Plate Tectonics - For animations of the concepts explained here go to Savage Earth from PBS

Plate tectonics and sea floor spreading is a theory developed mainly since about 1960 that attempts to explain the evolution of the outermost part of Earth, its crust. It is a theory developed to explain observations about the earth such as:

Plate tectonic theory now seems like something that has always been around. It is clearly the best explanation for many phenomena observed on Earth, isn't it? This has not always been true.

One of the pioneers that paved the way to plate tectonics was Alfred Wegener, a German arctic explorer and geophysicist who lived in the early 1900's. He proposed the idea of continental drift to explain the apparent fit of continents on either side of the Atlantic Ocean. Click here to learn more about him and his revolutionary (for the time) theories that set some of the groundwork for our understanding of Earth systems today.

In order to visualize crustal history, animations have been constructed that show the motion of continental crustal blocks with time.

Another site with animations of plate motions.

The mechanisms driving motions of Earth's crust are the result of the transfer of heat from the interior of the Earth to the surface.

There are three kinds of plate edges - spreading, transform and subducting.

Spreading edges:
- Hot rising matter at the center of the spreading = ocean ridge. This new crust spreads and cools.
- After these edges start they always lie in oceans

• irregular 'tears' form along the spreading ridges. These are called transform faults. As they get longer, these faults lead to edges where plates slide past . Diagram at right from http://www.pmel.noaa.gov/vents/geology/MOR.html

　

　

　

Transform or sideways sliding edges:
- These have earthquakes but no volcanoes.

From Dylan Prentis, Dept. of geography, UCSB and http://earth.rice.edu/mtpe/geo/geosphere/hot/san_andreas.html

Subducting edges:
- cold crust sinks (subducts)- some is resorbed into mantle
- subducted material is heated and melts leading to volcanic arcs

Diagram at left from http://www.llnl.gov/str/Kersting.html

• evidence spreading - mid-Atlantic, Red Sea
• evidence transform - California - San Andreas
• evidence convergent - Juan de Fuca, Alaska
• Places where three plates come together called triple junctions
  example - Mendocino

Each plate moves in a way that can be described by rotation around an axis
- the transform edges are lines of latitude with respect to that rotation axis

Characteristics of spreading plate edges

spreading centers lie in ocean basins after they form

Diagnostic rocks of early phases of spreading:

Rocks on trailing continental margins at edges of widening ocean are a wedge shaped sediment pile, formed by quiet deposition in shallow water. Pile sinks 2' for every 3' sediments (isostacy + compaction)

slabs of ocean floor rocks, now exposed = ophiolite complexes. These consist, from bottom up, of:

• ultramafic rocks often with tectonic fabric
• gabbro and diabase
• sheeted diabase dikes
• basalt flows, often pillowed
• deep sea sediments (chert, ooze)

Characteristics of transform boundaries

almost all in ocean basins

Diagnostic features

• This is the zone of most visible consequences of plate tectonics as most effects result in above sea level features

Diagnostic rocks -

• results of rapid sediment accumulation in trenches - thick marine sediments, turbidites
• forearc basin - sediments derived from the volcanic arc in crumple zone
• melange
• high P, low T metamorphism where crustal rocks are being dragged down
• med T, P metamorphism due to thickening as the edge of the continental crust buckles.

further back but parallel to the convergent plate boundary:

• magma from subducting slab
• high T low P metamorphism from heat brought up by the magma

Would you like to calculate actual current plate motions? To do so, select two plates that are in contact, for example Eurasia and North America, then locate the latitude and longitude of a point on the boundary and then go to the Relative Plate Motion Calculator to find the motion.

Most of plate interiors - dull places.

Continental interiors - Are affected by vertical movements connected with plate movements

• failed rifts
• domes and basins due thermal effects
• plateaus - Tibet and Basin and Range
• deformation transmitted from margins

sediments - mature clastics, sandstone, shale and carbonate

For additional information - visit: developing the theory and the US Geological Survey web site on plate motions