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Transcript
3.4.2 - Continental Puzzle http://www.imaginiquebengals.com/condriplatec.pdf Background: The activity you just completed showed you that the continents actually fit together like puzzle pieces. But that isn’t even close to all of the evidence that is hidden within the continents that scientists began to recognize in the early-mid 1900s. Let’s look through some of those. Fit of the continents: In the 1960s, it was recognized that, especially if you take the continental slope into account, the continents fit together incredibly well. Just look at South America and Africa to the right. Fossil Evidence across continents: Alfred Wegener could not propose an idea like plate tectonics without having more evidence than simply continent shape. To help his cause, Wegener also discovered that there were fossils located directly opposite each other on widely separated continents. For example, referencing the South America and Africa picture to the left, the fossils of Mesosaurus can be found on both sides of the Atlantic Ocean. South Carolina actually has evidence of this, as the upstate has a band of trilobite fossils that are not natural to North America, but instead reigned on African soil. This indicates that parts of South Carolinian crust actually originated as African continental crust. Similarly, marsupial (kangaroos, wallabies, koala, possums, opossums, wombats, etc.) fossils are found in Australia, Antarctica, South America, and North America. If you look at the image to the right below, this is Earth 80 million years ago when marsupial mammals first appeared. Notice that Australia, Antarctica, and South America are all connected at this point, allowing marsupials to populate all three continents before they separated. We also continue to find fossils throughout the world of plants or animals that could not have existed in that environment (tropical plants in Antarctica, sea fossils in the middle of the desert, etc…) Rock Type and Structural Similarities: We find similar rock types on continents on opposite sides of the Atlantic Ocean. These rocks in both the Appalachian Mountains, the British Isles, and Caledonian Mountains all have similar age, composition, and structure. Comparing the image below on the left, to the image on the right, if North America, Europe, and Africa are reassembled, these mountain ranges form a continuous mountain range. Paleoclimatic Evidence: Glacial till (unsorted sedimentary rock deposited by glacier movement) of the same age is form in southern Africa, South America, India, and Australia. This would be nearly impossible to explain currently without the knowledge that at one point nearly 200 million years ago, the supercontinent Pangae existed, which placed these 5 current day continents next to each other. The image to the right should give you an idea of their placement, and should give a prelude to Australia, Antarctica, and South America being connected in the marsupial migration issue referenced in the fossil section. Paleomagnetism: We saw during the Mid-Atlantic Ridge seafloor spreading lab, that when igneous rock is created, any magnetic minerals in that rock crystalize and align themselves within the Earth’s current magnetic field. This is important because if the continents had not moved, igneous rocks on every continent should point towards the current magnetic North Pole. However, this is not the case. When you compare current day North America and Europe, they are not pointing in the same direction. If you compare the two images below, you will notice as you rotate North America counter-clock wise as you move from Figure A to Figure B, the green and red lines that represent magnetic orientation begin to line up. The latitude that the rock is created at also influences magnetic orientation of igneous rock. As you can see in the image below, because of the magnetic waves surrounding the Earth, the magnetic crystals formed would orient themselves similar to the dip needles in the image and make latitude easy to figure out. Pangea: The above evidence all points towards all of the land on Earth being combined into a supercontinent called Pangaea that formed about 270 million years ago and started to break up about 200 million years ago. It was not the first supercontinent (there have been at least 6 proposed supercontinents and upwards up 15 previously) and it will likely not be the last. The Americas are separating from Africa, The Americas are traveling towards Eurasia, Oceania (including India) are driving north, and Africa is still pushing into Eurasia. Check out this website real quick for an animation of the entire process of Pangea beaking up: http://geologyguy.com/teaching/iac/animations/pangaea.htm Review Questions 1. List, and briefly explain, the 5 major pieces of evidence listed above that exist that indicate that Earth’s plates are moving and were once combined into . 2. When did Pangaea form and when did it break up? 3. Was Pangaea the only supercontinent that has existed? If not, how many have existed in the past? 4. For the animation linked above, how long ago (in millions of years) did the animation start? 5. How frequently(every how many millions of years) did it pause for a moment?