Rock Formation and the Rock Cycle

What are Rocks?

  • A rock is a naturally occurring solid mixture of one or more minerals or organic matter.
  • Rocks are classified by their formation, composition, and texture.
  • Rocks change over time through the rock cycle.

Rock Breakdown

  • Rocks become smaller through:
    • Erosion
    • Wind
    • Earthquakes
    • Rain
    • Floods
    • Slides
    • Human activities

Weathering vs. Erosion

  • Weathering: The breaking up of rocks and soil.
  • Erosion: The moving of weathered rocks and soil.
  • Weathering allows erosion to take place.

Forces of Change

  • Rocks change through:
    • Earthquakes
    • Volcanoes
    • Heat and pressure

Three Main Categories of Rocks

  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks

Igneous Rocks

  • Hardest and heaviest rock on Earth
  • Referred to as the parent rock because all other rock types formed from them.
  • Formed by:
    • Cooling
    • Solidification
  • Formed when hot, molten magma at 600600 to 1,300°C1,300 °C (1,1001,100 to 2,400°F2,400 °F) cools and crystallizes either at the Earth’s surface or inside the crust.

Types of Igneous Rocks

  • Intrusive Igneous Rocks: Magma pushes into surrounding rock below the Earth’s surface.
  • Extrusive Rocks: Forms when magma erupts onto the Earth’s surface (lava), cools quickly with very small or no crystals formed.

Formation of Igneous Rocks

  • Extrusive Igneous Rock:
    • Magma comes out and cools on the surface.
    • Cooling Time: Seconds to months
    • Texture: Fine-grained, lacks crystal growth
  • Intrusive Igneous Rock:
    • Magma cools beneath the surface.
    • Cooling Time: Thousands of years
    • Texture: Coarse-grained, shows crystal growth

Igneous Rock Examples

  • Basalt
  • Pumice
  • Obsidian
  • Rhyolite
  • Scoria
  • Dacite
  • Granite
  • Gabbro
  • Diabase
  • Diorite
  • Pegmatite
  • Peridotite

Sedimentary Rocks

  • Formed by erosion.
  • Sediments are moved from one place to another.
  • Sediments are deposited in layers, with older ones on the bottom.
  • Layers are formed through compaction and cementation.
  • Formed at or near the Earth’s surface.
  • No intense heat and pressure are involved.
  • Strata: Layers of rock
  • Stratification: The process in which sedimentary rocks are arranged in layers.

Formation of Sedimentary Rocks

  1. Erosion & Transportation: Eroded sediments end up in the water and begin to settle down.
  2. Deposition: Over time, more sediments are added to newly formed sedimentary layers.
  3. Compaction: The release of moisture from the sedimentary layers makes the layers compact.
  4. Cementation: Salt crystals glue the layers to form more compact sedimentary rocks.

Sedimentary Rock Examples

  • Breccia
  • Caliche
  • Chalk
  • Chert
  • Coal
  • Conglomerate
  • Diatomite
  • Limestone
  • Sandstone
  • Shale
  • Dolomite
  • Siltstone
  • Rock Salt
  • Gypsum
  • Ironstone
  • Coquina

Metamorphic Rocks

  • Form due to the transformation of pre-existing rocks in response to environmental changes such as heat, high pressure, and mechanical stress.
  • The process of change is called “metamorphism.”
  • Formation takes place deep within the Earth

Formation of Metamorphic Rocks

  • Massive pressure due to the weight of overlying rock layers, heat, and molten magma.

Metamorphic Rock Examples

  • Marble
  • Anthracite
  • Gneiss
  • Hornfels
  • Mariposite
  • Novaculite
  • Quartzite
  • Phyllite
  • Schist
  • Skarn
  • Slate
  • Soapstone

Rock Cycle

  • Volcanic eruption leads to Igneous rock formation.
  • Weathering and erosion of rocks leads to sediment formation.
  • Sediments compact and cement to form Sedimentary rock.
  • Heat and pressure transforms rocks to form Metamorphic rock.
  • Melting forms Magma.

What is a Rock Cycle?

  • The process that describes the gradual transformation between the three types of rocks.
  • It occurs continuously in nature through geologic time.

What Causes the Rock Cycle?

  • Plate Tectonics Activity
  • Erosional Weathering Processes

Steps of the Rock Cycle

  1. Magma
  2. Igneous rock (e.g., basalt, granite) formed through cooling.
  3. Sediments (e.g., sand, gravel) formed through weathering and erosion.
  4. Sedimentary rock (e.g., sandstone, limestone) formed through deposition, compaction, & cementation.
  5. Metamorphic rock (e.g., quartzite, marble) formed through heat & pressure.

Skipping the Rock Cycle

  • Rocks can move directly to other stages by cutting steps.

The Rock Cycle Diagram

  • Magma
    • Cooling, Crystallization → Igneous Rock
    • Melting ← Igneous Rock
  • Igneous Rock
    • Weathering and Erosion → Sediment
    • Heat and Pressure → Metamorphic Rock
  • Sediment
    • Deposition, Compacting and Cementing → Sedimentary Rock
  • Sedimentary Rock
    • Weathering and Erosion → Sediment
    • Heat and Pressure → Metamorphic Rock
  • Metamorphic Rock
    • Weathering and Erosion → Sediment
    • Melting → Magma

Formation Processes

1. Formation of Igneous Rocks

  • Melting, Cooling, and Crystallization
  • Magma, the molten rock present deep inside the earth, solidifies due to cooling and crystallizes to form igneous rocks.
  • Cooling can occur slowly beneath the surface or rapidly at its surface.

2. Formation of Sedimentary Rocks

  • Weathering, Erosion, Deposition, Compaction, and Cementation
  • Igneous rocks are broken down to form sediments in the form of gravel, sand, silt, and clay, which get mixed and pressed together to form sedimentary rocks.

3. Formation of Metamorphic Rocks

  • Metamorphism
  • Sedimentary and igneous rocks are buried deep underground due to tectonic plate movement, exposed to high heat and pressure, changing them into metamorphic rock.

4. Weathering

  • Igneous, sedimentary, and metamorphic rocks on the Earth's surface are constantly broken down by wind and water over a long time.

5. Erosion (Transportation)

  • Carrying away of broken rocks by rain, streams, rivers, and oceans to a distant place from their origin.

6. Deposition

  • During the carriage of rocks by rivers, the rock particles sink and become a layer of sediment.
  • Sediments build up and form small accumulations, which turn into sedimentary rock over time and pressure.
  • Melting of underground metamorphic rock forms magma, which on crystallization forms igneous rock, thus continuing the cycle.

Importance of the Rock Cycle

  • Aids in soil formation, sustaining life on Earth.
  • Forms life-sustaining minerals (sodium, iron, potassium, calcium) in the biosphere.
  • Creates Earth’s energy reserves like fossil fuels and radioactive sources.
  • Provides building materials such as iron, limestone, marble, granite, and basalt.
  • Provides raw materials for currency, investments, and adornments such as gold, diamonds, rubies, and emeralds.