3.2 Rock Cycle and Rock Types

Rocks are the building blocks of Earth's crust, constantly changing through the rock cycle. This process, driven by plate tectonics, transforms rocks between igneous, sedimentary, and metamorphic types through various geological processes.

Understanding rock types and their formation is crucial for grasping Earth's dynamic nature. From magma cooling to form igneous rocks, to weathering and erosion creating sedimentary rocks, to high pressure and temperature forming metamorphic rocks, each type tells a unique story of Earth's history.

The Rock Cycle

Processes and Drivers

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• The rock cycle is a continuous process that transforms rocks between three main types: igneous, sedimentary, and metamorphic
• This process is driven by plate tectonics, which involves the movement and interaction of Earth's lithospheric plates

Formation of Igneous Rocks

• Magma, molten rock beneath Earth's surface, can cool and crystallize to form intrusive igneous rocks
• When magma reaches the surface as lava and cools, it forms extrusive igneous rocks

Weathering, Erosion, and Sedimentation

• Weathering, both physical and chemical, breaks down rocks into smaller fragments
  • Physical weathering involves mechanical breakdown without changing chemical composition (frost wedging, abrasion)
  • Chemical weathering alters the chemical composition of rocks through reactions with water, air, or organic acids (dissolution, oxidation)
• These weathered fragments are then transported by erosion and deposited as sediments
• Sediments accumulate in layers and undergo compaction and cementation through the process of lithification to form sedimentary rocks

Metamorphism and Melting

• When rocks are subjected to high temperatures and pressures deep within the Earth, they can transform into metamorphic rocks without melting completely
• Metamorphic rocks can melt if the temperature is high enough, forming magma and restarting the rock cycle
• Igneous and sedimentary rocks can also melt to form magma under appropriate conditions

Igneous, Sedimentary, and Metamorphic Rocks

Igneous Rocks

• Form from the cooling and solidification of magma or lava
• Classified as intrusive (plutonic) if they cool slowly beneath the surface, resulting in large, visible crystals (granite, gabbro)
• Classified as extrusive (volcanic) if they cool rapidly on the surface, resulting in small crystals or a glassy texture (basalt, rhyolite)

Sedimentary Rocks

• Form through the processes of weathering, erosion, deposition, and lithification of rock fragments, organic matter, or chemical precipitates
• Characterized by layered structures (bedding) and often contain fossils
• Examples include sandstone, limestone, and shale

Metamorphic Rocks

• Form when pre-existing rocks are subjected to high temperatures, pressures, or chemically active fluids, causing the rocks to change their physical and chemical properties without melting
• Characterized by foliated (layered) or non-foliated textures, depending on the type of metamorphism and the original rock composition
• Examples include gneiss, schist, and marble

Formation of Igneous Rocks

Magmatic Processes and Cooling Rates

• Igneous rocks form from the cooling and solidification of magma (beneath Earth's surface) or lava (on Earth's surface)
• The rate of cooling and the composition of the magma or lava determine the characteristics of the resulting igneous rock

Intrusive (Plutonic) Igneous Rocks

• Form when magma cools slowly beneath Earth's surface, allowing large crystals to grow
• Have a coarse-grained texture and are typically composed of minerals like quartz, feldspar, and mafic minerals (pyroxene, amphibole)
• Examples include granite, diorite, and gabbro

Extrusive (Volcanic) Igneous Rocks

• Form when lava cools rapidly on Earth's surface, resulting in small crystals or a glassy texture
• Have a fine-grained or aphanitic texture and may contain gas bubbles (vesicles) formed by escaping gases during cooling
• Examples include basalt, andesite, and rhyolite

Classification Based on Composition

• Igneous rocks are classified based on their silica content and the relative proportions of light-colored (felsic) and dark-colored (mafic) minerals
• Felsic rocks (granite, rhyolite) are rich in silica and light-colored minerals
• Mafic rocks (gabbro, basalt) have lower silica content and are dominated by dark-colored minerals
• Intermediate igneous rocks (diorite, andesite) have compositions between felsic and mafic rocks, with moderate silica content and a mixture of light and dark minerals
• Ultramafic igneous rocks (peridotite, komatiite) are very low in silica and composed almost entirely of mafic minerals, typically forming in the Earth's mantle

Sedimentary Rock Formation

Weathering

• Weathering is the breakdown of rocks and minerals at or near Earth's surface by physical, chemical, or biological processes
• Physical weathering involves the mechanical breakdown of rocks into smaller fragments without changing their chemical composition (frost wedging, abrasion)
• Chemical weathering alters the chemical composition of rocks through reactions with water, air, or organic acids (dissolution, oxidation)

Erosion and Deposition

• Erosion is the process by which weathered rock fragments, soil, and sediments are transported from their original location by water, wind, ice, or gravity
• Common erosional agents include rivers, glaciers, waves, and wind
• Deposition occurs when the energy of the transporting medium (water, wind, or ice) decreases, causing the transported sediments to settle and accumulate in layers
• Depositional environments include river deltas, floodplains, beaches, and ocean basins

Lithification

• Lithification is the process by which loose sediments are converted into solid sedimentary rocks through compaction and cementation
• Compaction occurs as the weight of overlying sediments squeezes out water and air from the spaces between grains
• Cementation involves the precipitation of minerals (calcite, silica) from water, binding the sediment grains together

Sedimentary Rock Characteristics

• The characteristics of sedimentary rocks, such as grain size, sorting, and composition, reflect the conditions of the depositional environment and the source of the sediments
• Well-sorted, rounded sand grains indicate a high-energy environment like a beach or desert
• Poorly-sorted, angular grains suggest rapid deposition in a low-energy environment like a river floodplain

Metamorphism and Its Effects

Types of Metamorphism

• Metamorphism occurs when pre-existing rocks are subjected to high temperatures, pressures, or chemically active fluids, causing the rocks to change their physical and chemical properties without melting
• Regional metamorphism occurs over large areas when rocks are subjected to high temperatures and pressures during mountain-building events (orogenies) or deep burial
• Contact metamorphism occurs when rocks are heated by nearby intrusions of magma or lava, resulting in localized metamorphism
• Hydrothermal metamorphism occurs when rocks are altered by hot, chemically active fluids (water, CO2) that circulate through fractures and pore spaces

Foliated and Non-Foliated Metamorphic Rocks

• Regional metamorphism typically results in foliated metamorphic rocks, such as slate, schist, and gneiss, which have a layered or banded appearance due to the alignment of platy or elongated minerals
• Foliation develops perpendicular to the direction of the greatest compressive stress and depends on the intensity of metamorphism and the composition of the original rock
• Contact metamorphism typically results in non-foliated metamorphic rocks, such as hornfels and quartzite, which have a granular or massive texture

Mineral Composition and Index Minerals

• The mineral composition of metamorphic rocks depends on the composition of the original rock (protolith) and the temperature and pressure conditions during metamorphism
• Limestone (calcium carbonate) can transform into marble (recrystallized calcium carbonate) under moderate metamorphic conditions or into calc-silicate rocks (containing calcium-bearing silicate minerals) under high-temperature metamorphism
• Index minerals, such as chlorite, biotite, garnet, and sillimanite, are used to determine the temperature and pressure conditions of metamorphism because they form at specific ranges of temperature and pressure
• The presence or absence of these minerals in a metamorphic rock can help geologists infer the metamorphic grade (low, medium, or high) and the depth of formation