Earth’s Structure, Plate Tectonics, and Rock Deformation
Rock Cycle
The rock cycle involves processes that modify rocks, transforming them into sediments. On the Earth’s surface, weathering breaks down rocks into sediments, which are transported by geological agents and accumulate in sedimentary basins. Within the Earth’s crust, high pressure and temperature transform sediments into rocks.
Factors within the Crust:
- Pressure: The weight of overlying rocks increases with depth.
- Temperature: Increases with depth, approximately 3°C per 100 meters.
- Compression and Distension: Caused by mantle movements.
Key Processes:
- Diagenesis: Transformation of sediments into sedimentary rocks due to pressure and temperature, resulting in cementation and compaction.
- Metamorphism: Changes in rocks under high temperatures and pressures, forming metamorphic rocks.
- Magmatism: Melting of rocks to form magma, which then solidifies into magmatic rocks.
Heat Sources:
- Geothermal Gradient: Temperature increase towards the Earth’s interior.
- Meteorite Impacts: Kinetic energy converts to thermal energy.
- Radioactive Decay: Subatomic particles increase thermal energy.
- Sinking of Denser Materials: Primarily iron.
Earth’s Structure
Crust
A thin, solid rock layer.
- Continental Crust: Composed mainly of granite.
- Oceanic Crust: Composed mainly of basalt and gabbro.
Mantle
Composed of peridotite rock.
- Upper Mantle
- Lower Mantle
Core
Metallic composition, primarily iron.
- Outer Core: Liquid
- Inner Core: Solid
Discontinuities
Areas within the Earth where seismic wave velocity changes due to composition or material state changes.
- Mohorovičić Discontinuity: Between crust and mantle (40-50 km).
- Repetti Discontinuity: Within the mantle (670 km).
- Gutenberg Discontinuity: Core-mantle boundary (2900 km).
- Lehmann Discontinuity: Within the core (5150 km).
Lithosphere
The rigid outermost part of the Earth, comprising the crust and the uppermost part of the mantle.
- Continental Lithosphere: Continental crust and upper mantle.
- Oceanic Lithosphere: Oceanic crust and upper mantle.
Plate Tectonics
Theories
- Fixist Theories: Denied horizontal continental displacement.
- Contractionist Theory: Suggested ridges and mountains formed by Earth’s contraction and cooling.
- Mobilist Theories: Accepted horizontal continental displacement.
- Continental Drift (Wegener): Proposed a supercontinent, Pangaea, which broke apart. Evidence included geographic fit, fossil distribution, paleoclimatic data, and geological formations.
Supporting Evidence and Concepts
- Astenosphere (Barrell): A plastic layer beneath the lithosphere.
- Isostasy: Gravitational equilibrium between the lithosphere and asthenosphere.
- Convection Currents (Holmes): Currents in the mantle driving plate movement.
- Seafloor Spreading (Hess): New oceanic crust is created at mid-ocean ridges and spreads outwards.
- Magnetic Stripes: Evidence of seafloor spreading and magnetic field reversals recorded in oceanic rocks.
Plate Tectonics Theory
The lithosphere is divided into plates that move due to mantle convection. This theory incorporates and expands upon continental drift.
Evidence for Plate Tectonics
- Benioff Zone: Earthquake distribution pattern at subduction zones.
- Mountain Ranges: Result of plate collisions.
- Lithospheric Plates: Composed of crust and upper mantle (approx. 100 km).
- Continental Plate: Continental crust.
- Oceanic Plate: Oceanic crust.
- Mixed Plate: Both continental and oceanic crust.
Plate Movements and Boundaries
Plate Boundaries:
- Convergent (Destructive): Subduction zones, trenches.
- Divergent (Constructive): Mid-ocean ridges, rifts.
- Transform (Conservative): Transform faults.
Types of Plate Boundaries:
Constructive Boundaries
Mid-ocean ridges where hot mantle material rises, creating new oceanic crust through fissure volcanism. Basaltic lava forms new crust, and extensional stresses cause divergence. Rift valleys form along the ridge axis, and hydrothermal activity is common.
Passive Margins
Transform faults are zigzag features along mid-ocean ridges. They accommodate differential spreading rates and are characterized by significant seismic activity.
Destructive Boundaries
Subduction zones where oceanic lithosphere descends into the mantle. Features include deep-sea trenches, accretionary prisms, high seismicity, and volcanism. Overriding plates can be oceanic or continental.
Collision Boundaries
Occur when two continental plates collide. Intense deformation, thickening of the crust, mountain building (orogens), metamorphism, and seismicity are characteristic.
Island Arcs
Volcanic island chains formed by subduction. They are seismically and volcanically active.
Thermal Orogens
Mountain ranges formed by subduction-related volcanism and crustal thickening (e.g., the Andes).
Accretionary Prisms
Sediments scraped off the subducting plate and added to the overriding plate.
Collision Orogens
Mountain ranges formed by the collision of two continental plates.
Intraplate Orogens
Deformation within a plate, often due to compression from distant plate boundaries or thermal plumes.
Thermal Plumes and Hotspots
Thermal plumes are columns of hot material rising from the mantle, causing volcanism. Hotspots can create volcanic island chains, contribute to mid-ocean ridges, or form continental plateaus.
Descending Convective Plumes
Cooling of thermal plumes leads to magmatism, volcanism, and tectonic deformation.
Rock Deformation
Rocks deform under stress, either elastically, plastically, or by fracturing.
Folds
Geometric Elements:
- Hinge: Area of maximum curvature.
- Limbs: Sides of the fold.
- Core: Central part of the fold.
- Axial Plane: Imaginary plane through the hinge.
Types of Folds:
- Anticlines: Oldest rocks in the core.
- Synclines: Youngest rocks in the core.
Fractures
Joints and Faults: Brittle deformation.
- Joints: Fractures without displacement.
- Faults: Fractures with displacement.
Types of Faults:
- Normal (Gravity) Faults: Extensional stress.
- Reverse Faults: Compressive stress.
- Strike-Slip Faults: Shear stress.
Thrust Nappes
Large-scale, low-angle reverse faults.