Rocks are the building blocks of Earth's crust, forming through various processes. The rock cycle explains how these rocks transform over time, driven by Earth's internal heat and surface processes. Understanding this cycle is key to grasping Earth's dynamic nature.

Igneous, sedimentary, and metamorphic rocks each have unique characteristics based on their formation. The rock cycle connects these types, showing how they change through processes like weathering, melting, and metamorphism. This constant transformation shapes Earth's surface and interior.

Rock Classification by Formation

Categories Based on Formation Processes

• Rocks are classified into three main categories based on how they form: igneous, sedimentary, and metamorphic
• The formation environment and processes that create a rock determine its unique set of characteristics
• Igneous rocks form from the cooling and solidification of magma (below Earth's surface) or lava (at Earth's surface)
• Sedimentary rocks form through the deposition and consolidation of weathered rock and mineral fragments (clastic), chemical precipitation from solution (chemical), or the accumulation of organic material (biologic)
• Metamorphic rocks form when pre-existing rocks are subjected to high temperatures, pressures, and/or chemically active fluids, causing physical and/or chemical changes

Identification and Classification Methods

• The texture, composition, and other physical properties of rocks are used to identify and classify them
• Geologists use a rock classification flow chart to systematically identify rock types based on observable characteristics
• Texture refers to the size, shape, and arrangement of mineral grains or crystals in a rock (fine-grained, coarse-grained, porphyritic)
• Composition refers to the types and proportions of minerals or materials that make up a rock (felsic, mafic, siliciclastic, carbonate)
• Other physical properties used for identification include color, hardness, density, and reaction to acid (limestone reacts with hydrochloric acid)

The Rock Cycle and Transformations

Continuous Transformation Process

• The rock cycle is a continuous process by which rocks transform from one type to another over geologic time
• Uplift and exposure of rocks at Earth's surface leads to weathering and erosion, which breaks down rocks into sediments that can eventually form new sedimentary rocks
• Burial and heating of rocks can cause metamorphism, and if heating continues, rocks can melt to form magma
• Magma can cool and crystallize to form new igneous rocks, which can be uplifted and exposed to surface processes, starting the cycle again

Weathering, Erosion, and Deposition

• Weathering is the breakdown of rocks and minerals at Earth's surface due to physical, chemical, and biological processes (frost wedging, acid rain, root wedging)
• Erosion is the removal and transportation of weathered rock and mineral fragments by water, wind, ice, or gravity (river erosion, glacial erosion, mass wasting)
• Deposition is the settling and accumulation of eroded sediments in a new location, such as a river delta, beach, or ocean basin (Mississippi River Delta, Bahamas carbonate platforms)
• Lithification is the process by which loose sediments are compacted and cemented together to form a solid sedimentary rock (sandstone, shale, limestone)

Earth's Processes Driving the Rock Cycle

Endogenic Processes

• The rock cycle is driven by a combination of Earth's internal (endogenic) and external (exogenic) processes
• Endogenic processes, such as plate tectonics, volcanism, and mountain building, are powered by Earth's internal heat and can lead to the formation of igneous and metamorphic rocks
• Plate tectonics is the movement and interaction of Earth's lithospheric plates, which can cause uplift, deformation, and metamorphism of rocks (Himalayan Mountains, Andes Mountains)
• Volcanism is the eruption of magma or lava onto Earth's surface, forming extrusive igneous rocks and volcanic landforms (Hawaii, Yellowstone)
• Mountain building, or orogeny, is the formation of mountain ranges through tectonic compression, uplift, and deformation of rocks (Appalachian Mountains, Rocky Mountains)

Exogenic Processes and the Water Cycle

• Exogenic processes, such as weathering, erosion, and deposition, are powered by energy from the Sun and gravity and can lead to the formation of sedimentary rocks
• The water cycle plays a crucial role in the rock cycle by causing weathering, transporting sediments, and facilitating chemical reactions
• Water is a powerful agent of physical weathering through processes like frost wedging and abrasion (Grand Canyon, Zion National Park)
• Water also drives chemical weathering by dissolving minerals and facilitating reactions like oxidation and hydrolysis (karst landscapes, cave formations)
• Running water is the primary means of eroding and transporting sediments from source areas to depositional environments (Nile River, Amazon River)

Igneous, Sedimentary, and Metamorphic Rocks

Igneous Rock Characteristics and Formation

• Igneous rocks form from the cooling and crystallization of magma or lava and are classified as intrusive (plutonic) or extrusive (volcanic) based on their cooling environment
• Intrusive igneous rocks, such as granite, cool slowly beneath Earth's surface, resulting in large, visible crystals
• Extrusive igneous rocks, such as basalt, cool quickly at Earth's surface, resulting in small crystals or a glassy texture
• Composition of igneous rocks ranges from felsic (high silica content, light-colored) to mafic (low silica content, dark-colored) (granite, basalt, obsidian)
• Texture of igneous rocks depends on cooling rate and can be phaneritic (large crystals), aphanitic (small crystals), or porphyritic (large crystals in a fine-grained matrix) (granite, basalt, porphyry)

Sedimentary Rock Characteristics and Formation

• Sedimentary rocks form through the deposition and lithification of sediments and are classified as clastic, chemical, or biologic based on their composition and formation process
• Clastic sedimentary rocks, such as sandstone, form from the compaction and cementation of weathered rock fragments
• Chemical sedimentary rocks, such as limestone, form from the precipitation of minerals from solution
• Biologic sedimentary rocks, such as coal, form from the accumulation and alteration of organic materials
• Texture of sedimentary rocks can be clastic (made of rock and mineral fragments) or non-clastic (crystalline or biologic) (sandstone, limestone, coal)
• Sedimentary structures, such as bedding, cross-bedding, and ripple marks, provide evidence of the depositional environment and processes (beach deposits, river deposits, tidal deposits)

Metamorphic Rock Characteristics and Formation

• Metamorphic rocks form when pre-existing rocks are subjected to high temperatures and pressures, leading to textural and mineralogical changes
• Foliated metamorphic rocks, such as gneiss, have a banded or layered appearance due to the alignment of minerals under directed pressure
• Non-foliated metamorphic rocks, such as marble, have a more uniform texture due to recrystallization under non-directed pressure
• Metamorphic grade refers to the intensity of metamorphism and is determined by the temperature and pressure conditions (low-grade, medium-grade, high-grade)
• Contact metamorphism occurs when rocks are heated by nearby magmatic intrusions, while regional metamorphism occurs over large areas due to burial and tectonic compression (hornfels, schist, slate)