The plates have been named

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1 Geology 12 February 10, 2015 Uniformitarianism This is the belief that the earth s history can be understood by assuming processes that are going on today (volcanism, earthquakes, etc) have always shaped the planet and will continue to shape the planet What is Plate Tectonics? Tectonicsis the study of the origin and organization of the structural (shapes) of the surface of the Earth. The plates have been named The basic idea of plate tectonics is that the Earth s surface is divided into a few large, thick plates that move slowly and change in size (moving at ~ 2 to 10 cm/year) It is popular in Geology because it explains many of the features we see on Earth (remember Uniformitarianism?) Plate Tectonics: It is the combination of two theories (called a unified theory): Continental drift and ocean floor spreading Continental Driftis the idea that continents move freely over the Earth s surface, changing positions relative to one another Sea Floor Spreading is the hypothesisthat the sea floor forms at mid ocean ridges, pushing the sea floor apart at these ridges (underwater mountains) The Early ideas for Plate Tectonics: In the early 1900s Alfred Wegenermade a case for continental drift because when he looked at a world map he notice that the continents looked like they fit together, like the pieces of a puzzle 1

2 The Early ideas for Plate Tectonics: Wegener also noticed that South America, Africa, India, Antarctica, and Australia had almost identical Paleozoic (250 to 500 Million years old)rocks and fossils Evidence against Continental Drift The idea was widely unpopularwhen proposed in the 19 th century It was strongly against the idea that the Earth was only 5700 years old Other scientists came up with suggestions that disproved, or so they thought, Wegener s theory 1. What made the continents move? Wegener proposed that the rotation of the Earth was causing the plates to move Calculations showed that this effect (called centrifugal force) was too small to make a large slab of the continent to move 2. The Oceans? People believed that the oceans could be moving around land, simply revealing different parts of the sea floor This formed a land bridgewhich occurs when the tides (or something similar) cause the ocean floor to be exposed and then allowing land animals to pass across it This could explain why similar fossils were found on different continents: the animals work there Renewed Interest: The Sea Floor With the development of SONAR (to detect submarines during the WWII) we also found out that we can see the featuresof the sea floor. big features The sea floor was anything but featureless 2

3 2/10/2015 Polar Wandering The position of the magnetic poles move from year to year This is further evidence of continental drift Trenches Ridges Hess s theory of Sea Floor Spreading In 1962 Harry Hess proposed that the sea floor may be moving too, calling it sea floor spreading The sea floor was moving like a conveyor belt away from a mid-oceanic ridge(mountain) towards the edges of the ocean disappearing (and being destroyed) beneath a continent (called a subduction zone) He suggested that this process was driven by convection currents Convection Current Convection Current Convection is the rising of hot fluids, cooling and the falling of cold fluids creating a cycle of flow How old is the sea floor? The sea floor can be as old as 200 million years Remember, the Earth is 4.54 billion years old ( years old) Sea floor is constantly being formed at ridges (spreading boundaries) Sea floor is constantly being destroyed at trenches, called subduction zones (Collision boundaries) Subduction is the sliding of one plate beneath another 3

4 What is a plate? A plate is a large, mobile slab of rock that is part of the Earth s surface They are the outer layer of the lithosphere The plate can be 10 to 100 km thick if an oceanic plate The plate can be 125km to 250km thick if a continental plate The plates move on a layer called the asthenosphere which acts like a layer of lubricant making it easier for the rocks to move. This layer is a plastic-like magma Types of plate movement There are three ways plates can move Divergent (spreading) Convergent (colliding) Transform (Sliding past each other) Magnetic Evidence of Plate Movement A Canadiangeologist Lawrence Morley made a very important observation about the sea floor We saw that there were differences in how the magnetic rocks were lined up on the sea floor. Some had their molecules pointing up, some had them pointing down The top of the Earth is now considered the North pole, making the bottom now the South pole As lava cools when it hits the cold ocean the molecules (usually iron) line up with the N-S pole of the Earth When the poles flip (~ years) the alignment in the rocks also flip so we can detect them 4

5 The pattern can be seen using simple magnetic scanning Notice that the right and left sides about the ridgeare the same! 5