THE ROCK CYCLE


WHAT IS THE DIFFERENCE BETWEEN A ROCK AND A MINERAL?

THERE ARE THREE TYPES OF ROCKS. THEY ARE:

1. Igneous Rocks : Formed from the cooling of molten rock.
A. Volcanic igneous rocks formed from molten rock that cooled quickly on or near the earth's surface.
B. Plutonic igneous rocks are the result of the slow cooling of molten rock far beneath the surface.
2. Sedimentary Rocks : Formed in layers as the result of moderate pressure on accumulated sediments.
3. Metamorphic Rocks : Formed from older "parent" rock (either igneous or sedimentary) under intense heat and/or pressure at considerable depths beneath the earth's surface.


THE INTERRELATIONSHIP AMONG THE ROCK TYPES IS REFERRED TO AS THE
ROCK CYCLE
.


THERE ARE FOUR MAIN LAYERS THAT MAKE UP THE EARTH:

1. Inner Core - A mass of iron with a temperature of about 7000 degrees F. Although such temeratures would normally melt iron, immense pressure on it keeps it in a solid form. The inner core is approximately 1,500 miles in diameter.
2. Outer Core - A mass of molten iron about 1,425 miles deep that surrounds the solid inner core. Electrical currents generated from this area produce the earth's magnetic field.
3. Mantle - A rock layer about 1,750 miles thick that reaches about half the distance to the center of the earth. parts of this layer become hot enough to liquify and become slow moving molten rock or magma.
4. Crust - A layer from 4-25 miles thick consisting of sand and rock.

The core, mantle and crust of the earth can be envisioned as a giant rock recycling machine. However, the elements that make up rocks are never created or destroyed although they can be redistributed, transforming one rock type to another. See our Periodic Chart of the Elements for additional information about specific elements.

The recycling machine works something like this. Liquid (molten) rock material solidifies either at or below the surface of the earth to form igneous rocks . Uplifting occurs forming mountains made of rock. The exposure of rocks to weathering and erosion at the earth's surface breaks them down into smaller grains producing soil. The grains (soil) are transported by wind, water and gravity and eventually deposited as sediments. This process is referred to as erosion. The sediments are deposited in layers and become compacted and cemented (lithified) forming sedimentary rocks. Variation in temperature, pressure, and/or the chemistry of the rock can cause chemical and/or physical changes in igneous and sedimentary rocks to form metamorphic rocks. When exposed to higher temperatures, metamorphic rocks (or any other rock type for that matter) may be partially melted resulting in the creation once again of igneous rocks starting the cycle all over again.



As you might expect - since most of the earth's surface is covered by water - molten material from inside the earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water resulting in the formation of igneous rocks. Some low grade metamorphism often occurs during and after the formation of the rock due to the intrusion of the material by the sea water. As the molten material flows from the fissure, it begins forming ridges adjacent to it.

If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that igneous rocks form on the sea floor as spreading ridges. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the oceanic plate eventually "dives" under the adjacent continental plate. As the oceanic plate travels deeper, high temperature conditions cause partial melting of the crustal slab. When that occurs, the surrounding "country rock" (existing adjacent rock) is metamorphosed at high temperature conditions by the contact. The molten material is either driven to the surface as volcanic eruptions, or crystallizes to form plutonic igneous rocks.

In the classroom, an apple can be used to represent the structure of the earth. The peel represents the crust, the white portion of the apple, the mantle. The core of the apple represents the core of the earth. The thickness of each "layer" of the apple approximates the relative thickness of the earth's structural features. Almost all of our minerals, oil and gas come from just a thin outer portion of the crust at maximum depths of about three miles.


For individual specimens, visit our Rock and Mineral Specimens page. We also have 15 specimen collections of igneous, metamorphic and sedimentary rocks and an Introductory Rock Collection which includes 5 examples of each of the three types of rocks. Detailed information is available on our Boxed Collections page.