plate tectonics. objectives to be able to describe the history and supporting evidence for plate...
TRANSCRIPT
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.
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)
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
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)
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…