Plate Tectonics Study Guide

Alfred Wegener = Continental Drift Continents were once part of a single land mass called Pangaea (all lands). During the Mesozoic Era Pangaea began to break up and drift apart

Evidence to support Continental Drift: Similarities in the coastlines of the continents. Fossil evidence = the same fossils were found on different continents that would have been joined during Pangaea. Example = Mesosaurus (S. America and W. Africa) Geological evidence = age and type of rocks in the coastal regions match. Example = W. Africa and E. Brazil Climate evidence = glaciers in S. Africa and S. America, Coal deposits in E. United States, Europe, and Siberia

The scientific community was not impressed…..they insisted that Wegener had to find evidence of some kind of force that would have put Continental Drift into motion.

1947 = scientists set out to map the Mid-Atlantic Ridge (an undersea mountain range) They discovered that rock samples brought up from the ocean floor were much younger than continental rocks. No oceanic rocks were older than 175 million years old. Continental rocks = 4 billion years old!!!

Harry Hess and Robert Dietz hypothesized that the valley at the center of the Mid-Atlantic Ridge was a break in the Earth’s crust and that magma was welling up through the rift. If the ocean floor was moving outward from the rift, then the continents might be moving too. Seafloor Spreading

Evidence for Seafloor Spreading = Paleomagnetism = As magma cools, the iron rich minerals align themselves with the Earth’s magnetic field and point north. However, some rocks have magnetic orientations that point south. HMMMMMMM……. (What does this mean?????)

Time to date some rocks!!! All rocks with magnetic fields pointing north dated to the same time period, all those with magnetic fields pointing south dated from the same time period. So what??? So, Earth’s magnetic field must have reversed itself at times during Earth’s history

The same alternating pattern of reversed magnetism was also found in the rocks on the ocean floor. This same pattern is found on both sides of the Mid-Atlantic Ridge. Seafloor spreading is the “force” behind Continental Drift

Theory of Plate Tectonics: All of this evidence for Continental Drift and Seafloor Spreading gave rise to a new, more encompassing theory Describes continental movement AND gives an explanation for why and how the continents move

So….how does this work? There are 2 types of Earth crust Oceanic Crust Continental Crust Oceanic crust + Continental Crust + the Mantle = the LITHOSPHERE

Lithosphere Aesthenosphere = solid rock that slowly flows (like putty) when under pressure.

The Lithosphere is broken into separate plates that ride on the Asthenosphere like blocks of wood floating on water. The continents and oceans are along for the ride

Lithospheric Plate Boundaries 1) Divergent Boundaries = two plates moving away from each other, allows the asthenosphere to flow upward and fill in the open space. Most of these are found on the ocean floor Create Rift Valleys Example = Red Sea formed by the separation of the African and Arabian Plates

2) Convergent Boundaries = the direct collision of one plate with another. There are 3 types of Convergent Boundaries. 2A) Subduction = when oceanic crust collides with continental crust. Oceanic crust is subducted (or forced under) the less dense continental crust. Forms ocean trenches

2B) A convergent boundary that occurs when two continental plates collide = serious mountain building. Example = the Himalayas

2C) A convergent boundary that occurs when oceanic crust collides with oceanic crust. A deep ocean trench forms when one of the plates is subducted, the water causes mantle rock to melt, which rises to the surface to form volcanic islands called an Island Arc.

3) Third type of Convergent Boundary is called a Transform Boundary. Forms when two plates are grinding past each other. Does not happen smoothly, but in sudden spurts of movement separated by long periods of inactivity. Example = San Andreas Fault in California

The whole system of plate movement is driven by Convection Currents inside the Earth: hot upwellings of Asthenosphere caused by the sinking of lithospheric plates