Geoscience Chapter 2 - Plate Tectonics

Continental Drift

The hypothesis that continents slowly move across Earth's surface.

Alfred Wegener

The German meteorologist who proposed continental drift in 1915.

Pangaea

A former supercontinent containing Earth's landmasses.

Glacial Evidence

Evidence from ancient glaciers found on continents that are now far from the poles.

Glacial Striations

Scratches or streaks in rock produced by moving ice.

Matching Coastlines

The similarity between continental coastlines, especially across the Atlantic Ocean.

Fossil Evidence

Evidence from identical fossils found on continents now separated by oceans.

Matching Geologic Units

Similar rock formations and mountain belts found on continents now separated by oceans.

Wegener's Problem

The inability of Wegener to explain how or why continents moved.

Paleomagnetism

The study of fossil magnetism preserved in rocks.

Magnetic Declination

The difference between geographic north and magnetic north.

Magnetic Inclination

The angle between a magnetic field line and Earth's surface.

Curie Point

The temperature above which a mineral loses its magnetism.

Magnetic Dipoles

Magnetic elements in minerals that become aligned with Earth's magnetic field as magma cools.

Magnetic Polarity

The direction of Earth's magnetic field.

Normal Polarity

Magnetic polarity like Earth's present-day field.

Reverse Polarity

A magnetic polarity opposite to the present-day field.

Magnetic Reversal

A change in Earth's magnetic field polarity.

Magnetic Anomaly

A variation in the magnetic properties of ocean-floor rocks.

Positive Magnetic Anomaly

An area where sea-floor rock has the same polarity as the present magnetic field.

Negative Magnetic Anomaly

An area where sea-floor rock has polarity opposite to the present magnetic field.

Chrons

Long magnetic-polarity intervals lasting about 500–700 thousand years.

Subchrons

Short magnetic-polarity intervals lasting about 200 thousand years.

Sea-Floor Spreading

The process by which new oceanic crust forms at mid-ocean ridges and moves outward.

Mid-Ocean Ridge (MOR)

A long underwater mountain range where new oceanic crust forms.

Magnetic Stripes

Symmetric bands of alternating magnetic polarity preserved in ocean-floor rocks.

Sea-Floor Bathymetry

The mapping and measurement of variations in seafloor depth and features.

Sonar

A method that maps the seafloor by measuring depth using sound waves.

Deep-Ocean Trench

A deep depression in the seafloor commonly associated with subduction.

Seamount

An underwater volcanic mountain.

Fracture Zone

A large fracture that segments or offsets portions of the mid-ocean ridge system.

Oceanic Crust

The crust beneath ocean basins, composed primarily of basalt.

Plate Tectonics

The theory that Earth's outer shell is broken into rigid plates that move.

Tectonic Plate

A rigid fragment of the lithosphere that moves relative to other plates.

Lithosphere

The rigid layer made of Earth's crust and upper mantle.

Asthenosphere

The partially molten, plastic layer beneath the lithosphere over which plates move.

Plate Boundary

The region where tectonic plates interact.

Divergent Boundary

A plate boundary where tectonic plates move apart.

Convergent Boundary

A plate boundary where tectonic plates move toward each other.

Transform Boundary

A plate boundary where tectonic plates slide sideways past each other.

Ridge-Push

A force in which an elevated mid-ocean ridge pushes lithosphere away from the ridge.

Slab-Pull

A force in which a dense subducting plate pulls downward.

Subduction

The process in which one tectonic plate sinks into the mantle.

Subduction Zone

A convergent boundary where one plate descends into the mantle.

Continental Collision

The collision of continents after oceanic subduction closes an ocean basin.

Continental Rifting

The process in which continental lithosphere stretches, thins, and breaks apart.

Rift

A region where continental lithosphere is being pulled apart.

Active Margin

A continental margin located near a plate boundary and commonly associated with earthquakes.

Passive Margin

A continental margin far from a plate boundary.

Volcanic Arc

A chain of volcanoes on the overriding plate above a subduction zone.

Island Arc

A volcanic arc formed where the overriding plate has oceanic crust.

Continental Arc

A volcanic arc formed where the overriding plate has continental crust.

Back-Arc Basin

A marginal sea that forms behind a volcanic arc.

Accretionary Prism

A feature associated with subduction where material accumulates near the trench.

Transform Fault

A fault along which plates slide laterally past one another.

Triple Junction

A point where three plate boundaries intersect.

Hotspot

A location where a deep mantle plume rises and partially melts the lithosphere independently of plate boundaries.

Hot-Spot Track

A chain of extinct volcanoes created as a plate moves over a relatively stationary hotspot.

Absolute Plate Velocity

The motion of a plate measured relative to a fixed point in the mantle.

GPS (Global Positioning System)

A satellite-based system used to accurately measure plate motion.

Wadati-Benioff Zone

A zone of earthquakes that reveals the descent of a subducting plate into the mantle.

Ocean-Ocean Convergence

A convergent boundary where one oceanic plate subducts beneath another.

Ocean-Continent Convergence

A convergent boundary where oceanic lithosphere subducts beneath continental lithosphere.

Continent-Continent Convergence

A collision between continental plates that deforms crust and produces mountains.

Oceanic Lithosphere

The lithosphere associated with oceanic crust; relatively dense and able to subduct.

Continental Lithosphere

The lithosphere associated with continental crust; relatively light and buoyant.

Pillow Basalt

Basalt formed when magma rapidly cools after erupting onto the seafloor.

Dike

A preserved magma conduit within oceanic crust.

Gabbro

A coarse-grained igneous rock formed from deeper magma beneath mid-ocean ridges.