficiently broken down. Small rock pieces become
smaller through movement, as they hit other rocks and chip away. Since sand
comes from a "Mother" or source rock, it is possible to determine
what type of rock produced the "baby rocks." The composition and
general color of both the "baby" and "mother" rock are
often very similar.
PROCEDURE:
- Show examples of "mother" or source rocks to the students
(granite, chert, serpentinite, and basalt). Describe the origin of each
source material to the students. Be sure to point out that quartz is a
mineral and not a rock.
There are five possible source materials for the sand in this lab.
Granite is a plutonic igneous rock. It occurs in varying shades of light
and dark colors depending on the specimen’s mineral composition.
Serpentinite is a metamorphic rock that is smooth, green, and scaly. It
is the state rock of California. Chert is a sedimentary rock that forms
from the shells of microscopic marine life in deep ocean environments.
It is very hard, and comes in many colors. Quartz is found in many rocks
including granite and chert, and is the main constituent of most sand.
Most quartz crystals are clear or white in color. Basalt is a dark,
fine-grained igneous rock.
- The students will describe the different types of sand by looking at
the sand using a hand lens and microscope. Each sand will resemble the
mother rock because the sand is of the same composition as the mother
rock.
- The students will be using hand lenses so you may want to show them
how to use the lenses. Remember the hand lens should be close to the eye
and then move the object into focus. Have the students observe the samples and
describe them using the words found on the lab sheet. Tell them to be
very descriptive as this will help them decide which sands may be
related.
- Have the students also look at the sand under the microscope to look
for more details. Give students an index card. Make them write the name
of each sand they are looking at. With an Avery label (round is
preferred) have them put a little sand on the "sticky" side
and then paste it to the index card next to its appropriate name. Look
at these sands under a Swift-GH Microscope.
- Rodeo Beach, Cleone, and Bodega are related because they have varying
amounts of chert, serpentinite, basalt, and quartz in them. Half Moon
Bay, Monterey, and Montara are similar because they have pieces of
quartz, feldspar (pink, orange, or white) and small black minerals in
them.
BODEGA, Sonoma County, California - .25-5mm; poorly sorted;
subrounded to angular. Contains chert, basalt, serpentinite, quartz,
feldspar, and greywacke. Along the Sonoma coast, you can see the high
energy waves eroding the rocks along the coast. The common rocks in this
general area are basalt, serpentinite, chert, and greywacke. The quartz
and feldspar are eroding from the chert or the greywacke.
CLEONE, Mendocino County, California -.25-0.5mm; well sorted;
subangular - subrounded. Contains quartz, feldspar, serpentinite, chert
and basalt. The dark color is due to over abundance of basalt,
serpentinite and chert. Also may contain pieces of shell material. Type
of rock in area are basalt, serpentinite and chert.
HALF MOON BAY, California - 0.1 - .25 mm; very well sorted,
subangular to subrounded. Contains quartz, feldspar, mica and minor
magnetite. The cliffs along Half Moon Bay reveals the Mother Rock for
this sand. The cliffs releases millions of sand grains to re-enter the
rock cycle.
MONTARA BEACH, California - 0.1 - 7mm; very poorly sorted;
subrounded. Contains quartz, feldspar, and small pieces of granite with
mica and hornblende. This sand is eroded directly from granitic rock
very close to the beach. This is near the famous "Devils
Slide" area, where very badly weathered granite has created very
spectacular cliffs.
MONTEREY, California - 025 -7mm; very poorly sorted; subangular.
Contains quartz, feldspar and pieces of granitic  rock. The Mother rock
is a granite, but unlike Montara Beach sand, Monterey has not been
chemically weathered. Abrasion of large boulders of granite along the
coast have mechanically broken this sand.
RODEO BEACH, Marin County, California -0.1 -7mm; poorly sorted;
angular - rounded. Contains chert, serpentinite, quartz, basalt,
magnetite. This beach is in a cove behind the Golden Gate Bridge, where
very high energy waves erode the rocks exposed along this coast. The
source rocks include chert, serpentinite, and basalt.
- You may want students to draw a picture of each of the sand samples
and color the grains the color that they see. This will help them really
look at the different sands more carefully.
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