Rocks and

Minerals4 September 2015

Rocks and Minerals: Definitions

– A mineral is solid inorganic material of the Earth that has both a known chemical composition and a crystalline structure that is unique to that mineral

– A rock is a solid aggregate of one or more minerals that have been cohesively brought together by a rock-forming process.

Earth Interior: Physical

Mineralsthe building blocks of rocks

Mineral Characteristics

• natural

• inorganic

• solid

• definite composition

• crystal structure

Mineral CompositionMinerals are grouped or classified based on

their composition. There are 6 groups

1. Carbonates - contain carbon, oxygen, and one ore more metallic element

2. Silicates- formed from silicon and

oxygen - elements combined to

form a silicon tetrahedron, 1 silicon atom and 4 oxygen atoms

- formed from cooling magma

- either near the surface (few crystals) or deep below surface (larger crystals)

3. Oxides

- Minerals that contain oxygen and one or more other element(s)

4. Sulfates and Sulfides

- Minerals containing sulfur

Gypsum

Pyrite

5. Halides

-minerals containing halogen ions plus one or more other elements

Halite

6. Native ElementsMinerals that exist in

a relatively pure form

i.e. Gold, silver, copper

Gold crystal structure

Rocks

– Elements are chemically combined to form minerals– Minerals are physically combined to form rocks.

Igneous Rocks

• Magma- molten material underground

• Lava- magma that reaches the surface

• Igneous rocks are formed from magma that has cooled and hardened either beneath the surface or from a volcanic eruption

2 Ways to Form Igneous Rock• Intrusive Igneous

Rocks- form when magma hardens beneath Earth’s surface

• Magma intrudes into existing rocks

• Extrusive Igneous Rocks- form when lava hardens on the surface of the Earth

• Extruded onto the surface

granite

rhyolite

• Magma contains some gases, including water vapor- this make it less dense, so it rises

• As magma rises, it cools and forms crystals

• The longer the cooling time the larger the crystals

• Granitic Composition- contain mainly quartz and feldspar, some with biotite mica and amphibole. Make up major rocks of continental crust

• Basaltic Composition- contain mainly dark colored minerals and feldspar, along with Mg and Fe. Darker and denser than granitic composition

Igneous rock classification scheme based on mineral composition and texture. There are other blends of minerals with various textures, many of which have specific names.

Granite is a coarse-grained igneous rock composed mostly of light-colored, light-density, nonferromagnesian minerals. The earth's continental areas are dominated by granite and by rocks with the same mineral composition of granite.

This is a piece of obsidian, which has the same chemical composition as the granite. Obsidian has a different texture because it does not have crystals and is a volcanic glass. The curved fracture surface is common in noncrystalline substances such as glass.

Sedimentary Rocks- formed from compacted and cemented sediments

• Weathering physically and chemically breaks rocks into small pieces called sediments

• Sediments are moved by wind, water, ice, and gravity• Eventually, they are dropped and form layers that are cemented together

Weathering, Erosion, and Deposition

• Weathering breaks rocks down

• Erosion involves weather and the removal of rock

• Deposition is the dropping of sediments • Sediments are

deposited according to size

Compaction and Cementation• Compaction is the

process that squeezes the water out of the sediments. It is caused by the weight of the sediments.

• Cementation takes place when dissolved minerals are deposited in the tiny spaces among the sediments.

• Cementing holds the sediments together.

Classification of Sedimentary Rocks- based on formation

• Clastic Sedimentary Rocks are formed from weathered bits of rocks and minerals

• Grouped according to size of the sediments in the rock

• Chemical Sedimentary Rocks are formed when dissolved minerals precipitate from water solution

Coquinalimestone

• Sedimentary rocks hold many clues to the Earth’s history

• Layers of sediments are records of geologic events on Earth

• Fossils are unique to sedimentary rocks

This is a sample of breccia, a coarse-grained sedimentary rock with coarse, angular fragments. Compare the grain sizes to the centimeter scale.

This is a sample of sandstone, a sedimentary rock that formed from sand grains in a matrix of very fine-grained silt, clay, or other materials. The grains in this sample are mostly the feldspar and quartz minerals, which probably accumulated near the granite from which they were eroded.

This is a sample of limestone, a sedimentary rock made of calcium carbonate that formed under water directly or indirectly from the actions of plants and animals. This fine-grained limestone formed indirectly from the remains of tiny marine organisms.

Metamorphic Rocks- form when existing rocks undergo change through

heat and pressure• Metamorphism means

to change• Most metamorphic

changes occur at high temperatures and pressure

• These occur deep below the Earth’s surface and extend into the mantle

Agents of Metamorphism

• Heat- causes existing minerals to recrystallize or new minerals to form

• Pressure- causes the spaces between mineral grains to close= more compact rock= greater density

Increasing metamorphic change occurs with increasing temperatures and pressures. If the melting point is reached, the change is no longer metamorphic, and igneous rocks are formed.

This is a sample of marble, a coarse-grained metamorphic rock with interlocking calcite crystals. The calcite crystals were recrystallized from limestone during metamorphism.

Rock Cycle

• Earth is a dynamic planet with the surface and interior in a constant state of flux.– Internal changes alter the surface by moving the

Earth’s plates, building mountains.– Seas advance and retreat over the continents

brining in new materials and taking other materials away.

– Rocks are continually being changed by Earth’s forces

A schematic diagram of the rock cycle concept, which states that geologic processes act continuously to produce new rocks from old ones.

The Rock Cycle

Planetary History

• Planets formed hot: Energy from collisions, radioactivity

• Heavier materials dropped to center: metals at core; basic, mafic rocks in mantle; lighter float to top (differentiation)

• Radioactive heating declines; crust solidifies• Tectonics, impacts, volcanism, erosion

modify surfaces (next lectures)