Reply to ASK-AN-EARTH-SCIENTIST

Subject: An Introduction to Rock Types

I need some information and ideas to teach 6th graders about rocks and (the) formation of them.

I am glad to hear that you are teaching your students about rocks and their origin. I suggest you check out a Physical Geology or Introductory Geology Textbook and refer to the description of the Rock Cycle. There are three families of rocks: (1) Igneous Rocks, (2) Sedimentary Rocks, and (3) Metamorphic Rocks. The Igneous Rocks are those that form from molten magma. Magmas are melted rock and they rise through the earth's crust. Those magmas that cool very slowly at great depths in the crust as giant magma chambers eventually solidify to form "Intrusive Igneous Rocks" such as granite (the granite is composed of an interlocking network of minerals rich in quartz and feldspars). These very large intrusions tend to formthe cores of major mountains, and can be later exposed by erosion. Classic examples occur as the Sierra Nevada Mountains of California, The Rocky Mountains and the Appalachians. If the magma actually makes it to the earth's surface before cooling, a volcano is born and the magma erupts at the earth's surface to form lava. Rocks that form in this way are thus called "Extrusive Igneous Rocks."
When mountains are weathered, broken down and eroded away and thus produce sediment. The sediment (such as sand and mud) is carried to inland basins and to the oceans where it is deposited. The sediment accumulates, is buried by succeeding layers of sediment and is eventually compacted and cemented together to form a sedimentary rock. Examples include conglomerates (very coarse, bouldery or pebbly deposits), sandstones (composed of sand sized grains of quartz), and mudrocks called shales. Sedimentary rocks can also form by biological processes, such as the formation of limestones as a result of the proliferation of coral reefs and their surrounding biology; such limestones have formed vast deposits over the continents during times when sea level stood much higher than today and the oceans flooded large areas of the continents (up to 125 meters abovepresent sea level). The may also form inorganically, such as when a large body of salt water dries up to form an evaporite deposit. An example isthat about 5 million years ago, the Mediterranean became isolated from the Atlantic Ocean as tectonic forces closed the Gibraltar Straits. The entire Mediterranean Ocean dried up, and all the salt of the sea water precipitated out to form extensive salt deposits there. You can try it yourself, by evaporating a gallon of seawater to completion.
The last family of rocks are called the Metamorphic Rocks. They get this name because the under go a metamorphism (a change in form). After great burial and due to the confining pressures of the overlying rocks (overburden), as well as to the great heat that occurs with depth in thecrust, any rock will be forced to recrystallize and transform into a newrock to conform with its new surroundings. In this way, and sandstone is metamorphosed to form a chert (quartzite), and a limestone is sed to form a rock called marble. However, at greater burial depths, the rocks all melt completely to form a new magma, and the process starts all over again. Thus we call this the rock cycle.

Craig R. Glenn, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822