Earth's Structure and Plate Tectonics

Flashcards covering Earth's internal structure, plate tectonics, continental drift, paleomagnetism, seafloor spreading, different types of plate boundaries, and related geological phenomena.

Earth's Core

Accounts for one-sixth of Earth
as volume but one-third of its mass due to its density.

Outer Core

A liquid layer, 2270 km thick, with a density of 9.9 g/cm3, composed mostly of iron with some nickel, based on the absence of S waves traveling through it.

Inner Core

A solid layer with a radius of 1216 km and density of 13 g/cm3, growing as Earth cools at the expense of the outer core, rotating faster, and moving independently of the crust and mantle.

Plate Tectonics

A system powered by Earth’s internal heat, where the rigid lithosphere is broken into moving plates above the plastic asthenosphere.

Asthenosphere

A more plastic layer than either the overlying lithosphere or the underlying lower mantle.

Lithosphere

Relatively cool and rigid layer broken into a mosaic of moving plates above the asthenosphere.

Plate Margins

The most active areas on Earth, characterized by intense volcanism, seismic activity, crustal deformation, and mountain building.

Continental Drift

Hypothesis proposed by Alfred Wegener in 1915, suggesting a supercontinent called Pangea began breaking apart about 200 million years ago.

Alfred Wegener

German meteorologist who proposed the continental drift hypothesis.

Pangea

A supercontinent, meaning 'all earth,' that began breaking apart about 200 million years ago.

Evidence for Continental Drift

Includes the fit of continents, fossil evidence, similar rock types and structures across continents, and paleoclimate evidence.

Objections to Continental Drift Hypothesis

Mainly due to Wegener's profession as a meteorologist, lack of a driving mechanism, and satisfaction with the fixist theory.

Paleomagnetism

The study of Earth's ancient magnetic field, recorded in magnetic minerals within rocks.

Earth's Magnetic Field

Consists of lines of force, similar to a giant bar magnet, generated by circulating currents of charged particles in the outer core.

Dynamo Theory

Explains the origin and longevity of Earth's magnetic field through interactions between the twisting flow of molten material in the outer core and generated electrical currents.

Magnetic Reversals

Periodic events where Earth’s north magnetic pole becomes the south magnetic pole, and vice versa, recorded in oceanic crust.

Seafloor Spreading Hypothesis

Proposed by Hess in 1962, stating that new crust forms near ridges in the middle of oceans and old crust is consumed at the edges.

Ocean Floor as a Magnetic Recorder

Symmetrical stripes of high- and low-intensity magnetism parallel to oceanic ridges, preserving a record of Earth's past magnetic fields.

Evidence for Seafloor Spreading

Includes youngest rocks at ridges, oldest rocks on seafloor ~180 m.y., sediments older away from ridges, and highest heat flow at ridges.

Global Heat Flow

Highest at mid-ocean ridges and decreases away from the ridges.

Plate Tectonics (Synthesis)

The unifying theory combining continental drift, seafloor spreading, and paleomagnetism, explaining that Earth’s lithosphere is broken into plates that move slowly and continually change shape and size.

Mantle Convection

The driving mechanism for plate tectonics, where heated, less dense mantle material rises, and cooler, denser material sinks.

Plate Boundaries

Zones where interactions between lithospheric plates occur, categorized as divergent, convergent, or transform.

Divergent Boundaries

Constructive margins where plates split and move apart, creating new ocean floor; marked by the mid-ocean ridge system.

Mid-Ocean Ridge System

An interconnected system exceeding 70,000 km in length, where new mafic igneous ocean floor is created through seafloor spreading.

Spreading Rates

Classified as slow (

Rift Valley

A valley formed at a divergent boundary, often connected to mid-ocean ridges, such as in Thingvellir National Park, Iceland.

Continental Rifting

The process where landmasses split into segments, leading to the initial formation of a rift valley, then a shallow sea, and ultimately a mid-ocean ridge and new ocean basins.

Passive Margins

Coastal areas not associated with plate boundaries, characterized by little volcanism and few earthquakes, consisting of a continental shelf, slope, and rise.

Continental Shelf

Flooded extension of a continent with an average slope of one-tenth of 1 degree.

Continental Slope

The boundary between continental and oceanic crust, with an average slope of about 5 degrees.

Continental Rise

The edge of the oceanic crust at a passive margin.

Convergent Boundaries

Destructive margins where plates move toward each other, resulting in deep trenches or high folded mountain belts, often with earthquakes and magma generation.

Ocean-Continent Convergence

Where oceanic lithosphere subducts beneath continental lithosphere, forming ocean trenches and generating magma that rises to create continental volcanic arcs (e.g., Andes, Cascades).

Subduction Zones

Areas where oceanic lithosphere returns into the asthenosphere at destructive plate margins.

Continental Volcanic Arc

A chain of volcanic mountains formed from magma rising at ocean-continent convergent boundaries (e.g., Cascade Range).

Ocean-Ocean Convergence

Where two oceanic plates converge, one subducts, leading to volcanoes on the ocean floor that may emerge as island arcs (e.g., Japan, Aleutian Islands).

Island Arc

A chain of volcanic islands formed at ocean-ocean convergent boundaries (e.g., Japan, Aleutian Islands).

Continent-Continent Convergence

Where two continents collide, forming a suture zone and high mountains (e.g., Himalayas) with no volcanoes due to the inability of the buoyant continental crust to subduct.

Suture Zone

The zone where two continents collide and are joined together.

Terranes

Small crustal fragments that collide with and accrete to a continental margin during convergence (e.g., along western North America).

Isostasy

The universal tendency of segments of Earth’s crust to establish a condition of gravitational balance, influenced by differences in both thickness and density.

Isostatic Adjustment

Changes in Earth’s crustal elevation due to gravitational balance, often associated with erosion or changes in crustal thickness/density.

Transform Boundaries

Conservative margins where plates slide past one another, neither creating nor destroying lithosphere, typically relating to mid-ocean ridge segments or complex plate movements.

Transform Faults

Plate boundaries where plates slide horizontally past one another, connecting convergent and divergent boundaries in various combinations (e.g., San Andreas Fault, Juan de Fuca Ridge transform).

San Andreas Fault

A major transform fault in California that connects a spreading center in the Gulf of California and the Mendocino Fault.

Hotspot

A mantle magmatic center not at a plate boundary, over which a plate moves, sequentially producing islands (e.g., Hawaii).

Mantle Plume

A localized column of hot magma rising from the mantle that can create hotspots and volcanic activity away from plate boundaries.