The Rock Cycle

The rock cycle is a fundamental concept in geology that describes the transitions of rocks through different rock types: sedimentary, metamorphic, and igneous.
It's a continuous process driven by Earth's internal heat, solar energy, and gravity.

Key Processes

Weathering: The breakdown of rocks at the Earth's surface through physical and chemical means.
- Physical weathering: Mechanical breakdown into smaller pieces (e.g., freeze-thaw, abrasion).
- Freeze-thaw occurs when water enters cracks in rocks, freezes, expands, and eventually breaks the rock apart.
- Abrasion involves the wearing down of rocks by the impact of other rocks or particles carried by wind, water, or ice.
- Chemical weathering: Decomposition of rocks through chemical reactions (e.g., oxidation, hydrolysis).
- Oxidation is the reaction of rock minerals with oxygen, often seen as rust on iron-rich rocks.
- Hydrolysis is the reaction of rock minerals with water, leading to the formation of new minerals and dissolution of the rock.
Erosion: The transport of weathered material by wind, water, ice, or gravity.
- Wind erosion is most effective in arid and semi-arid regions, where it can transport fine-grained sediments over long distances.
- Water erosion includes both the impact of raindrops (splash erosion) and the flow of water over the surface (sheet erosion, rill erosion, gully erosion).
- Glacial erosion involves the grinding and plucking of rocks by moving ice.
- Gravity erosion includes landslides, rockfalls, and soil creep.
Sedimentation: The deposition of eroded material in layers.
- Sediments are typically deposited in areas such as riverbeds, lakes, and oceans.
- Accumulated sediments undergo compaction and cementation to form sedimentary rocks.
Lithification: The process by which sediments are compacted and cemented together to form sedimentary rocks.
- Compaction involves the squeezing together of sediments by the weight of overlying layers.
- Cementation involves the precipitation of minerals between sediment grains, binding them together.
Metamorphism: The transformation of existing rocks into new forms through heat and pressure.
- Occurs deep within the Earth.
- Changes the mineralogy, texture, and chemical composition of the parent rock.
- Contact metamorphism occurs when rocks are heated by nearby magma intrusions.
- Regional metamorphism occurs over large areas due to tectonic forces.
Melting: The process by which solid rock is heated to its melting point to form magma.
- Melting typically occurs in the Earth's mantle or lower crust.
- The composition of the magma depends on the composition of the parent rock and the conditions under which melting occurs.
Crystallization/Solidification: The cooling and solidification of magma or lava to form igneous rocks.
- Magma: Molten rock beneath the Earth's surface.
- Lava: Molten rock erupted onto the Earth's surface.
- Intrusive igneous rocks form from magma that cools slowly, allowing large crystals to grow.
- Extrusive igneous rocks form from lava that cools quickly, resulting in small crystals or a glassy texture.

Rock Types

Igneous Rocks: Formed from the cooling and solidification of magma or lava.
- Intrusive Igneous Rocks: Formed from magma that cools slowly beneath the Earth's surface, resulting in large crystals (e.g., granite).
- Granite is a coarse-grained rock composed mainly of quartz, feldspar, and mica.
- Extrusive Igneous Rocks: Formed from lava that cools quickly on the Earth's surface, resulting in small crystals or a glassy texture (e.g., basalt).
- Basalt is a fine-grained rock composed mainly of plagioclase feldspar and pyroxene.
Sedimentary Rocks: Formed from the accumulation and lithification of sediments.
- Clastic Sedimentary Rocks: Formed from fragments of other rocks (e.g., sandstone, shale).
- Sandstone is composed of sand-sized grains of quartz and feldspar.
- Shale is composed of fine-grained clay minerals.
- Chemical Sedimentary Rocks: Formed from the precipitation of minerals from solution (e.g., limestone, rock salt).
- Limestone is composed mainly of calcium carbonate ( $CaCO_3$ ).
- Rock salt is composed of halite (sodium chloride, $NaCl$ ).
- Organic Sedimentary Rocks: Formed from the accumulation of organic matter (e.g., coal).
- Coal is formed from the accumulation and compression of plant material.
Metamorphic Rocks: Formed from the transformation of existing rocks through heat and pressure.
- Foliated Metamorphic Rocks: Have a layered or banded appearance due to the alignment of minerals (e.g., gneiss, schist).
- Gneiss is a coarse-grained rock with distinct bands of light and dark minerals.
- Schist is a medium-grained rock with platy minerals such as mica aligned in parallel layers.
- Non-Foliated Metamorphic Rocks: Lack a layered appearance (e.g., marble, quartzite).
- Marble is formed from the metamorphism of limestone or dolostone.
- Quartzite is formed from the metamorphism of sandstone.

Pathways Through the Rock Cycle

Any rock type can transform into any other rock type through various processes.
Example:
- Igneous rock can be weathered and eroded into sediment.
- Sediment can be lithified into sedimentary rock.
- Sedimentary rock can be metamorphosed into metamorphic rock.
- Metamorphic rock can be melted into magma.
- Magma can crystallize into igneous rock.

Driving Forces

Earth's Internal Heat: Drives plate tectonics, volcanism, and metamorphism.
- Radioactive decay of elements in the Earth's interior generates heat.
- This heat drives convection currents in the mantle, which in turn drive plate tectonics.
Solar Energy: Drives weathering and erosion.
- Solar energy provides the energy for the hydrological cycle, which includes precipitation, runoff, and evaporation.
- These processes contribute to weathering and erosion.
Gravity: Drives erosion and sedimentation.
- Gravity pulls weathered material downhill, leading to erosion.
- Gravity also