Rock Cycle - Rocks (3B)

  • Comparing different sand samples.
  • Contrasting the components of sand.
  • basalt
  • chert
  • erosion
  • granite
  • quartz
  • serpentinite

Students compare rocks with sand.

Colorful sand grains from Rodeo Beach, California.


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.

  1. 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.

  2. 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.
  3. 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.
  4. 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 reflecting microscope.
  5.  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, and dark lithic fragments. The dark color is due to about 20% of lithic fragments. Also may contain pieces of shell material. Type of rock from drainage area is greywacke.

    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.

  6. 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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