Geology: Rocks and Minerals
Minerals
For a material to be considered a mineral, the substance in question must meet the following criteria:- Naturally occurring (not man-made)
- Solid (not a liquid or a gas)
- Inorganic (not from living organisms)
- Definite chemical composition (specific chemical make-up)
- Definite crystalline structure (specific arrangement of atoms)
How do we identify minerals?
- Color- Most visible characteristic, but unreliable because many minerals share the same color and many minerals exist in different colors.
- Streak- The color of the mineral in powdered form (use a “streak plate”). Very reliable tool for identifying samples. Note: the color of the powdered form is often different form the color of the solid form.
- Fracture/ Cleavage- Cleavage is the tendency of a mineral to split along one or more smooth, flat surfaces. If a mineral does not display cleavage, it is said to have fracture, which means it breaks unevenly.
- Hardness- The mineral’s resistance to being scratched. Minerals are compared to the ten minerals on the “Moh’s Scale of Hardness”. Minerals are often compared to glass (hardness: 5.5).
- Luster- Either metallic (shiny, like a polished metal) or nonmetallic (dull, with no shine). Types of nonmetallic luster include glossy, pearly, greasy, earthy, etc.
- Other characteristics that can be tested include: magnetism, reaction with chemicals, taste, specific gravity, crystal form, fluorescence, optics.
All physical characteristics are a result of the mineral’s internal arrangement of atoms.
Igneous Rocks
Igneous rocks form from hot, molten rock that has solidified.- Hot molten rock inside the Earth is called magma.
- Once it reaches the surface, it is called lava.
Igneous rocks form when, deep inside the Earth, the temperatures are high enough to melt rock. As this molten rock material rises to the surface, it cools and solidifies into a solid rock. Sometimes this occurs underground, sometimes it occurs at the surface.
When magma cools and solidifies (crystallizes) while still inside the Earth, it forms an intrusive, or plutonic igneous rock. Because they take longer to cool, intrusive igneous rocks have large crystals (the longer the cooling time, the larger the crystals).
When lava cools and solidifies (crystallizes) after erupting onto the surface, it forms an extrusive, or volcanic igneous rock. Because they cool very quickly, extrusive igneous rocks have small, or even no crystals.
Characteristics of Igneous Rocks...
Felsic igneous rocks are:
- light in color
- contain feldspars and silicon
Mafic igneous rocks are:
- dark in color
- contain magnesium and iron
Igneous rocks are often described using the term texture. Common textures include:
- glassy (no crystals)
- fine (small crystals)
- coarse (medium crystals)
- very coarse (large crystals)
- Some extrusive igneous rocks cool so quickly that air becomes trapped in the rocks forming small gas bubbles. These rocks are called vesicular
Metamorphic Rocks
Metamorphic rocks form when existing rocks are exposed to intense heat and/or pressure inside the earth These intense conditions will alter the rocks, changing (morphing or recrystallizing) them into a new rock.There are two main types of metamorphism.
Regional Metamorphism
- Occurs over large areas
- Can happen over large areas (even hundreds of miles)
- Caused by intense pressure involved in plate tectonics
- Crystals are squished and altered
Contact Metamorphism
- Occurs along the edges of magma or lava
- Can occur above or beneath ground
- When lava or magma comes into contact with rocks, the heat alters those rocks
Metamorphic rock characteristics include...
- Foliation- There may be alignment or banding of minerals
- Distortion as a result of exposure to pressure