1.3 The rock cycle and Earth's dynamic systems

Rocks are the building blocks of Earth's crust, formed through various processes. The rock cycle shows how igneous, sedimentary, and metamorphic rocks transform over time due to heat, pressure, and erosion. These changes are driven by Earth's dynamic systems, including plate tectonics.

Weathering and erosion break down rocks, while deposition builds new layers. These processes shape Earth's surface over geologic time. Scientists use dating methods to understand Earth's history, dividing it into eons, eras, periods, and epochs. Earth's systems interact constantly, influencing each other in complex ways.

The Rock Cycle and Earth's Dynamic Systems

Types of rocks

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• Igneous rocks form from the cooling and solidification of magma or lava
  • Intrusive (plutonic) igneous rocks crystallize slowly beneath Earth's surface (granite)
  • Extrusive (volcanic) igneous rocks form from rapidly cooled lava at or near Earth's surface (basalt, obsidian)
• Sedimentary rocks form from the compaction and cementation of weathered and eroded sediments
  • Clastic sedimentary rocks composed of rock fragments (sandstone, shale)
  • Chemical sedimentary rocks form from precipitation of minerals (limestone, rock salt)
• Metamorphic rocks form from the transformation of pre-existing rocks under high heat and pressure
  • Foliated metamorphic rocks have a layered appearance due to mineral alignment (gneiss, schist)
  • Non-foliated metamorphic rocks lack a layered appearance (marble, quartzite)

Rock cycle transformations

• Igneous to sedimentary involves weathering, erosion, and deposition of igneous rocks
• Sedimentary to metamorphic occurs through burial and exposure to high heat and pressure
• Metamorphic to igneous happens when metamorphic rocks melt to form magma
• Igneous to metamorphic takes place when igneous rocks are exposed to high heat and pressure without melting

Plate tectonic boundaries

• Divergent boundaries occur where plates move away from each other, creating new seafloor (seafloor spreading, mid-ocean ridges)
• Convergent boundaries form where plates collide or one plate subducts beneath another (subduction zones, mountain building, volcanic arcs)
• Transform boundaries exist where plates slide past each other (strike-slip faults, offset landforms)

Surface features from tectonics

• Mid-ocean ridges, rift valleys, and seafloor spreading form at divergent boundaries
• Mountains, volcanic arcs, and subduction zones form at convergent boundaries
• Strike-slip faults and offset landforms form at transform boundaries

Weathering, Erosion, and Geologic Time

Physical vs chemical weathering

• Physical weathering is the mechanical breakdown of rocks without chemical changes
  • Caused by temperature changes (frost wedging, thermal expansion), pressure release (exfoliation), and biological activity (root wedging)
• Chemical weathering is the alteration of minerals through chemical reactions
  • Processes include dissolution (limestone), oxidation (rusting), and hydrolysis (feldspar to clay)

Erosion and deposition processes

• Erosion removes and transports weathered materials
  • Water erodes through rivers, waves, and glaciers
  • Wind erodes in arid environments (sand dunes)
  • Ice erodes through glacial plucking and abrasion
• Deposition is the settling and accumulation of eroded materials
  • Leads to formation of sedimentary rocks (compaction, cementation)
  • Environments include rivers (floodplains), oceans (continental shelves), and glaciers (moraines)

Principles of relative dating

• Superposition states that in an undeformed sequence, younger layers are deposited on top of older layers
• Original horizontality assumes sedimentary layers are deposited in nearly horizontal positions
• Cross-cutting relationships show that a geologic feature that cuts across another must be younger
• Inclusions indicate that rock fragments within another rock must be older than the host rock

Methods of absolute dating

• Radiometric dating uses the decay of radioactive isotopes to determine the age of rocks or minerals
  • Carbon-14 dating for organic materials up to ~50,000 years old
  • Potassium-40 to Argon-40 dating for rocks older than ~50,000 years
  • Uranium-238 to Lead-206 dating for rocks older than ~1 million years
• Other methods include dendrochronology (tree rings) and varve analysis (sedimentary layers)

Divisions of geologic time

• Eons are the largest divisions of geologic time (Phanerozoic, Proterozoic, Archean, Hadean)
• Eras represent major changes in life forms (Cenozoic, Mesozoic, Paleozoic)
• Periods are subdivisions of eras based on specific life forms or events (Quaternary, Cretaceous, Cambrian)
• Epochs are subdivisions of periods (Holocene, Pleistocene, Eocene)

Interactions of Earth's systems

• Geosphere influences others through weathering, erosion, and volcanism
  • Weathering and erosion provide sediments to the hydrosphere and nutrients to the biosphere
  • Volcanic eruptions release gases into the atmosphere and create new landforms
• Hydrosphere affects weathering, erosion, and sediment transport
  • Water is a major agent of weathering (hydrolysis) and erosion (rivers, waves)
  • Ocean currents redistribute heat and nutrients, influencing climate and biological productivity
• Atmosphere impacts weathering through temperature, precipitation, and wind
  • Atmospheric CO2 levels influence climate and weathering rates
  • Wind erodes and transports sediments (loess, sand dunes)
• Biosphere contributes to weathering through biological processes
  • Plant roots and microbial activity enhance weathering
  • Organic matter accumulation leads to formation of fossil fuels and biochemical sedimentary rocks