Plate Tectonics: Plates and Earth's Structure

• Plate tectonics explains the distribution of volcanoes, earthquakes, continents, and topography.

Plates

• The lithosphere, Earth's outermost rocky surface, is broken into approximately 15 large pieces called plates.
• Plates range from 50 km to 280 km thick and consist of the crust and stiff upper mantle.
• Crust types include continental crust (30 km thick, up to 100 km under mountain belts) and oceanic crust (10 km thick).
• The lithosphere sits atop the asthenosphere, a weak layer in the mantle with a small percentage of molten rock (0.5%).
• The asthenosphere's weakness facilitates the movement of lithospheric plates.
• Below the asthenosphere are the lower mantle and the core, which consists of a solid inner portion and a liquid outer portion made of iron and nickel.
• The liquid outer core convects, generating Earth's magnetic field.
• The arrangement of atmosphere, ocean, crust, mantle, and core reflects Earth's differentiation by density.
• Plate sizes vary, with the Eurasian and Pacific Plates being the largest.
• The Juan de Fuca Plate is a small plate, and the Arabian Plate is the smallest primarily continental plate.
• Plates are "curvitabular," meaning they are slab-like but also curved.
• Plate boundaries can be straight, curved, zigzagged, or diffuse, indicating relative plate motion.

Tectonics

• Plate tectonics describes the building of mountain belts, volcanic island arcs, ocean basins, and earthquake locations through plate motion.

Boundaries

• Plate boundaries are classified as convergent, divergent, or transform, based on the relative motion of neighboring plates.
• The type of boundary determines the resulting geological processes and phenomena.

Transform Boundaries

• Transform boundaries involve plates sliding past each other horizontally, conserving lithosphere.
• Friction prevents constant motion; when overcome, earthquakes occur.
• Earthquakes at transform boundaries are shallow (20 km or less) and range up to ~M7.5 magnitude.
• Offset features such as rock bodies, stream valleys, and human-built structures indicate transform boundaries.
• Faults are either right-lateral (San Andreas Fault, North Anatolian Fault) or left-lateral (North American Plate's southern margin).
• Crushed rock leads to linear landscape features like valleys and coastlines.
• Fault breccia indicates a shallow fault, while mylonite indicates deeper deformation with mineral flow.
• Stepovers in fault traces create local compression or tension zones.
• Right steps in right-lateral faults cause extension (sag ponds, troughs), and left steps cause compression (pressure ridges).
• Oceanic transform faults offset segments of oceanic ridges.

Divergent Boundaries

• Divergent boundaries involve plates moving apart, causing normal faulting, crustal thinning, and lateral stretching.
• They manifest as rift valleys in continental crust and oceanic ridges in oceanic crust.

Continental Rifting

• Continental rifting results in grabens or rift valleys due to normal faulting.
• The Great Rift Valley of east Africa is a modern example.
• High topographic relief at rift valley edges leads to clastic sediment deposition.
• Immature sediments like alluvial fans and arkosic sand accumulate, and lakes become salty due to evaporation.
• Decompression of the mantle causes partial melting, generating mafic magma and basalt floods.
• Interaction with crust can produce intermediate magmas and composite volcanoes.
• Continental rifting can lead to seafloor spreading, as seen in the Red Sea and Gulf of Aden.

Seafloor Spreading

• Decompression melting of the mantle produces mafic magma at oceanic ridges.
• Magma cools to form new oceanic crust, creating bathymetrically high oceanic ridges.
• Mafic magma rises into the crack between diverging plates, forming pillow basalts, dikes, and gabbro.
• This sequence of rock types is known as an ophiolite sequence.
• Seafloor spreading rates vary (East Pacific Rise is faster than the Mid-Atlantic Ridge).
• Active divergence sections are offset by transform faults, creating a staircase shape.
• Cooling magma records Earth's magnetic field polarity.
• Oceanic crust cools and becomes denser over time, causing subsidence and forming abyssal plains.
• Seafloor age can be determined by radioactive isotope dating, fossil content of sediments, and magnetostratigraphy.

