Plate Tectonics Drifting continents:Theory of Continental Drift
Alfred Wegener suggested that all continents were joined together at some time in the past. This is called the hypothesis of “continental drift”. According to continental drift, continents have moved slowly to their current locations. He called this large landmass as Pangaea (“all land”)
Landforms• Mountains in South Africa line up with mountains in Argentina• Coal mines line up on different continents
Mountain Structures Similar rock structures are found on different continents. For example, parts of Appalachian mountains are similar to those found in Greenland and western Europe.
Appalachian Mountainsstrike through theeastern United States andCanada then end at thesea off of Newfoundland.The CaledonidesMountains are found ineastern Greenland,Ireland, Great Britainand Norway.These two mountainranges are the same agewith the same rock typesand structures.
Fossils Besides the puzzlelike fit of continents, fossils provided support for continental drift. For example: fossils of the reptile Mesosaurus have been found in South America and Africa.
Another fossil that supports the continental drift is Glossopteris which has been found in Africa, Australia, India, South America, and Antarctica.
Climate Clues Fossils of tropical plants were found in Spitsbergen, an ice covered island in the Arctic Ocean (north of Norway).
Glacier Activity Glacial deposits and rock surfaces scoured and polished by glaciers are found in South America, Africa, India and Australia.
Why was it not accepted? Wegener could not provide an adequate explanation for the force that pushes or pulls the continents.
Although Wegener provided evidence to support hishypothesis, he couldn’t answer two questions:1. What was causing the continents to move? Wegener said that this force might be the rotation of Earth, however, physicists were able to show that this force was not great enough to move continents.
2. How were the continents moving? Wegener proposed that the continents were plowing through a stationary ocean floor. But, his peers argued that continents could not push through the ocean floor without fracturing, because crustal rock is too brittle. And no evidence of fracturing had been found.
And the rest of the story….Undaunted by rejection, Wegener devoted the restof his life to doggedly pursuing additionalevidence to defend his theory. He froze to death in1930 during an expedition crossing the Greenlandice cap, but the controversy he spawned raged on.However, after his death, new evidence fromocean floor exploration and other studies rekindledinterest in Wegeners theory, ultimately leading tothe development of the theory of plate tectonics.http://bumileluhur.blogspot.com/2011/01/this-dyniamic-earth-story-of-plate.html
The controversy continues!Plate tectonics has proven to be as important to the earthsciences as the discovery of the structure of the atom was tophysics and chemistry. Even though the theory of platetectonics is now widely accepted by the scientificcommunity, aspects of the theory are still being debatedtoday. Ironically, one of the chief outstanding questions is theone Wegener failed to resolve: What is the nature of theforces propelling the plates? Scientists also debate how platetectonics may have operated (if at all) earlier in the Earthshistory and whether similar processes operate, or have everoperated, on other planets in our solar system. Will you be the one to solve the mystery?
Mapping TheOcean Floor Scientists began using sound waves on moving ships to map large areas of ocean floor in detail. Sound waves echo off the ocean bottom – the longer the sound waves take to return to the ship, the deeper the water is.
Mapping the Mid-Ocean Ridge Also known as the Mid- Atlantic Ridge Longest mountain chain in the world Divided by a trench
Most is hundreds of meters under water, but part reaches above the ocean surface. It is almost twice as deep as the Grand Canyon. Iceland is part of the mid-ocean ridge
In the early 1960s, Harry Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying seafloor away from the ridge in both directions. This theory is known “seafloor spreading”.
In 1968, a research ship, Glomar Challenger, began gathering rock samples from mid-ocean ridges. They made a remarkable discovery as they studied the ages of rock samples. The younger rocks are closer to the mid-ocean ridges and older rocks are farther from the ridges.
Evidence of Sea-Floor Spreading Molten Material Magnetic Stripes Drilling Samples
Magnetic Clues Iron-bearing minerals, such as magnetite, that are found in the rocks of the seafloor can record Earth’s magnetic field direction when they form. The magnetic alignment in the rocks reverses back and forth over time in strips parallel to the mid-ocean ridges.
Subduction of Deep-Ocean Trenches Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.
Subduction and Earth’s Ocean Subduction and sea-floor spreading can change the size and shape of the oceans. The ocean floor is renewed about every 200 million years.
The Pacific Ocean is shrinking; a trench swallows more oceanic crust than the mid-ocean ridge can produce. The Atlantic Ocean is expanding. There are only a few trenches in the Atlantic. The continental crust is attached to the ocean floor, so as the Atlantic expands and moves, the continents move with it.
Section 3: theory of Plate tectonicS In the 1960s, scientists developed a new theory that combined continental drift and seafloor spreading . According to the theory of plate tectonics, Earth’s crust and part of upper mantle are broken into sections. These sections are called plates, move on a plastic-like layer of the mantle.
Where did the theory come from? J. Tuzo Wilson combined Earth’s plates, continental drift, and sea-floor spreading into a single theory---PLATE TECTONICS
A Theory of Plate Tectonics Plate tectonics states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle, and it explains the formation, movement, and subduction of Earth’s plates.
Basically…. The plates on the lithosphere float on the asthenosphere, convection currents rise in the asthensophere and spread out beneath the lithosphere. http://youtu.be/ryrXAGY1dmE
Earth’s crust and a part of upper mantle combined are the lithosphere. (100km=62 mile thick) The plastic-like layer below the lithosphere is called asthenosphere. The rigid plates of the lithosphere float and move around on the asthenosphere.
Causes of Plate Tectonics Convection currents cause the movements of plates Hot, less dense liquid or gas is forced upward, as it reaches the surface, it cools down and sinks back down. This entire cycle of heating, rising, cooling and sinking is called a convection current.
No plate can budge without affecting the other plates surrounding it!
Plate Boundaries When plates move, they can interact in several ways. Three different moving types of plates are: Plates moving apart (Divergent) Plates moving together (Convergent) Plates slide past each other (Transform)
Faults break in Earth’s crust where rocks have slipped past each other. Plate boundaries are where the edges of different pieces of the lithosphere meet.
Transform Boundary Two plates slip past each other, moving in opposite directions. (Do not push or pull) Crust is neither created nor destroyed near a transform boundary. Earthquakes occur frequently along these boundaries.
Divergent Boundary Two plates move apart or diverge. Most occur at the mid-ocean ridge; they can also occur on land (poor Africa). A rift valley is a deep valley that forms along the divergent valley. The Great Rift Valley in Africa marks a deep crack in the African continent that runs for about 3,000 km. When divergent boundaries develop on land, two of Earth’s plates pull apart.
Convergent Boundary Two plates come together or collide (converge) Results: earthquakes, trenches, volcanoes, mountains When two plates collide the denser of the two plates will sink or slide beneath the other.
The continents’ slow dance The plates move at slow rates about 1 to 10 cm per year.http://youtu.be/hSdlQ8x7cuk
To prevent confusion Not all volcanoes occur at plate boundaries. For example, the Hawaiian islands formed from a hot spot volcano. As the Pacific plate moves across a fixed hot spot (mantle plume), new volcanoes (and islands) form.
Summary Theories: Continental Drift Theory of Plate Theory TectonicsForces of Plate Cycle of Ridge Push/Gravity Slab PullMovement: Convection ****** CurrentsTypes of Plate Divergent Convergent TransformMovement/Interactions:(Boundary)Processes or Sea-floor Subduction EarthquakesTypes of Spreading (Volcanoes) andMovement Mountains(Results):Stress: Tension Compression ShearingFaults: Normal Reverse Strike Slip