Download Rock Notes: Description of Igneous, Sedimentary, Metamorphic

Rock Notes:
Description of Igneous, Sedimentary, Metamorphic rocks and how each rock was
formed.
Igneous rocks:
Igneous rock forms when hot, liquid rock, or magma, cools and solidifies.
Igneous rock begins as magma. When magma cools down enough, it "freezes" to
form igneous rock. Igneous rocks differ in what they are made of and how fast
they cool. The longer magma takes to cool, the more time crystals have to grow
and the coarser the rock's texture becomes.
Coarse-grained cools slowly and fine-grained cools quickly
The light-colored rocks are less dense than the dark-colored rocks are. The lightcolored rocks are rich in elements such as aluminum, potassium, silicon, and
sodium. These rocks are called felsic rocks. The dark-colored rocks, called mafic
rocks, are rich in calcium, iron, and magnesium, and poor in silicon.
Intrusive Igneous Rock
When magma intrudes, or pushes, into surrounding rock below the Earth’s surface
and cools, the rock that forms is called intrusive igneous rock. Intrusive igneous
rock usually has a coarse-grained texture because it is well insulated by
surrounding rock and cools very slowly. The minerals that form are large, visible
crystals.
Extrusive Igneous Rock
Igneous rock that forms from magma that erupts, or extrudes, onto the
Earth’s surface is called extrusive igneous rock. Extrusive rock is common
around volcanoes. It cools quickly on the surface and contains very small crystals
or no crystals.
Sedimentary Rocks:
Wind, water, ice, sunlight, and gravity all cause rock to physically weather
into fragments. Through the process of erosion, these rock and mineral fragments,
called sediment, are moved from one place to another. Eventually, the sediment is
deposited in layers. As new layers of sediment are deposited, they cover older
layers. Older layers become compacted. Dissolved minerals, such as calcite and
quartz, separate from water that passes through the sediment to form natural
cement that binds the rock and mineral fragments together into sedimentary rock.
Sedimentary rock forms at or near the Earth’s surface. It forms without the heat
and pressure that are involved in the formation of igneous and metamorphic rocks.
The most noticeable feature of sedimentary rock is its layers, or strata. A single,
horizontal layer of rock is sometimes visible for many miles. Road cuts are good
places to observe strata. Many features can tell you about the way sedimentary
rock formed. The most important feature of sedimentary rock is stratification.
Stratification is the process in which sedimentary rocks are arranged in layers.
Strata differ from one another depending on the kind, size, and color of their
sediment.
Clastic Sedimentary Rock
Clastic sedimentary rock forms when rock or mineral fragments, called
clasts, are cemented together. Chemical sedimentary rock forms when minerals
crystallize out of a solution, such as sea water, to become rock. Organic
sedimentary rock forms from the remains of once-living plants and animals.
Clastic sedimentary rock is made of fragments of rocks cemented together by a
mineral such as calcite or quartz. Clastic sedimentary rocks can have coarsegrained, medium-grained, or fine-grained textures.
Chemical Sedimentary Rock
Chemical sedimentary rock forms from solutions of dissolved minerals and
water. As rainwater slowly makes its way to the ocean, it dissolves some of the
rock material it passes through. Some of this dissolved material eventually
crystallizes and forms the minerals that make up chemical sedimentary rock.
Halite, one type of chemical sedimentary rock, is made of sodium chloride, NaCl,
or table salt.
Organic Sedimentary Rock
Most limestone forms from the remains, or fossils, of animals that once lived in the
ocean. For example, some limestone is made of the skeletons of tiny organisms
called coral. Coral are very small, but they live in huge colonies called reefs. Over
time, the skeletons of these sea animals, which are made of calcium carbonate,
collect on the ocean floor. These animal remains eventually become cemented
together to form fossiliferous limestone.
Rocks can also go through a process called metamorphism. The word
metamorphism comes from the Greek words meta, which means “changed,” and
morphos, which means “shape.” Metamorphic rocks are rocks in which the
structure, texture, or composition of the rock have changed. All three types of rock
can be changed by heat, pressure, or a combination of both.
Composition of Metamorphic Rock
Metamorphism occurs when temperature and pressure inside the Earth’s crust
change. Minerals that were present in the rock when it formed may not be stable in
the new temperature and pressure conditions. The original minerals change into
minerals that are more stable in these new conditions.
Look at Figure 2 to see an example of how this change happens.
All metamorphic rock has one of two textures—foliated or nonfoliated.
Foliated Metamorphic Rock
The texture of metamorphic rock in which the mineral grains are arranged in
planes or bands is called foliated. Foliated metamorphic rock usually contains
aligned grains of flat minerals, such as biotite mica or chlorite. Shale is a
sedimentary rock made of layers of clay minerals. When shale is exposed to slight
heat and pressure, the clay minerals change into mica minerals. The shale becomes
a foliated metamorphic rock called slate.
Metamorphic rocks can become other metamorphic rocks if the environment
changes again. If slate is exposed to more hear and pressure, the slate can change
into a rock called phyllite. When phyllite is exposed to heat and pressure, it can
change into schist. More heat and pressure cause minerals to separate into distinct
bands in a metamorphic rock called gneiss.
Nonfoliated Metamorphic Rock
The texture of metamorphic rock in which the mineral grains are
not arranged in planes or bands is called nonfoliated. Notice that
the rocks shown below do not have mineral grains that are
aligned. This lack of aligned mineral grains is the reason these
rocks are called nonfoliated rocks.