PTR - The Rock Cycle Lesson

 

The Rock Cycle

With the exception of a small amount of atmosphere escaping into space and the few small meteorites crashing to Earth, the matter with which we currently interact has been here since the Earth first formed. The water that you drink has been evaporated, condensed, and precipitated countless times over, and the components of the air that you breath has been circulated through living organisms in the form of nitrogen, oxygen, and carbon cycles. The rocks and minerals that compose the crust are moved and transformed by a similar cycle.

Minerals are naturally occurring, inorganic solids that have a specific chemical composition and a crystalline structure:

1. "Naturally occurring" meaning that people didn't make it,
2. "Inorganic" meaning that it was not formed by organisms (wood and pearls are not minerals),
3. "Solid" meaning not a liquid or gas at standard temperature and pressure (25 degrees Celsius and 1 atm),
4. "Specific chemical composition" meaning that atoms are chemically bonded together in specific ratios to form the mineral - it has a chemical formula, such as NaCl (halite),
5. "Crystalline structure" meaning that the molecules arrange themselves in an orderly fashion, forming patterns that make up various crystal structures.

Rocks are mixtures of minerals. Granite, for example, is a mixture of feldspar, quartz, and mica minerals that are locked together by their crystal structures.

Igneous rocks are formed when molten material cools and hardens. Molten material that cools underground (magma) is known as intrusive igneous rock, while molten material that escapes to the surface (lava) before it cools is known as extrusive igneous rocks. Lava cools much quicker above-ground than magma does below-ground, so the properties of extrusive and intrusive igneous rocks are different. The slow cooling underground allows for the formation of larger crystals as the minerals solidify, while rapid cooling above ground does not permit large crystals to grow. An example of intrusive igneous rock is granite, while an extrusive igneous rock is basalt.

Sedimentary rock forms through a process that involves several steps. First, existing rock must be broken down into smaller pieces through the process of weathering. Weathering can be accomplished through different actions:

1. moving water can break down rock (directly by dissolving away some minerals and indirectly by tumbling other rocks onto it),
2. water freezing in cracks in the rocks can swell and break them apart,
3. plant roots can also force their way into cracks, breaking the rock apart,
4. rocks can become dislodged and fall onto other rocks,
5. sandstorms can wear away rocks,
6. and so on.

Once this material has been broken down, it is then relocated and spread out through the action of wind and water. This step is known as erosion. The amount of erosion that occurs depends on several factors, including the size of the rocks and the amount and duration of force (sand will move farther than pebbles in the same stream, for instance).

As more layers build up over time, the weight of the upper layers compresses the lower layers - called compaction - which reduces the space between the individual particles. Water is still able to penetrate these voids, though, and as it evaporates it leaves behind any minerals that were dissolved in it. These minerals crystalize and lock the surrounding particles together (cementation). The resultant material is sedimentary rock. Examples include sandstone, breccia, and conglomerate.

Metamorphic rock is any rock that has been changed due to intense heat and pressure, but not to the point where it has melted into magma. These transformations usually occur deep below the Earth's surface, such as the areas near subduction zones.

There are two different types of metamorphic rock. Foliated metamorphic rocks have layers or a banded appearance. Examples include slate (formed from shale) and gneiss.

Non-foliated metamorphic rocks do not have layers or bands; examples include quartzite (formed from sandstone) and marble (formed from limestone).

 

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