GE147

The Rock Cycle

Referenced Website: http://ttt.ssd.k12.wa.us/projects/web/rockteam/WebSite/rockcycle.htm.htm

Rocks are formed, deformed, and broken down through many different processes collectively known as the rock cycle. Over the course of this cycle, rocks are altered both physically and chemically and are transported a variety of ways over the land and even through the earth's interior. As can be seen by this diagram it is conceivable that material that was once at the top of a mountain can end up thousands of feet below the ocean's surface, or 50 kilometers within the earth's crust.

Plate Tectonics

The Plate Tectonic Theory is the theory that the earth's crust is divided into individual movable pieces called plates that move on response to internal heat currents that originate beneath the earth's crust. These currents are called CONVECTION CURRENTS and act much like the weather systems that form in the atmosphere, causing the earth's plates to collide and separate. The map below shows these plate boundaries.

Heat rises. This is a fact that many of us are very familiar with. Material is heated by the earth's core and in turn rises towards the crust. As this material cools it begins to sink again. This phenomenon forms a circular pattern shown on the attached figure and this motion causes the plates to move.

A Global View of Plate Tectonics

Referenced Website: http://www.extremescience.com/PlateTectonicsmap.htm \

Movement of Crustal Plates

Tectonic Plates can collide or separate. Colliding plates are called CONVERGENT and separating plates are called DIVERGENT. Examples of these are shown on the attached figure.

Convergent plates can lead to SUBDUCTION ZONES where the sea floor, which is very dense pushes under the relatively light continental crust (Example: Western United States). Convergent plates lead to the formation of mountains either by physically pushing up the continental crust or by the melting and buoyant rise of material during subduction. This phenomenon is depicted on the attached figure. Convergent plates can also cause continents to collide into one another. Since continents are of similar densities their collision does not lead to subduction. Instead they push up against each other forming a mountain range (Example: India/Asia)

Divergent plates lead to sea floor spreading (Mid-Atlantic Rift) where new oceanic crust is being made continuously. This phenomenon is also shown on the attached figure.

Major Rock Types

There are three major rock types that make up the primary components of the Rock Cycle. These groups are called IGNEOUS, SEDIMENTARY, and METAMORPHIC and are labeled for the environment in which they were formed.

IGNEOUS ROCKS:

Igneous rocks are formed by the cooling of magma (molten rock material formed within the earth). The appearance of igneous rocks are a function of how fast the magma cools. INTRUSIVE igneous rocks are formed under the earth's surface and cool slowly (Example: Granite). Slowly cooling igneous rocks have large crystals (coarse grained). EXTRUSIVE igneous rocks are exposed to the earth's surface and cool rapidly (Example: Basalt, Lava). Rapidly cooling igneous rocks have small crystals (fine grained). Intrusive rocks are brought to the surface by rising through the continental crust (buoyant rise). Extrusive rocks are brought to the surface as lava blasted out of volcanoes or oozing out of ocean ridges.

SEDIMENTARY ROCKS:

Sedimentary Rocks are form by the compressing and heating of unconsolidated material (sand, clay, shells, coral) so that they become chemically cemented together. Sediments that form these rocks are materials that have been transported (usually via water) to points of deposition (usually flat lying areas such as valley or the ocean floor) giving the rocks that are formed a layered or stratified appearance. Sedimentary rocks such as LIMESTONE are composed of carbonate (CaCO₃) materials such as shells and corals.

METAMORPHIC ROCKS:

Metamorphic rocks are formed when existing rocks (Igneous, Sedimentary, or other Metamorphic) are subjected to high pressure and high heat. These conditions cause the rock to change chemically and re-form as an altered form of its old self. This can occur where convergent plates are colliding into one another or where semi-molten material is rising through the crust.

EXCERCISE:

Peg Matite is an advantageous scientist who likes to go hiking. One summer she decided to venture up into the Cascade Range in Washington State to take a look at Mount Rainier, a dormant volcano whose last known eruption was in 1894. Being a rock enthusiast she had hoped that despite the 35 sq. miles of snow and ice sheπd be able to see some of the bedrock from the summit. After hours of breathtaking scenery and hiking, she had arrived at the summit and was lucky enough to find an outcrop of exposed rock. She noticed that the rock looked similar as many other summits sheπd been, but wondered if it was possible for parts of rock from other mountain ranges to end up here on the summit of a volcano in Washington State. As she began to think about the possibilities for why the similarity could be, she remembered plate tectonics and the rock cycle. Using only these two processes, describe the transport of a grain from the Rockies in northern Colorado to Mount Rainier. Keep in mind that both of these places are in the western US. This is important when describing the plate tectonic processes. Make your descriptions at least one page and turn in at the beginning of lab next week.