Plate Tectonics

Current theory which explains many of the earth’s surface features and its dynamic nature Provides model of earth in which tectonic plates of crustal and upper mantle material are moved across the earth’s surface by mantle convection Updates Wegener’s theory of continental drift (1912)

Plate Tectonics Movement of continents –200 million years ago: original single land mass (“Pangaea”) –Current continents include North America, South America, Eurasia, and Africa –Evidence for movement Sea-floor spreading –Atlantic Ocean becoming 2 inches wider each year Magnetic reversals –300 reversals of magnetic fields recorded in rock Dating ages of rock –Calculate that 140 million years ago Europe and the Americas were joined

Plate Tectonics PLATE FORMATION AND MOTION –Surface of Earth broken up into about 12 large pieces as well as additional smaller pieces (“plates”) –Consists of crust and part of upper mantle Oceanic plates covered with salt water (approx. 70% of earth’s surface) Continental plates covered with land –Mantle convection Plates move over a partially molten, underlying section of earth’s mantle Heat energy powers motion in mantle Heat derives from initial bombardment of earth & presence of radioactive elements Plates move along with convection cells underneath them (much like a film of oil would move along on top of boiling water)

Plate Tectonics TYPES OF PLATE MOVEMENT –DIVERGENT –Plates move apart –New crust is created at divergent plate boundaries –Forms volcanic ridges –CONVERGENT –Plates come together –Old crust returns to the mantle in subduction zones –3 possibilities »When ocean plates converge, deep trenches are formed »When an ocean plate subducts beneath a continental plate, offshore trenches and a chain of continental volcanoes parallel to shore occur »When 2 continental plates converge, mountain ranges of crumpled continental rock are formed –TRANSFORM –Two plates scrape past each other –Causes earthquake generated waves throughout the earth

Plate Tectonics EARTH’S DYNAMIC NATURE –MOUNTAINS Mountains do not last more than a few hundred million years –Constantly worn down by erosion –New mountains continuously developing Develop as a result of volcanism –e.g. Mt. Olympus Develop as a result of continental plates colliding –e.g. Himalayas –VOLCANOES Develop in 3 geological situations –Along divergent plate boundaries »e.g. Mid-Atlantic Ridge (earth’s longest mountain range is under the Atlantic Ocean!) –Near convergent plate boundaries »e.g. “Ring of Fire” in Pacific Ocean; Cascade Mountains in NE United States –Above “hot spots” »e.g. Hawaiian Islands –EARTHQUAKES –May be felt near any plate boundary »Shallow earthquakes near divergent plate boundaries »Stronger earthquakes near subduction zones, such as in Japan »Earthquakes of various strengths at transform plate boundaries, such as the San Andreas Fault in the US –Occasional earthquakes in middle of plates »Not well understood »Examples in US in Missouri –“Seismology” studies and measures vibrations due to earthquakes