In many books, the Rock Cycle is oversimplified. Statements like
"igneous rocks can become sedimentary and metamorphic;" "
sedimentary rocks can become igneous and metamorphic;" or
"metamorphic can become sedimentary and igneous" are common.
The Rock cycle is much more complicated. This is illustrated on the
Rock Cycle diagram. The large counterclockwise arrows show the general trend of
the Rock Cycle. First, igneous rock forms from magma. This rock is then uplifted,
weathered, and eroded on the Earth’s surface, forming
sedimentary rock. The sedimentary rock is eventually buried within the crust
of the Earth, where pressure and temperature finally change it into
metamorphic rock. Eventually, some metamorphic rock may melt, beginning the
However, the smaller arrow indicate other paths of rock
Essentially, any rock type can be melted, weathered, or metamorphosed to
make any other rock type. Moreover, a rock can be remade into the same type
of rock, i.e., a metamorphic rock can be remetamorphosed. Rock formation is
dynamic and very complicated.
The Rock Cycle is tightly interwoven with the Plate Tectonic Cycle, in
that most rocks form at plate boundaries. In general, igneous rocks and
metamorphic rocks form most abundantly at divergent and convergent plate
boundaries. Sedimentary rocks can form anywhere on the Earth’s surface,
but the thickest accumulations are associated with convergent plate
boundaries, where volcanoes and mountain ranges form. Sedimentary rocks also
cover most of the ocean floor.
- Go over the Rock Cycle diagram on the student’s worksheet. Fill in
the processes like "melt," "erosion," or any other
word from the Rock Cycle diagram.
- Set up examination specimens of each of the rocks listed below. Write
the list of rock types and symbols (below) on the board. Explain the
origin and environment of formation of each rock. Be sure to tell the
students that these are just some of the places where these rock types
can form. Tell the students to locate where each type of rock would form
on the plate tectonic diagram. The rock cycle and plate tectonic
diagrams below show the answers. There may be multiple answers for each
limestone (L) = sedimentary rock; fine grained, calcium
carbonate, formed mainly in oceans, usually with organisms that make
shells; fizzes in HCl (releases carbon dioxide)
conglomerate (C) = sedimentary rock; course grained, formed in
sandstone (SS) = sedimentary rock; medium grained, formed in
oceans, lakes, rivers
marble (M) = metamorphic rock; from limestone, formed under
increased pressure and temperature
schist (Sc) = metamorphic rock; shiny surface, temperature and
pressure; usually formed from basalt and other fine grained volcanic
gneiss (Gn) = metamorphic rock; banded, pressure and temperature;
usually formed from granite or other course grained plutonic rocks
pumice (P) = igneous rock; light in weight, volcanic eruption with
high percentage of gas; volcanic
granite (G) = igneous rock; large minerals; formed in magma chamber
obsidian (O) = igneous rock; cooled quickly from lava; volcanic
basalt (B) = igneous rock; small, dark colored minerals, volcanic
- You may want students to write a description of each rock on their
worksheet. Use a hand lens to look at the size of the
crystals. Students may also want to draw a picture of the enlarged
- Review the answers with the whole class. Emphasize that the diagram
the students have completed shows just some of the possible range of
- For more information on rocks you may want to read the "Secondary"
units on the individual rocks. You may also want to purchase our general
Minerals and Rocks Kits for a more detailed explanation.