The rock cycle is a fundamental concept in Earth science, describing how rocks transform and cycle through different forms. It connects igneous, sedimentary, and metamorphic rocks through processes like weathering, erosion, melting, and metamorphism.

Understanding the rock cycle helps us grasp Earth's dynamic nature. It explains how landscapes form, minerals concentrate, and Earth's surface constantly changes. This knowledge is crucial for fields like geology, resource exploration, and environmental science.

Rock Types

Igneous Rocks

• Form from the cooling and solidification of magma (molten rock beneath Earth's surface) or lava (molten rock on Earth's surface)
• Classified based on their mineral composition and texture
  • Felsic igneous rocks (granite) are rich in silica and light-colored minerals
  • Mafic igneous rocks (basalt) are rich in magnesium, iron, and dark-colored minerals
• Intrusive igneous rocks (diorite) cool slowly beneath Earth's surface, forming large crystals
• Extrusive igneous rocks (obsidian) cool rapidly on Earth's surface, forming small crystals or a glassy texture

Sedimentary Rocks

• Form from the accumulation and lithification of sediments, which are loose particles of rock, mineral, or organic matter
• Classified based on their texture and composition
  • Clastic sedimentary rocks (sandstone) form from the cementation of rock and mineral fragments
  • Chemical sedimentary rocks (limestone) form from the precipitation of minerals from solution
  • Organic sedimentary rocks (coal) form from the accumulation and compression of organic matter
• Often contain fossils, which are the preserved remains or traces of once-living organisms

Metamorphic Rocks

• Form from the transformation of pre-existing rocks under high temperature and pressure conditions within Earth's crust
• Classified based on their texture and the type of metamorphism they have undergone
  • Foliated metamorphic rocks (gneiss) have a layered or banded appearance due to the alignment of minerals
  • Non-foliated metamorphic rocks (quartzite) have a uniform texture without visible layering
• Metamorphic grade refers to the intensity of metamorphism, with higher grades resulting in more significant changes to the rock's mineralogy and texture

Weathering and Erosion

Weathering Processes

• Physical weathering involves the mechanical breakdown of rock into smaller fragments without changing its chemical composition
  • Freeze-thaw weathering occurs when water freezes and expands in rock crevices, causing the rock to break apart
  • Thermal expansion and contraction can cause rocks to crack due to temperature changes
• Chemical weathering involves the alteration of rock's chemical composition through reactions with water, air, or organic compounds
  • Dissolution occurs when minerals dissolve in water, such as the formation of caves in limestone
  • Oxidation involves the reaction of minerals with oxygen, such as the rusting of iron-rich rocks
• Biological weathering is caused by the actions of living organisms, such as the growth of plant roots or the production of organic acids by lichens

Erosion and Transportation

• Erosion is the removal and transportation of weathered rock material by agents such as water, wind, or ice
• Water erosion includes the action of rivers (Grand Canyon), waves, and glaciers
  • Rivers erode their banks and beds, creating valleys and canyons
  • Waves erode coastlines, forming cliffs and beaches
  • Glaciers erode the land beneath them, creating U-shaped valleys and fjords
• Wind erosion is most effective in arid environments, creating features such as sand dunes and rock formations (Arches National Park)
• Mass wasting is the downslope movement of rock and soil under the influence of gravity, including landslides and rockfalls

Sedimentary Processes

Deposition

• Deposition is the settling and accumulation of sediments in a new location
• Sediments are transported and deposited by agents such as water, wind, or ice
  • Water deposits sediments in layers, forming features such as deltas (Mississippi River Delta) and floodplains
  • Wind deposits sediments in arid environments, creating sand dunes and loess deposits
  • Ice deposits sediments as glaciers melt, forming moraines and outwash plains
• The size and shape of sediments depend on the energy of the depositional environment, with larger, more angular fragments deposited in high-energy environments (gravel in a fast-flowing river) and smaller, more rounded particles deposited in low-energy environments (clay in a lake)

Lithification

• Lithification is the process by which loose sediments are converted into solid sedimentary rock
• Compaction occurs as the weight of overlying sediments causes the grains to be pressed together, reducing pore space
• Cementation involves the precipitation of minerals (calcite or silica) from groundwater, binding the sediment grains together
• Lithification can occur over millions of years, depending on factors such as the composition of the sediments, the presence of cementing fluids, and the depth of burial

Igneous and Metamorphic Processes

Melting and Magma Generation

• Melting occurs when a rock's temperature exceeds its melting point, causing it to change from a solid to a liquid state
• Magma is molten rock beneath Earth's surface, formed by the melting of the mantle or crust
  • Decompression melting occurs when hot mantle rock rises and experiences a decrease in pressure, causing it to melt (mid-ocean ridges)
  • Flux melting occurs when water or other volatile compounds are added to rock, lowering its melting point (subduction zones)
  • Heat transfer from hot magma or nearby intrusions can also cause melting of surrounding rock
• The composition of the resulting magma depends on the source rock and the extent of melting, with partial melting producing more silica-rich magmas (rhyolite) and complete melting producing less silica-rich magmas (basalt)

Crystallization and Igneous Rock Formation

• Crystallization is the process by which magma cools and solidifies to form igneous rocks
• As magma cools, minerals crystallize in a specific order based on their melting points, with high-temperature minerals (olivine) forming first and low-temperature minerals (quartz) forming last
• The rate of cooling determines the texture of the resulting igneous rock
  • Slow cooling beneath Earth's surface allows for the growth of large crystals (phaneritic texture in granite)
  • Rapid cooling on Earth's surface results in small crystals or a glassy texture (aphanitic texture in basalt or obsidian)
• The composition and texture of igneous rocks provide information about their formation conditions and the tectonic setting in which they formed

Metamorphism and Metamorphic Rock Formation

• Metamorphism is the transformation of pre-existing rocks in the solid state due to changes in temperature, pressure, or chemical environment
• Regional metamorphism occurs over large areas, typically associated with mountain-building events (Himalayas), and results in the formation of foliated metamorphic rocks (schist)
  • The intensity of regional metamorphism increases with depth, resulting in a sequence of metamorphic rocks known as a metamorphic facies series
• Contact metamorphism occurs when magma intrudes and heats the surrounding rock, resulting in the formation of non-foliated metamorphic rocks (hornfels) near the contact
• Hydrothermal metamorphism involves the interaction of rocks with hot, mineral-rich fluids, often resulting in the alteration of the rock's chemical composition (metasomatism)
• The mineralogy and texture of metamorphic rocks reflect the type and intensity of metamorphism they have undergone, as well as the composition of the parent rock