Plate Tectonics & Earth Structure

• The theory of plate tectonics explains the distribution of volcanoes, earthquakes, continents, and topography.
• The Earth's lithosphere consists of about 15 curvitabular plates, ranging in thickness from 50 km to 280 km.
• Plates consist of stiff upper mantle and either continental or oceanic crust.
• Oceanic lithosphere is thinner and denser than continental lithosphere.
• Continental crust averages 30 km thick, while oceanic crust is around 10 km thick.
• Oceanic ridges are prominent features that rise from the seafloor.
• The lithosphere sits atop the asthenosphere, a weak layer in the mantle with a small amount of melt that reduces its strength.
• The core is composed of a solid inner portion and a liquid outer portion, both made mostly of iron.
• Convection in the outer core generates a magnetic field.
• Atmosphere and oceans sit above the lithosphere due to their lower density.
• Continental lithosphere is more buoyant than oceanic lithosphere.
• Plates vary in size, with the Eurasian and Pacific Plates being the largest and the Arabian Plate being the smallest continental plate.
• Plate boundaries can be straight, arc-like, zigzag, or broad diffuse zones.
• Plate tectonics explains volcanic island arcs, ocean basins, earthquake locations, and lithospheric plate motions.

Boundaries

• Plate boundaries are where most of the dynamic action of plate tectonics occurs.
• The three main types of plate boundaries are convergent, divergent, and transform, defined by the relative motion of neighboring plates.

Transform boundaries

• These boundaries plates slide past each other, neither creating nor destroying lithosphere
• Friction acts there, earthquake happen when it is overcome
• Earthquakes here are shallow (less than 20 km deep) and range up to ~M7.5
• Features can be recognized by offset landscape features that cross the plate boundary fault zone
• Faults can be right-lateral or left-lateral depending on the direction of the plate on the opposite
• The San Andreas Fault in California and the North Anatolian Fault in Turkey are both right-lateral faults.
• Fault breccia indicates a shallow fault, while mylonite, with its smeared texture, indicates deeper deformation under higher pressure and temperature.
• Continental transform boundaries exist, but transform faults in oceanic lithosphere between segments of oceanic ridge are more common.
• A right step in a right-lateral transform fault creates a zone of extension, while a left step creates a zone of compression.

Divergent boundaries

• Plates move away from each other.
• Normal faulting dominates causing crust extension, thins vertically stretching laterally.
• They come in two varieties are rift valleys in continental crust and oceanic ridges in oceanic crust
• In continental crust, normal faulting drops a block of crust, producing a rift valley, like the Great Rift Valley of east Africa.
• Rift valley edges encourage clastic sediment that forms alluvial fans of poorly-sorted gravel.
• The crust decompression of the mantle allows melting generating mafic magma.
• Basaltic magma interacts with crust and produces intermediate magmas resulting in composite volcanoes
• A narrow body of oceanic crust sometimes separates continents after the crust thins
• New oceanic crust filling the gap via seafloor spreading, as seen in the Red Sea and the Gulf of Aden.

Seafloor spreading

• Decompression melting of the mantle happens to produce mafic magma.
• Fresh oceanic crust forms a high feature on the seafloor called an oceanic ridge.
• As plates diverge, mafic magma squirts into the crack cooling to produce pillow basalts erupting undersea.
• Basalt dikes cool quickly, while gabbro bodies cool slowly
• Ophiolite sequence is the order of rock types, useful later for convergent boundaries
• The East Pacific Rise generates twice as much new seafloor as the Mid-Atlantic Ridge.
• Active divergences offset along transform faulting, producing the plates' edge stair-step.
• Mafic magma helps record changes in the polarity of Earth's magnetic field
• The oceanic crust cools, becoming denser, and the lithospheric mantle thickens.
• Density-driven subsidence happens as the weight of sedimentation atop the crust creates the smooth abyssal plains.
• Analyzing the fossil content of sediments and magnetostratigraphy of oceanic crust helps determine crust age
• Gabbro of the deeper crust has zircon crystals with uranium lead timekeepers.

Convergent boundaries

• When two plates are headed toward each other, the boundary is described as convergent.
• Several different situations can result based on plate lithosphere
• Subduction, where a plate descends into the mantle, occurs when oceanic lithosphere converges with a continent.
• An oceanic trench marks subduction, but earthquakes are generated at megathrust faults and down into the plate, building up in a thick jumbled pile called an accretionary wedge.
• The subducted plate releases water back to the mantle melting with ultramafic rock generating mafic magma.
• Melt rises, pools, and transfers heat to continental crust which melts to intermediate magmas, and creates volcanoes.
• Development of a continental volcanic arc parallel to the trench indicate convergence.
• Higher pressures and temperatures metamorphose the descending plate which does not warm up faster leading to blueschist and eclogite metamorphic facies.
• Cascades in the Pacific Northwest in USA and Andes in the western edge of South America exemplifies these boundaries.

