Plate Tectonics and Volcanoes

Plate Tectonic Theory

Unifying theory that explains the dynamic nature of the geosphere.

Continental drift

Hypothesis that continents move relative to each other.

Plate

A discrete piece of lithosphere that moves relative to other pieces.

Paleomagnetism

Magnetism preserved in minerals at the time they formed.

Ridge

An underwater mountain range formed by plate tectonics.

Trench

A deep, narrow depression in the ocean floor formed by subduction.

Seafloor spreading

Process by which new oceanic crust is formed at mid-ocean ridges.

Magnetic Reversals

Changes in Earth's magnetic field direction over geological time.

Fault

A fracture in the Earth's crust along which movement has occurred.

Divergent Plate Boundary

Boundary where two tectonic plates move away from each other.

Convergent plate boundaries

Boundary where two tectonic plates collide or move towards each other.

Subduction

Process where one tectonic plate moves under another plate.

Subduction Zone

Region where one tectonic plate is being forced below another.

Transform plate boundary

Boundary where two tectonic plates slide past each other.

Accretion

Process by which material is added to a tectonic plate.

Compression

Stress that occurs when tectonic plates push together.

Collision

Event where two tectonic plates crash into each other.

Pangea

The supercontinent that included all of Earth's continents.

Gondwanaland

Supercontinent that included the southern continents.

Fossil assemblage

A group of fossil species found together.

Glacial deposits

Sediments left by glaciers that provide evidence of past ice ages.

Apparent polar wander curve

A line connecting the seemingly changing locations of the magnetic pole.

Alfred Wegener

First to formulate a detailed, global explanation of how continents assumed their present locations and shapes.

Distinctive fossil seed fern Glossopteris

Fossil found in southern continents or Gondwanaland that supports the theory of continental drift.

Wandering Magnetic Poles

The concept that the magnetic poles have shifted over time.

Seafloor Spreading

The process by which new oceanic crust is created at mid-ocean ridges.

Magnetic Stripes

Patterns of magnetic anomalies on the ocean floor that provide evidence for seafloor spreading.

Earthquakes

Sudden shaking of the ground caused by the movement of tectonic plates.

Ridge

An underwater mountain range formed at a spreading center.

Trench

Deepest parts of the oceans where a tectonic plate sinks into the mantle.

Seafloor spreading

The movement of newly formed oceanic crust away from mid-ocean ridges.

Mid-ocean ridges (MORs)

Locations where new oceanic crust forms as lithosphere pulls apart and magma from the mantle wells up, cools, and solidifies.

Magnetic Signature

Magnetic field stripes arranged symmetrically about the center of mid-ocean ridges.

Magnetic Stripes

Alternating regions of strong and weak magnetic strength on the seafloor.

Magnetic Reversals

180 degree shift in the Earth's magnetic field, where north and south trade places.

Fault

A place where rock has broken and the blocks on opposite sides have moved.

Transform fault

A type of strike-slip fault that converts movements at plate boundaries into lateral sliding.

Earthquakes

Key evidence needed to further support the emerging theory of plate tectonics.

Divergent Plate Boundary

Region where two plates move away from each other.

Rifting

Hot asthenosphere material begins to rise, causing lithospheric thinning and normal faulting.

Normal Fault

Angled breaks in the lithosphere where the upper block has moved down.

Seafloor Elevation

The height of the seafloor relative to sea level.

Clues for Plate Tectonics

Evidence used to understand the movement of tectonic plates.

Contribution to Plate Tectonics

Hess recognized a mechanism to explain continents being separated through spreading and sinking.

Volcanoes

Common along fast-spreading ridges like the Mid-Atlantic Ridge.

Analysis of rocks on land

Made it possible to construct a timetable of magnetic polarity reversals.

Oceanic crust

Forms where continents once joined during rifting.

East Africa-Arabia

An example of modern rifting where the African continent is being separated from Arabia.

Modern tectonic plates

Move slowly, averaging several cm (a few inches) per year, approximately as fast as your fingernail grows.

Three Major Types of Plate Boundaries

Divergent, Convergent, Transform.

Lithosphere

The rigid outer layer of the Earth, including the crust and upper mantle.

