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