Waco Gem and Mineral Club, Waco, Tx

10.2 Plate Tectonics and the Rock Cycle


10.2: Plate Tectonics and the Rock Cycle

Some have described our Earth as a round soft-boiled egg with a partly solid/partly liquid hot core surrounded by a syrupy hot layer of soft rock known as the mantle, both contained within a thin and brittle outer crust of hardened rock.


Source: United States Geological Survey website, "Education" link.

On Earth, the rocks making up the crust are constantly moving through a cycle of formation and change through processes involved with plate tectonics. The thin, brittle crust of the Earth is not an even shell, as with our soft-boiled egg example, but rather, is cracked and divided into a number of plates that float and travel over the more fluid mantle. Much of the earth‘s seismic activity (earthquakes, volcanic eruptions, mountain building) occurs at the boundaries of these plates, where plates collide (as in the Himalaya Mountains), diverge (as along the mid-Atlantic ridge), slide past one another (as at the San Andreas fault), or where one overrides another (as along the east coast of Japan). Several web sites offer animations and/or instructions on making earthquake models so that you can demonstrate their effects and illustrate the different sorts of plate boundaries. For instance, see:

How to Make an Earthquake Model for Kids

USGS Science Fair Project Ideas

Earth Science 3D Paper Models and Toys heading

For photos of various earthquake events, you might check out the web site Yup…rocks.

As a result of these tectonic processes, with plates colliding, diverging, overriding, or sliding past one another, new rock is formed, old rock is worn down and re-deposited as sediment, and other rocks are changed through heat and pressure. You can use various types of rocks to illustrate this rock cycle.

Igneous rocks formed from hot, molten magma, either deep underground (e.g., granite) or extruded onto the planet‘s surface (e.g., basalt). Igneous processes can form volcanoes and mountains that lift land up and create new land.

Sedimentary rocks, on the other hand, result from processes that wear the earth down. Gravity, combined with the weathering properties of wind, rain, and freezing, disintegrates rocks, breaks them into smaller components, and transports them into valleys and basins as gravel, sand, or mud, where they pile up in layers and eventually harden into the sedimentary rocks known as conglomerate, sandstone, and mudstone or shale. Sedimentary rocks also form chemically, as when calcium carbonate precipitates out of tropical seas to form limestone or when seas or lakes evaporate, leaving behind deposits of halite or gypsum.

Sometimes, igneous and sedimentary rocks get buried under other rocks and get caught up into immense forces involved in plate tectonics and mountain building. When this happens, these rocks get heated and squeezed, and the pressures can change their structures and transform them into whole new rocks, known as metamorphic rocks. These include rocks such as gneiss, schist, slate, or marble.

Here are some illustrations of how rocks move through a Rock cycle

Granite is an igneous rock that hardened and crystallized from molten magma deep beneath the earth. You‘ll see bits of crystallized quartz in granite. When granite weathers, these quartz crystals get worn down into grains of sand. When deposited in a valley, lakebed, or ocean, sand can harden into the sedimentary rock called sandstone. If the sandstone is buried and subjected to heat and pressure, it will transform into the metamorphic rock called quartzite.

Granite Sandstone Quartzite
igneous   sedimentary   metamorphic

Also, the bits of flaky mica and the feldspar in igneous granite can get worn down into silt and clay. When that hardens, it becomes sedimentary shale. And when shale is subjected to heat and pressure, the original mica re-crystallizes to form flat, platy layers of metamorphic slate or schist.

Granite Shale Slate or Schist
igneous   sedimentary   metamorphic

Here‘s a simplified diagram of the rock cycle. Your kids should be able to find other diagrams in geology books that they can get from the library or a bookstore or from sites on the World Wide Web. Have them create a large poster of the rock cycle in which they list different sorts of rocks they might expect to find at different points along the cycle. To create a three dimensional poster, they might glue small specimens of some of the different types of rocks alongside their lists.

Rock Cycle.jpg

Source: United States Geological Survey.