Convergent Boundaries

• Convergence occurs when two plates move toward each other.
• Outcomes depend on whether the leading edge of the plates consists of oceanic or continental lithosphere.

Oceanic/Continental Convergence

• Subduction occurs when oceanic lithosphere descends beneath continental lithosphere.
• An oceanic trench marks the start of subduction.
• Earthquakes occur along the subduction zone, and sediment may be scraped off to form an accretionary wedge.
• The subducted plate releases water, causing melting in the mantle and generating mafic magma.
• Magma rises, pools, and transfers heat to the continental crust, resulting in intermediate magmas and volcanoes which form a continental volcanic arc.
• High pressure and relatively low temperatures cause blueschist and eclogite metamorphic facies.
• Modern examples include the Cascadia volcanic arc and the Andes.

Oceanic/Oceanic Convergence

• Volcanic island arcs form when two oceanic plates converge, with the older, denser plate subducting.
• Subduction leads to trench formation and magma generation due to dehydration reactions in the mantle.
• Magma rises through the overlying crust, creating a volcanic island arc.
• A deep sea trench parallels belts of volcanoes.

Continental/Continental Convergence

• After all oceanic lithosphere is subducted, continents collide and crumple upward into mountains.
• A "keel" of thickened lithosphere extends downward.
• This process is called orogeny, and the classic modern example is the Himalayas.
• Continent-continent collision leads to large, shallow earthquakes but no volcanism.
• Regional metamorphism occurs, generating rocks such as phyllite, schist, and gneiss.
• Metamorphic minerals like chlorite, garnet, kyanite, staurolite, and sillimanite grow.
• Mountain belt flanks deform through folding and thrust faulting, examples including the Pyrenees, the Appalachians and the Alps.

Plate Interiors

• Plate interiors move as coherent blocks with minimal tectonic activity.
• Passive margins are the trailing edges of continents with minimal topographic relief.
• Subsided cool crust allows mature sedimentary strata to accumulate.
• Examples include eastern South America, Western Africa, Western Australia, and southern India.

Variations and Mashups

• Plate boundaries can exhibit mixed behaviors such as transpression (oblique convergence) and transtension (oblique divergence).

Transpression

• Transpression involves convergence at an oblique angle, causing squishing and shearing.
• Rocks develop an S-C fabric indicating compression and shearing.
• Pressure ridges may form due to upward movement of Earth materials.

Transtension

• Transtension involves divergence at an oblique angle, often evolving into a zigzag pattern of rift valleys and transform faults.
• The Gulf of California is a recent example of transtension between Baja California and mainland Mexico.

The Historical Record of Plate Interactions

• Mountain belts are formed at convergent plate boundaries between continental blocks.
• Mountain belts leave durable signatures in rocks, structures, and topography.

Mountain Belts

• While high relief landscapes are unlikely to persist over geologic time, the signatures of mountain-building may be preserved.
• These signatures include folded/thrust-faulted sedimentary strata, foliated metamorphic rocks, migmatite and felsic magma bodies which cool to form granite plutons.
• Erosion of mountains leads to clastic sediment deposition in adjacent areas, marine sediment is dubbed flysch and terrestrial sediment is dubbed molasse.

Cratons and the Accretionary Growth of Continents

• Over time, collisions between continental crust masses lead to larger, composite crust blobs.
• Old crustal components are called "cratons," while recently added components are called "terranes."
• Continents grow through accretion, as seafloor sediment, volcanic island arcs, and other small masses merge.

Ophiolites

• Ophiolites are scraps of oceanic crust found on dry land within mountain belts.
• From bottom to top, there is mantle peridotite, gabbro, sheeted dikes of basalt, and pillow basalt (potentially topped with deep sea sediments).
• Ophiolites are remnants of oceanic lithosphere that separated terranes and were destroyed through subduction.

