WEEK Six: Rockin' our World

Some Background Information from

EARTH BOOK
© 2011 AIMS Education Foundation

Igneous Rocks

The word igneous means born from fire. Igneous rock is formed from magma that has hardened and crystallized. Magma is molten rock that also contains early-formed crystals and dissolved gases. Lava is magma that reaches the surface of the Earth, usually through erupting volcanoes.
Geologists classify igneous rocks into two main types—intrusive and extrusive. Intrusive igneous rocks form underground, which causes them to cool slowly. This often gives them a coarse texture with large mineral crystals. Granite and gabbro are examples of intrusive igneous rocks.
Extrusive igneous rocks form when magma reaches the surface of the Earth or the ocean floor, cooling rapidly. This causes them to have very small crystals. Basalt, obsidian, and pumice are examples of extrusive igneous rocks.
In terms of mineral composition, intrusive and extrusive igneous rocks can be the same. For example, basalt, which makes up much of the ocean floor, is identical in composition to gabbro, which is formed deep in the Earth’s crust. The former is extrusive and the latter is intrusive, so they have very different appearances despite their identical compositions.

Sedimentary Rocks

Sedimentary rocks are made up of materials that were once a part of another rock. These parts, called sediments, were deposited as layers of loose materials. Most sediments are deposited on ocean floors or at the bottoms of rivers and lakes. Over time, the loose sediments are formed into solid rocks.
Sedimentary rocks are classified into three groups based on what the original sediments were. Clastic sedimentary rocks form from layers of sediments. Pressure causes the water around the sediments to be squeezed out and the sediments are cemented together. Sandstones, shales, and conglomerates form this way.
Chemical sedimentary rocks form when rock fragments dissolve in water. Over time the water evaporates and the minerals that were in the rock crystallize into large deposits. Rock salt and gypsum form this way.
Organic sedimentary rocks form from the sedimentary remains of shells, skeletons, and other plant and animal parts. Limestones and coals form this way.

Metamorphic Rocks

Metamorphic rocks are formed when rocks are changed due to heat and pressure. These changes take place deep underground where heat from the Earth and pressure from the layers of rock above create extreme conditions.
There are two types of metamorphic rocks—foliated and nonfoliated. Foliated rocks appear to have layers like a sedimentary rock. These layers are actually bands of minerals that have been aligned. Slate, schist, and gneiss are the three main types of foliated metamorphic rock.
Nonfoliated metamorphic rocks originate from rocks composed predominantly of one mineral, such as sandstone and limestone. When these rocks are subjected to heat and pressure, the resulting metamorphic rocks have a more granular texture. Quartzite, marble, amphibolite, metaconglomerate, and hornfels are the main types of nonfoliated metamorphic rock.
Determining the rock from which a metamorphic rock originated can be complicated. The same rock type can become many things depending on the intensity of the heat and pressure applied. The sedimentary rock shale becomes the metamorphic rock slate when subjected to relatively low heat and pressure. Slate can become schist with more intense heat and pressure, and schist can become gneiss with even more heat and pressure. Gneiss is also formed from the igneous rock granite, and has a similar mineral composition.

MOHS Scale

Quartz, feldspar, and mica are examples of minerals. Minerals are naturally occurring inorganic substances. Inorganic means they are made up of things that are not alive. Minerals have very specific chemical and physical properties. Some minerals consist of only one element, but most are compounds. Minerals are usually identified by their physical properties. Some of these properties are hardness, density, and specific gravity.
Most rocks are natural combinations of two or more minerals. Rocks are classified based on the conditions under which they formed.
Sedimentary Rocks
Sedimentary rocks are formed by the action of wind, water, ice, or organisms. They cover about three-fourths of the Earth’s surface. Most are depos- ited as sediments on the bottom of rivers, lakes, and seas. Many sedimentary rocks have been moved by water, wind, waves, currents, ice, or gravity. The most common sedimentary rocks are sandstone, limestone, conglomerates, and shale.
Igneous Rocks
Igneous rocks are formed at very high tempera- tures. They come from magmas, which are molten mixtures of minerals. These magmas come from deep within the Earth. If they cool off below the surface, they form intrusive rocks, which may later be uncov- ered through the process of erosion. When magmas reach the surface and cool, they form extrusive rocks. Granite and basalt are examples of igneous rocks.

Metamorphic Rocks
Metamorphic rocks are those that have been changed from some other type of rock. They change as a result of heat, pressure, or chemical action. All kinds of rocks can be changed. The result is a new crystal structure, the formation of new minerals, or a change in the rock’s texture. Slate was once shale. Marble comes from lime- stone. Granite can become gneiss.