Earth's Surface, Tectonic Processes, and Earthquakes

Vocabulary terms covering Earth's internal structure, plate tectonic boundaries, subduction zone anatomy, and earthquake mechanisms and hazards.

Lithosphere

The rigid outer crust of the earth, consisting of rocks that are less dense and more rigid than those found in the asthenosphere.

Sial

The continental crust, which is less dense than oceanic crust and predominantly composed of granitic rocks.

Sima

The oceanic crust, which is denser than continental crust and predominantly composed of basaltic rocks.

Mesosphere (mantle)

The Earth layer extending from the base of the lithosphere to a depth of $2900\,\text{km}$, constituting approximately $83\%$ of the Earth's volume and consisting predominantly of peridotite.

Mohorovicic Discontinuity (Moho)

The boundary separating the lithosphere from the mantle (classified strictly as a boundary, not a zone).

Asthenosphere

A weak, plastic-like, partly molten layer of the upper mantle directly below the lithosphere where convection currents drive tectonic plate movement.

Outer Core

A permanently molten layer of the centrosphere composed of iron and nickel whose flowing currents are thought to create Earth's magnetic field.

Inner Core

A solid ball of iron and nickel where temperatures reach approximately $3,700^\circ\text{C}$, remaining solid due to extreme pressure.

Condensation (Planetary Formation)

The process during early Earth development where loose superheated dust particles began to increase in density.

Accretion

The developmental phase where condensing particles joined together over time to form a solid, compact planet.

Differentiation

The planetary reorganization where heavy elements like iron and nickel sank toward the center to form the core, while lighter elements rose to create a thin crust.

Isostasy

The slow, continuous vertical adjustment of buoyant continental blocks of sial floating on dense basaltic sima, sinking when weight is added and rising when weight is removed.

Convection Currents

Slow, circular movements of mantle material caused by heating and cooling (rising as it heats and dropping as it cools) that propel tectonic plates at $2\text{--}3\,\text{cm/year}$.

Mid-Ocean Ridge

A constructive or diverging plate boundary found almost exclusively on the ocean floor where two plates move apart and new lithosphere is formed.

Sea-Floor Spreading

The process where two oceanic plates move in opposite directions from a rift, allowing upwelling magma from the asthenosphere to solidify and continuously create new crust.

Normal Polarity

The present magnetic orientation of Earth, in which all newly formed rocks cooling at mid-ocean ridges are currently magnetized.

Transform Faults

Cracks and horizontal boundaries that connect offset segments of mid-ocean ridges, where plates grind horizontally past one another in opposite directions.

African Rift Valley

A constructive continental plate boundary running approximately $10,000\,\text{km}$ from Turkey south to the Zambezi River, slowly widening at a few centimeters per year.

Subduction Zone

A destructive or converging plate boundary where a denser plate is forced downward beneath another plate into the mantle to be reclaimed.

Ocean Trench

A long, narrow, steep-sided depression marking the deepest parts of the ocean, created when a subducting plate drags down the leading edge of the overriding plate.

Island Arcs

A curved chain of volcanic islands, such as the Philippine and Tonga Islands, formed when two oceanic plates converge and excess subducted material rises to the surface.

Fold Mountains

Massive mountain ranges, such as the Himalayas and the Rocky Mountains, created when two buoyant continental plates collide and buckle upward.

Continental Shelf

A shallow, relatively flat region of sedimentary rock extending outward from the shoreline to the drop-off into a trench or deep ocean basin.

Continental Slope

The steep descent extending downward from the edge of the continental shelf toward an ocean trench.

Benioff Zone

The earthquake zone where a subducting plate descends into the mantle, characterized by high friction, frequent small tremors, and occasional major earthquakes when stuck plates slip.

Andesite Volcanoes (Composite Cones)

Steep-sided, snow-capped volcanic peaks formed when subducting oceanic crust melts in the mantle and rises to the surface as andesite lava.

Tear (Strike-Slip) Fault

A plate boundary where two lithospheric plates slide horizontally past one another without creating or destroying crustal surface area.

Earthquake

Ground shaking produced by seismic waves that are released when stress-deformed rocks fracture or undergo sudden movement via elastic rebound.

Focus

The exact underground location where a rock fracture and sudden shift originate, reaching depths of up to approximately $700\,\text{km}$.

Epicenter

The geographic location on the Earth's surface positioned vertically directly above the earthquake focus.

Focal Depth

The straight-line vertical distance between an earthquake's focus and its surface epicenter.

Elastic Rebound

The sudden release of built-up strain energy when locked rock along a fault ruptures and snaps back to an unstressed position.

Isostatic Readjustment (as an Earthquake Cause)

Earth tremors generated when land masses sink or rise in response to mass shifts, such as the subsidence caused by reservoir water impounded behind large dams.

Seismograph

An instrument designed to detect and record seismic waves produced by earthquakes.

Richter Magnitude Scale

A scale ranging from $1$ to $8.9$ that measures earthquake strength, where each whole number increment represents a $10\times$ increase in magnitude.

Primary Wave (P-wave)

A compressional shock wave that moves by alternately compressing and expanding the ground; it is the fastest seismic wave and the first to arrive at recording stations.

Secondary Wave (S-wave)

A shear seismic wave originating at the focus that moves ground in a side-to-side motion and travels slower than a primary wave.

Surface Wave

A slow seismic wave formed when primary and secondary waves reach the surface; it ripples like water waves and causes the ground shaking responsible for major damage.

Aftershocks

Secondary earthquakes occurring along a fault plane for several days after the primary mainshock, capable of collapsing weakened structures.

Liquefaction

The process by which violent shaking destabilizes water-saturated sandy soil in river deltas or ancient lake beds, causing the ground to act like quicksand.

Tsunami

A massive ocean wave triggered by sudden vertical displacement of the sea floor along a subduction zone, capable of traveling at $1000\,\text{km/h}$ in open ocean.