The Record of Subduction

• Subduction zones are identified by metamorphic, igneous, and structural features.
• Parallel oceanic trenches and volcanic arcs have distinct pressure and temperature blends.
• High pressure/low temperature metamorphism occurs near trenches, while moderate pressure/high temperature metamorphism occurs near magmatic arcs.
• Paired metamorphic belts distinguish records of subduction.

Sedimentary Basins

• Sedimentary basins preserve sediments over geologic time in different plate boundary settings.

Transform

• Transform boundaries do not tend to form substantial basins due to the conservation of land surface.

Wrench Basins

• Wrench basins form at jogs in fault traces.
• These basins dilate and create topographic low spots for sediment.
• The Dead Sea is an excellent modern example.

Divergent

• The primary basin type at divergent settings is a rift basin.

Rift Basins

• Fresh rift basins accumulate immature clastic sediment.
• Sediments in large rift basins may mature over time. Volcanism is also common.
• The Great Rift Valley system of east Africa is a modern example.
• Ancient examples include the Culpeper Basin of central Virginia and the Belt Supergroup.

Convergent

• Convergent plate boundaries can induce foreland, forearc, and backarc basins to form.

Foreland Basins

• Foreland basins form on continental crust due to the load of heavy mountain ranges.
• Thrust faulting causes adjacent crust to sag, creating space for sediment.
• The Persian Gulf is a modern example of a foreland basin.

Forearc Basins

• Forearc basins form on the trench side of volcanic arcs.
• Subduction keeps the crust low, and the neighboring volcanic arc sheds copious sediment to fill the basin.
• The Great Central Valley of California is a classic example.

Back-Arc Basins

• Back-arc basins require divergence to form, as they result from tensional forces caused by upwelling mantle and trench rollback.
• A good modern example of back-arc spreading is the Sea of Japan.

Geophysical Phenomena

• Plate tectonics have several manifestations best examined through the lens of geophysics.

Earthquakes

• Most earthquakes occur at plate boundaries.
• They are caused by the sudden slip of rock bodies past one another.
• Earthquake locations at subduction zones trace the subducted slab.

Volcanoes

• Most volcanoes are associated with plate boundaries.
• Volcanoes may form during subduction of oceanic lithosphere under continental lithosphere, subduction of oceanic lithosphere under oceanic lithosphere, rifting of continental lithosphere, or seafloor spreading of oceanic lithosphere.
• Volcanoes can also form in non-plate boundary settings, due to hot spots or mantle plumes, either through oceanic or continental lithosphere.

GPS & InSAR

• GNSS such as the Global Positioning System (GPS) can precisely document crustal motions, the motion is described in vectors.
• InSAR uses microwaves to measure Earth's surface shape, detecting changes between satellite passes and displaying vertical displacement on interferograms.

Driving Forces

• Lithospheric plate movement results from forces acting on the plate and resistance to those forces.
• Mantle convection contributes, but ridge push and slab pull are also important.
• Eclogite is particularly dense metamorphic rock on the subducting slab that contributes to the slab pull force.

Wilson Cycles

• Plate boundary dynamics are not constant; a given spot on Earth's surface can shift between divergent and convergent settings.
• Wilson cycles describe the cyclical opening and closing of ocean basins and the formation and breakup of supercontinents.

Mid-Atlantic Wilson Cycles

• The eastern margin of the United States demonstrates shifts from convergent to divergent to passive settings driven by supercontinent formation and breakup.
• There have been four major batches of active tectonism in this area.

The Grenville Orogeny

• From 1.2 to 1.0 Ga, mountain-building dominated, making the supercontinent Rodinia.

Iapetan Rifting

• From 700 Ma to 565 Ma, rifting dominated, breaking Rodinia up into fragments, and opened the Iapetus Ocean between them.

Appalachian Mountain-Building

• From 300 to 250 Ma, mountain-building dominated, marked by granites, metamorphism, and deformation, making the supercontinent Pangaea.

Atlantic Rifting

• From 200 to 180 Ma, rifting dominated, breaking Pangaea up into fragments, and opened the Atlantic Ocean between them.