Plate Tectonics

Objectives

• To be able to describe the history and supporting evidence for plate tectonic theory

• To be able to explain the basic principles of plate tectonic theory

• To be able to describe & draw the 3 types of plate boundaries

• To be able to describe features on Earth explained by plate tectonic theory

Definition• from the Greek τέκτων; tektōn, meaning “builder” or

“mason”

The main features of plate tectonics are: • The Earth's surface is covered by a series of crustal

plates.

• The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.

• Convection currents beneath the plates move the crustal plates in different directions.

• The source of heat driving the convection currents is radioactivity deep in the Earths mantle.

History• Alfred Wegener first proposed concept of continental drift (1915)

• Wegener said all continents were joined together in one landmass called Pangaea

• He used circumstantial evidence (puzzle-like fit of continents, fossil data, similar rock types and ages, glacial striations) to support his theory

• He proposed a “pole-fleeing force” to explain Pangaea’s drift away from the South Pole toward the equator and gravitational forces of the sun and moon to explain the westward drift of North America

• His theory was rejected; he died in Greenland in 1930. His body is presumed to be buried under >300 feet of ice.

Evidence Cited by Wegener

History (cont.)• Post World War II – technological advances led to detailed mapping

of the seafloor; ridges and trenches discovered

• Age dating of oceanic crust showed seafloor rocks are youngest at the ridges and oldest toward the trenches

• Paleomagnetic data showed a record of reversals preserved in the volcanic rocks of the seafloor

• Seafloor spreading proposed in 1961-62 by Howard Hess and R. Dietz

• Convection model proposed as mechanism explaining crustal motion; HEAT drives the system

• Plate tectonic theory accepted by the late 1960’s

Basic Principles

• The lithosphere exists as separate and distinct plates (<10 major plates; up to a few dozen including microplates)

• Plates ride on the fluid-like asthenosphere

• Plate motions range from ~1-4 cm/yr (as fast as fingernails grow) to ~16 cm/yr (as fast as hair grows)

Compositional vs. Mechanical Layering

The Lithosphere and Earth’s Interior

   Oceanic crust Continental crust

Average thickness 5-10 km 30-70 km

Density 3.2 g/cm3 2.7 g/cm3

CompositionRich in Fe, MgPoor in Si, Al

Rich in Si, AlPoor in Fe, Mg

Plate Tectonic Map

Tectonic Plate Motions

California Seismicity

Types of Plate Boundaries

• Divergent (AKA constructive; e.g. Salton Sea in Southern California)

• Transform (e.g. San Andreas Fault)

• Convergent (AKA destructive)– ocean-ocean (e.g. Mariana Islands)– ocean-continent (e.g. Pacific Northwest

Cascadia Subduction Zone)– continent-continent (e.g. India-Asia)

Divergent

Transform

Convergent

Phenomena on Earth Explained by Plate Tectonic Theory

• Mountains• Trenches• Midocean Ridges• Earthquakes• Volcanoes• Mineral/Ore Distribution• Resource Distribution (i.e. geothermal, etc.)• Island chains and arcs• Faulting and Folding of Rocks• Et Cetera…

Summary

• Alfred Wegener continental drift led to plate tectonics

• Supporting evidence includes: puzzle-like fit of continents; fossil, rock, glacial, and paleomagnetic data; seafloor spreading

• Driven by convection

• Explains most geologic phenomena on Earth