Oceanic/Oceanic Convergence

• Subduction results when oceanic lithosphere plates converge between each other
• Convergence density depends on age where older is denser, and so old subducts
• Trenches and accretionary wedges are created again.
• Rock of the slab undergoes dehydration releasing water back to the mantle which triggers melting.
• Magma can then pierce its way up through islands called volcanic islands
• As a general rule, a there is often an overlap of volcanos in deep sea trenches

Continental/Continental convergence

• After all the oceanic lithosphere has been subducted, the crumpling makes mountains on both continents due to buoyancy.
• This mountain building act is called orogeny.
• Collisional mountain belts can cause regional heating and metamorphism to neighboring rocks.
• Foliations can show how the shape and atoms reacted to changing conditions, pressure, temperature
• Strong compressional stresses generate rock deformation through folding and thrust faulting.
• Examples include the Pyrenees, the Cape Fold Belt in South Africa, the Valley & Ridge province of the Appalachians, etc.

Plate interiors

• Coherent blocks that translate in a common direction or rotating around some axis of motion.
• Passive margins are the trailing edges of continents within a plate.
• Virginia is the edge of a continent and the middle of the plate where no active mountains are being built but old mountains are being eroded away.
• The topographic relief is minimal.
• Modern passive margin examples include eastern South America, Western Africa, Western Australia, and southern India.

Variations and Mashups

• Some boundary plate settings mix elements of convergence and transform motion
• Sometimes it divergent and transformation as well
• This happened when plate motion is perpendicular nor parallel to the boundary plate
• Transtension is divergence that is oblique

Transpression

• Convergence is oblique
• In short compression happens and rocks shear in this scenario
• S-C fabric occurs at this level where both foliation and small shear exist
• Transpression has landscape effects and is durable
• Rocks move up and high, with mountains

Transtension

• Transtension is oblique divergence, tends not to last Quickly changed itself into a "zigzag" pattern of vallyes and transform faults.
• Very commonly found in the Gulf of California.

Historical record of plate interactions

• Plates do various things when they touch each other. Convergence, divergence, etc.
• Some may not turn out in the record
• However things like rock formations and mountain can change quickly

Mountain Belts

• Occurs at convergent plate boundaries with continental lithosphere
• Topographical relief is unlikely to exist in the record over time
• Over time these roots can be examined for high temps/pressures in a region and mineral data like isotope systems

Continent growth

• Over time continental crust smashes in that location.
• Crustal components include old "cratons" old and Terrance.
• Converge happens and these parts are fused
• Seafloor sediment, volcanic island arcs, and other small masses of continental crust is also merged.

Ophiolites

• Described with oceanic lithosphere at divergent boundary
• This sequence helps us find small remnants of rocks that can be found in the heart of mountain belts

Subduction records

• Can be seen in past locations
• We look for structural , igneous features in this location
• Volcanic arc can show past subductions in history

Sedimentary basins

These are basically loci where deposition and sediments can be found long term.

• Transform
• Because conservation and opposite directions, sedimentation is tricky, except there can be small "jogs" and there may be basins in the wrench
• Wrench basins come when a right passes to the right.
• Wrench basins are not just modern, but are in the Geological record as well.
• Diversion is also a feature of deposition and locations

Divergent

• Principal valley is a rift basin where faults and pressure lowers leading to sediments and erosion

Rift basins

• East Africa has outstanding examples

Convergent

• Load to the crust in a mountain causes this to make sediments
• There are three key aspects that need considered, including...
• Foreland basins
• Form of the plates as material loads down onto the plate.
• This is modern Persiana gulf
• Ordovician and the mid-Atlantic region is another past region
• Forearc Basin
• Subduction keeps sediments low.
• Is Jurassic/Cretaceous age.
• Subduction is a coast range

Back-arc basins

• From tensions in back-arc coupled regions
• ironic it needs divergence to form
• Rising mantle with oceanic trench rollback
• The arc crust can show divergence
• Sea of Japan is a modern example

Geophysical phenomena

• To better examine plate tectonics. Earthquakes Earthquakes are caused by the sudden sip of two bodies of matter. Tectonic stress happens over rock. Most quakes are near plate boundaries.

Volcano and location

. Subduction of it occurs under oceanic crust.

GPS and InSAR

• Help navigate satellite systems.
• To analyze GPS, a geodesist can choose a system.
• This helps find the spot where Earth is over long term.

Driving forces

• What could make a massive rock overcome the movement during geological time?
• Earth's mantle convection is key

Wilson Cycles

• It's vital that folks understand the change over time with the plate boundaries
• Continental have there own way of shifting called Wilson Cycle.

Middle Wilson Cycle

• East coast is key in shifting
• From time plate will shift convergently, divergently and passively repetitively.

Wilson Rock

• The oldest rock on the east coast is intrusive The Old Rock Grenville, a granite of of the coast in Virginia.

Lapstan Rift

• Flood basalt of the east
• The weakest link for the continent and seafloor.

Appalachian mountain building