The Basics The lithosphere is made up of numerous plates (14 major, 38 minor) These plates float atop the asthenosphere, where they can move (albeit very slowly) Plate tectonics describes the formation, movement and interactions between the plates. Early Evidence of Plate Tectonics The eastern coast of North and South America seems to fit with the western coast of Africa and Europe. This, by itself, might be a coincidence, but the similar fossil records and rock formations on at various places on each coast suggest that it is more than coincidence. A theory was proposed where these continents have been slowly moving apart. This was called continental drift. Further evidence exists in the locations of recorded earthquakes and volcanoes. 1
Current Evidence of Plate Tectonics The simplest way to prove plate tectonics is to measure the position of the plates and show that they have moved in a predictable way from where they used to be. With the advent of satellites, like GPS, precise data now shows exactly how parts of Earth are moving, confirming the plate tectonic theory. Why do the plates move? Between certain plates, magma is pushed through a crack in the lithosphere called a ridge. The magma cools and becomes rock, which expand, and push both plates away from each other. 2
Plate Boundaries There are 3 types of boundaries between plates: divergent (described last slide), convergent (plates move towards each other), and transform (plates move past each other). Transform Boundaries Unlike divergent boundaries where crust is made, and convergent boundaries where crust is destroyed, transform boundaries do not result in a change in the amount of crust. The most famous transform boundary occurs in California along the San Andreas fault. Large amounts of friction cause the fault to stick, then when pressure builds up, it slips, causing an earthquake along with plate movement. Plate Boundaries Convergent Boundaries How plates converge depends on where they converge, since continental plates (land) are less dense than oceanic plates. In all cases, the more dense plate will sink, or be pushed under, the less dense plate in a process called subduction. This occurs at a subduction zone. The subducted material re enters the mantle, where it melts and the rock cycle continues. This process puts strain on the lithosphere, causing cracks in which magma can push through. The result is a large number of volcanoes. 3
Evidence Supporting Plate Tectonics The recent development of satellite technologies (i.e., GPS) has increased the accuracy and confirmed many theories. However, the main theories for Tectonics existed prior to this. So what information did scientists use to make their predictions and theories? Evidence: Continental Outlines From observations, the coasts of South America and Africa look like they match very closely. This is also true for a number of other continents. Also, by looking at where certain types and ages of fossils are found, scientists noticed another pattern that supported the idea that these continents were once connected. 4
Evidence: Oceanic Ridge Around the time of WWII, researchers discovered the Mid Atlantic Ridge, and found that the seabed was newer near the ridge, and that overall the crust was thinner. They concluded (correctly) that the crust was being continuously produced. If there was new crust being made, and the size of the Earth wasn't increasing (they checked!), it also meant that crust was being destroyed. This process meant there had to be movement in the Earth's crust. Magnetic 'striping' in the seabed gave further evidence to the continual creation of new crust over a very long timeline. Apr 16 8:59 AM 5