Asthenosphere

The semi-fluid layer beneath the lithosphere that allows for tectonic movement.

Crustal extension

The process where pieces of continent migrate away from each other during rifting.

Hess

Geophysicist who recognized the mechanism explaining the separation of continents.

Mid-Atlantic Ridge

A fast-spreading ridge where new oceanic crust forms.

Divergent Plate Boundaries

Regions where two tectonic plates move apart, forming new seafloor at spreading centers.

Spreading Centers

Locations in the ocean where new seafloor is formed.

Black Smokers

Mineral-rich hot waters from hydrothermal vents that form dark plumes on the seafloor.

Convergent Plate Boundaries

Regions where two tectonic plates move toward each other.

Subduction Zones

Inclined parts of convergent plate boundaries where oceanic lithosphere sinks into the mantle.

Reverse Fault

Angled breaks in the lithosphere where the upper block has moved up.

Thrust Fault

A low angle reverse fault.

Subduction

The process of lithospheric plate sinking into the molten mantle.

Aleutian Trench

A convergent plate boundary in Alaska where oceanic Pacific plate sinks beneath North American plate.

Suture Zone

The area formed when two continental plates collide and neither is dense enough to sink.

Strike-Slip Fault

A fault where two tectonic plates slide past each other horizontally.

Transform Plate Boundary

A boundary where two tectonic plates move past each other.

San Andreas Fault

A transform fault in California known for its significant offsets during earthquakes.

Scarp

A steep slope or bank on Earth's surface created by movement on a fault.

Earthquake Distribution

The pattern of earthquakes that indicates the location of tectonic plate boundaries.

Tsunamis

Seismic sea waves generated by offshore earthquakes.

Mt. Pinatubo

A volcano located at a convergent plate boundary.

Mountain Building

The process influenced by plate tectonics that involves accretion, compression, and collision of tectonic plates.

Bingham Canyon Mine

A copper mine in Utah formed within subduction-related volcanoes.

Rainshadow Effect

A phenomenon resulting from moist air moving up the Andes, cooling, and condensing, causing precipitation.

Chugach Mountains

Mountains formed by offscraping of seafloor sediments against an overriding plate at a convergent margin.

Accretion

The process of adding pieces of the geosphere along the continental margin.

Compression

The process occurring at convergent plate boundaries that contributes to mountain building.

Magmatic Processes

Processes that help form large mountains and mountain ranges where oceanic plates converge and subduct.

Volcanic Arcs

Chains of volcanoes formed at convergent plate boundaries.

Island Arcs

Curved chains of volcanic islands that form along subduction zones.

Deep-Sea Trenches

Long, narrow depressions in the ocean floor formed at convergent plate boundaries.

Earthquake Effects

The impact of earthquakes, which are most devastating around the Pacific Ocean rim.

Volcano

a place where molten rock rises to the surface

Magma

molten rock underground

Magma Characteristics

magma contains dissolved gases such as H2O vapor, CO2, SO2

Magma Rising

As magma rises, the confining pressure of the overlying rocks decreases, causing gases to bubble out of solution and release the pressure

Explosive Magma

the more dissolved gas, the more explosive the magma

Lava Flow

Magma quickly cools and solidifies into igneous rock

Volcanic Eruptions

1300-1500 volcanoes on land with probable eruptions in the last 10,000 years

Eyewitness Accounts

only about 550 volcanic eruptions have eyewitness accounts

Average Eruptions

50-70 volcanoes erupt on land in an average year

Potentially Active Volcanoes

several potentially active volcanoes dangerously close to large population centers in the continental United States

Types of Magmas

Eruptions range from spectacular explosions to oozing red-hot lava flows

Viscosity

resistance to flow, depends on temperature and composition

Composition Influence

Composition has the greatest influence on magma viscosity

High SiO2

high SiO2 results in more viscous magma that solidifies at lower temperatures

Gas Content in Lavas

H2O is the most abundant gas dissolved in magma, along with CO2, SO2, and others

Gas Separation

Gases may separate from magma due to partial crystallization, temperature decrease, or decrease in confining pressure

Explosive Eruptions

Explosive eruptions can occur when gases separate from magma