Figure 1-8a Tectonic Plates

Figure 1-8a Tectonic Plates

Evidence for Continental Drift: Puzzle

According to Wegner, the continents are sections of a past super continent called Pangea, which broke apart and drifted to their present locations.

Now-Extinct Life Forms Preserved in the Geologic Record tell a story

Source: J. C. Carton/Carto/Bruce Coleman, Inc. New York

Evidence for Continental Drift: Fossils

Source: William E. Ferguson

Evidence for Continental Drift: Rock Record

Pangaea 200 to 300 Millions of Years Before the Present

• Axis of the oceanic ridge is offset by transform (strike-slip) faults which produce lateral displacement. Ridges and rifts indicate movement.

3-2Sea-Floor Spreading: Movement at ridges

Segmented Ocean Ridge

Earth’s geomagnetic field is recorded as new crust cools.

3-3 Global Plate Tectonics

Driving Mechanisms for Plate Motions

New crust. Parallel bands ofcrust with the same magnetism form along the ridge.

Geomagnetic Polarity Reversals

When new crust materials crystallizes, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.Periodically Earth’s magnetic field polarity (direction) reverses poles.

Magnetic Anomalies

1_14

Modern humans

Extinction of dinosaurs

Flowering plants and grasses

First mammals

Earliest dinosaurs

Early reptiles

Primitivefish

Geologic Time

• Rocks forming at the ridge crest record the magnetism existing at the time they solidify.

• Sea floor increases in age and is more deeply buried by sediment away from the ridge because sediments have had a longer time to collect.

• Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating magnetic anomaly stripes of the sea floor and measuring their distance from the ridge crest.

• Continents are moved by the expanding sea floor.

3-2 Sea-Floor Spreading

Because Earth’s size has not changed, expansion of the crust in one area requires destruction of the crust elsewhere.•Currently, the Pacific Ocean basin is

shrinking as other ocean basins expand.

•Seismicity is the frequency, magnitude and distribution of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs.

•Tectonism refers to the deformation of Earth’s crust.


•Destruction of sea floor occurs in subduction zones.

•Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere.


• Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.


South Figi Basin and Cross Section Showing Benioff Zone

Earth’s surface is composed of a series of lithospheric plates. Plate edges extend through the lithosphere and are defined by seismicity.•Plate edges are trenches, oceanic ridges

and transform faults.

•Seismicity and volcanism are concentrated along plate boundaries.


Movement of plates is caused by thermal convection of the “plastic” rocks of the asthenosphere which drag along the overlying lithospheric plates.


Driving Mechanisms for Plate Motions

Plate Rifting (cont'd): Earths Internal Heat Engine

Figure 1-10 Mid-Ocean Ridge

•Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a “hot spot”.


Mantle Plume

Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins.• Stages in basin history are:

– Embryonic - rift valley forms as continent begins to split.

– Juvenile - sea floor basalts begin forming as continental fragments diverge.

– Mature - broad ocean basin widens, trenches eventually develop and subduction begins.

– Declining - subduction eliminates much of sea floor and oceanic ridge.

– Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain.


The Wilson Cycle

3-4 Transform Faults

• If these plate motions continue, Baja will splinter off California.

The San Andreas fault in southern California is a transform fault that connects the sea-floor spreading ridge of the Gulf of California with the spreading ridge off Oregon and Washington.

Because the San Andreas fault has an irregular trace, strike-slip motion can cause local compression or tension.

3-4 Transform Faults

Fault Geometry

•The Red Sea is a juvenile ocean basin that is forming as the African plate diverges from the Arabian plate.

•New basaltic ocean crust is just beginning to form in the center of the Red Sea.

3-4 Juvenile Ocean Basin

Hot, salty groundwater is dissolving metals from the rocks and depositing them as metal sulfides in dense brine pools like the Atlantis II Deep.

3-4 Juvenile Ocean Basin

Atlantis II Deep

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Figure 1-8a Tectonic Plates