Basic Geology An Introduction to Rocks and Minerals
Jan 03, 2016
Basic GeologyAn Introduction to Rocks and Minerals
What is a Rock?
Rocks and minerals are often referred to as the same thing. In fact, they are not the same.
A mineral is a chemical compound or a mixture of compounds.
A rock is an aggregate of minerals. An aggregate is a mixture that is cemented or mechanically joined together.
In other words, rocks are composed of minerals.
Examples of Minerals
Quartz
Potassium Feldspar
Biotite Mica
Rock: An Aggregate of Minerals
Granite
A rock composed of minerals:
Quartz
Potassium Feldspar
Biotite Mica
Classification of Rocks
Rocks are classified by their method of formation.
There are three classes of rocks:
1. Igneous
2. Sedimentary
3. Metamorphic
Igneous Rock
Igneous means “rock from fire”. Igneous rocks form from a melt, a complex
mixture of liquids and gases, also known as lava or magma.
Melts form deep within the Earth and can have temperatures over 2000 o C.
Magma is melt material that never reaches the Earth’s surface before it cools.
Lava is melt material which erupts to the surface and cools.
Magma and Lava
Identification of Igneous Rock
Igneous rock has two types:
1. Intrusive
2. Extrusive
These types of igneous rock are determined by where the rock forms and the time it takes to cool.
Intrusive Igneous Rock
Intrusive igneous rock forms from magma which never reaches the surface.
The type is determined by looking at the size of mineral crystals in the rock.
When magma does not reach the surface, it cools slowly. This allows minerals in the rock to form large crystals.
A common intrusive igneous rock is granite.
Stone Mountain, Georgia
Extrusive Igneous Rock
Extrusive igneous rock forms from lava cooling on the surface.
Cooling can be due to exposure to air (on land) or water (on the seafloor).
Rapid cooling keeps the minerals in the rock from growing so they remain small and dense.
Examples of extrusive igneous rock are basalt and rhyolite.
Extrusive Igneous Rock
Basalt
Rhyolite
Sedimentary Rock
Water is the common factor in the formation of all sedimentary rock.
Seventy-five percent (75%) of all the visible rock on the Earth’s surface are sedimentary.
Sedimentary rock often has a layered pattern when exposed. These layers, called strata, results from gradual deposition of sediments over a long period of time.
Grand Canyon, Arizona
Formation of Sedimentary Rocks
The most common method of formation of sedimentary rock is weathering and erosion of existing rock.
Rain or moving water carries eroded sediments until the flow of water slows and the sediments are deposited.
If the water dries up, the sediment dries out and becomes cemented, a process called lithification.
This type of formation is called clastic sedimentation.
Sedimentary Rock FormationsSandstone arch in Utah Shale strata in New Mexico
Other Types of Sedimentary Rocks
Chemical sedimentary rocks are formed when compounds dissolved in water precipitate out or the water evaporates.
Organic sedimentary rocks are formed from the remains of plants and animals, including shells, that lithify to form solid rock.
Other types of Sedimentary Rock
Chemical Sedimentary
Halite Gypsum
Organic Sedimentary
Limestone Coquina
Metamorphic Rock
Metamorphism means change. Metamorphic rock is formed by changing
existing rock with heat and pressure. All rocks, igneous, sedimentary or metamorphic,
under the right conditions, can become metamorphic.
Metamorphism can change the shape, texture and mineral composition of an existing rock.
Formation of Metamorphic Rock
Identification of Metamorphic Rock
Metamorphic rock can be identified as being one of two types:
1. Foliated Texture
2. Nonfoliated Texture
Foliated Texture
A foliated rock forms when heat melts the minerals then pressure rearranges them in bands on cooling.
An example would be granite becoming gneiss.
Nonfoliated Texture
Nonfoliated metamorphic rock forms when the minerals are melted and dissolved then reform as a single mass with no layering.
An example would be limestone morphing into marble.
Uses of Metamorphic Rock Marble (limestone → marble)
Slate (shale → slate)
The Rock Cycle
What should be clear by now is that no rock lasts forever and that existing rocks can be changed .
This cycle of change, or recycling, is called the Rock Cycle.
Even though this is a continuing cycle, the time required is longer than the human lifetime. Keep in mind that when looking at geologic time, nothing is permanent!
Although the Rock Cycle can start with any type of rock, it is easiest to start with igneous rock.
1. Magma cools over time and begins to be weathered and eroded to form sediment.
2. Sediments compact and lithify to form sedimentary rock.
3. The sedimentary rock gets buried deeper and is exposed to heat and pressure becoming metamorphic rock.
4. The metamorphic rock can be carried deeper toward the mantle where it melts again to magma.
Rock Cycle
In Summary