EAOS 111 Plate Tectonics

Unifying Theory of Geology

The outer layer of the Earth consists of separate plates that move with respect to one another.

Differentiation

When heavier elements such as iron and nickel sink to the center of a celestial object to form a core and lighter elements like silica and oxygen create an outer crust.

Primary Waves (P-Waves, Compressional Waves)

Most quickly moving seismic waves, the first to arrive at a receiver, patterns of compression and expansion which travel through solids and liquids. They travel fast through cold material, and slower through hot material. Refracted by changes in density.

Secondary Waves (S-Waves, Shear Waves)

More slowly traveling seismic waves, the second earthquake waves to arrive at a receiver, waves moving perpendicularly in a shearing motion which cannot travel through liquid.

Continental Crust (Compositional Layer)

Average composition of granite, 35-40 km thick, with a density of about 2.8 g/cm³.

Oceanic Crust (Compositional Layer)

Average composition of basalt, 7-10 km thick, with a density of about 3 g/cm³.

Mantle (Compositional Layer)

Made of peridotite (solid rock), with a density of about 3.3 g/cm³.

Core (Compositional Layer)

Iron nickel alloy, made of a liquid outer layer and a solid inner layer.

Lithosphere (Strength Layer)

0-100 km deep. Strong, cool, and rigid, made up of the crust and upper mantle.

Asthenosphere (Strength Layer)

100-350 km deep. Weak, easily deformed, and solid, made up of mantle.

Mesosphere (Strength Layer)

350-2883 km deep. High temperature and pressure, strong and solid. Made up of mantle.

True or False: The mantle is molten.

False, the mantle is solid rock that deforms in a ductile (plastic) manner at high pressures and temperatures.

Divergent Plate Margins

Tectonic plates move away from one another, creating mid-ocean ridges or continental rifts and upwelling of asthenosphere. This process can create fresh continental or oceanic crust.

Convergent Plate Margins

Tectonic plates move toward one another, creating subduction or collision zones. This process can create mountain ranges or deep ocean trenches as well as earthquakes and volcanic activity.

Transform Plate Margins

Tectonic plates move horizontally against one another, often causing strike-slip seismic activity.

Decompression Melting

Melting that occurs as pressure is released due to diverging plate boundaries.

Flux Melting

Melting that occurs as water vapor is released from sinking rock and melts the rock it’s subducting beneath.

Volcanic Arc

A chain of volcanic islands formed above a subducting plate, resulting from rising magma.

Accretionary Wedge

A pile of sediment and rock that accumulates at the boundary between a subducting plate and the overriding plate, forming features like deep-sea trenches.

Continental Transform Boundaries

San Andreas Fault, CA, Alpine Fault, NZ.

Transtension

Simultaneous horizontal translation and rifting or divergence at one point on a plate boundary.

Transpression

Simultaneous horizontal translation and compression or subduction at one point on a plate boundary.

Oblique Collision

A tectonic interaction where two plates collide while also sliding past each other, causing a combination of compressional and shear forces.

Fiordlands

Subduction, frequent large earthquakes and landslides, and shallow to deep seismicity.

Fault Displacement

The movement of rocks along a fault line, resulting in a shift of the Earth's surface.

Liquefaction

A phenomenon where saturated soil significantly loses strength and stiffness in response to an applied stress, often during seismic activity, causing instability.

River Avulsion and Aggradation

River avulsion and aggradation refer to the processes where a river changes its course (avulsion) and accumulates sediment (aggradation), impacting the surrounding landscape and ecosystem.

Reverse or Thrust Fault

A type of fault where the land above the fault moves upward relative to the land below the fault, typically caused by compression.

Canterbury

Reverse and strike-slip, with frequent earthquakes.

New Zealand Plate Boundary

The boundary where the Pacific and Australian tectonic plates interact, characterized by complex geological activity due to the oblique collision. Compression occurs in the east coast of the North Island and Fiordland, strike-slip in the middle, and stable crust in the far north and south.

Pacific Ring of Fire

A region encircling the Pacific Ocean characterized by frequent earthquakes and volcanic eruptions due to tectonic activity.

Subsidence

The sinking of land during an earthquake due to the rapid release of tension which has previously caused uplift.

Tsunami

Powerful ocean waves produced by underwater earthquakes and volcanic activity.

Ground Rupture

Displacement of Earth’s surface from an earthquake or divergent plate boundary.

Hazard

A natural event or process which may result in loss of life or property.

Consequences

Potential loss of life and property.

Risk

Hazard times consequences. An area may have high seismic activity but low population density, resulting in low overall risk.

Building Response to Ground Shaking

Small buildings are most sensitive to rapid shaking while tall buildings are most sensitive to long rolling motions.

Volcanic Hazards

Ashfall, pyroclastic density currents, lahars, lava flow, or gas discharge.

Pyroclastic Density Current

A quickly-moving current of gas and volcanic material ejected from a volcano and flowing downhill.

Lahar

A destructive mudflow of volcanic ash and debris.

Maximum-Size Event

The most catastrophic event that could reasonably occur in a given area, often assessed for risk management and planning.

Te Riu a Maui

Zealandia, the submerged continent surrounding New Zealand, it began to break away from Gondwana about 100 Ma.

Terrane Accretion

Packages of continental crust which are too buoyant to subduct and are scraped off the subducting plate and added to the other plate.

Active Continental Margins

Margins that coincide with plate boundaries such as subduction zones.

Passive Continental Margins

Margins that are far from plate boundaries and tend to have wider continental shelves.

Continental Shelf

The submerged portion of a continental margin, extending from the shore to the continental slope and characterized by shallow ocean.

Continental Slope

The rapid transition from continental shelf to abyssal plain, characterized by a steep gradient.