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 Composition Rich in Fe, Mg Poor in Si, Al Rich in Si, Al Poor 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