plate tectonic theory

Plate tectonic theory provides a plausible mechanism for continental drift, explaining the concentration of earthquakes and 90% of volcanoes around the 'ring of fire'
(image at right –see interactive map of This Dynamic Planet).

Tectonism involves a variety of geological phenomena and structures associated with the large-scale movement of tectonic plates:
◘ earthquakes
◘ explosion and extrusion of lava, and intrusion of magma (hypabassal, plutonic)
◘ faulting - normal, thrust, transform, listric fault – duplex structures, horse, horst & graben, klippes, nappes, shear zones, windows,
◘ folding and deformation – décollement folds,
◘ orogeny associated with continental-continental or continental-oceanic collision margins
◘ magmatism
◘ metamorphism
◘ mid-oceanic ridge spreading centers and creation of oceanic crust
◘ rifting
◘ shear zones
◘ subduction zones and submarine trenches associated with continental-oceanic collision margins causing consumption of oceanic crust; associated with accretionary prisms, mélanges.
◘ volcanic island arcs above subduction zones, and associated with generation of a variety of subduction zone magmas
◘ volcanoes

Continental drift explains the observed close fit between coast lines at opposite sides of the Atlantic (right) as well as the paleontological observation that equivalent, contemporaneous fossilized plants and animals can be found in continents and land masses that are now separated by oceans and seas.

The ocean floor is younger than the continental crust, and is arranged in parallel "stripes" of alternating magnetization parallel to the mid-oceanic ridges.

Considerable evidence supports tectonic theory:
● Earth's crust is divided into several crustal plates (left - click to enlarge image) bounded by extensional, transform, or compressional stresses
● ocean floors arise in chains of mid-oceanic ridges, and spread outward from these spreading centers, subducting beneath continental margins, where oceanic crust is melted (and subsequently regenerated at the ridges).
● sub-plate convection currents in the fluid asthenosphere circulates as convection currents underneath the solid lithosphere and move the crustal plates relative to one another (image below left)
● radioactivity deep in the Earth's mantle provides the source of heat that drives the convection currents

As plates bump and grind, stresses build in the brittle lithosphere and the stored energy is ultimately released as earthquakes:
● at extensional boundaries where plates pull apart, such as spreading ridges, earthquakes are shallow, usually less than magnitude 8, and are aligned strictly along the axis of spreading.
● at transform faults where plates slide past each other, earthquakes are shallow (down to 25 km), usually smaller than magnitude 8, and exhibit strike-slip motion.
● at compressional boundaries between continental and oceanic crust, the plates collide with one another and one plate slides under the other (subduction), producing the largest earthquakes (some greater than magnitude 9) at depths that range from the very near surface to several hundred kilometers in depth (cool subducting plates can experience brittle failure as deep as 700 km)

◙ subduction zone magmas ◙

As an aid to understanding plate motion, divide the skin of an orange into square 'plates' and shift the sections relative to each other. To pull apart (extend) or push together (compress) the sections will require that edges at roughly 90 degrees to the direction of motion will slide past one another (transform).


history of plate tectonics theory : plate tectonics: mechanism :

images: This Dynamic Planet and tectonic plates courtesy of USGS