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Chapter 3

Chapter 3.docx

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Steve Joordens

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Chapter 3: Plate Tectonics What are the major features of earth?  7 continents make up most of most of the surface.  Features of the ocean include: deep trenches, submarine mountain ranges.  Part of the seafloor have mountains, the largest one form islands (Hawaii)  Island and seamount chains (seafloor mountains): islands and seamounts that are in long belts  Much of the ocean floor is moderately very deep and has fairly smooth surface-called abysall plains.  some continents continue outwards from the shoreline under shallow sea water forming submerged benches (narrow flat land) called continental shelves  All continents contain large interior regions with topography. Some continents have flat coastal plain while some have mountain around the edges  Deep mountain trenches make up the deeper parts of the ocean. Some ocean trenches follow the edge of the continents, while others are isolated. (located near pacific ocean)  Island arcs, curving chains, cross the seafloor. Most of them are very reactive  Mid-ocean trenches and there fracture zones encircle much of the globe  Ocean plateus: ocean containing several broad, evaluated regions 3.2 Why do Some Continents Have Matching Shapes  Alfred Wegener observed the fit of these continents and the hypothesis that he created was the: continental drift  He argued that the continental were once together but later drifted apart Were the continents once joined together?  The southern continents were interpreted to have fit together  If the continents were once together, this lead to the hypothesis that there were similar rocks and geological structures. They found similarities in rocks in southern Australia with the rocks in the Antarctica  Gondwana: the hypothetical combination of the southern continents into a single large continent Is the Distribution of Fossils consistent with Continental Drift?  Another evidence supporting this theory was the correspondence of fossil of plants and land animals on continents that were separated by huge distances  The dinosaur, Mesosarus, this animal found not swim in oceans and is currently separated by continents  The distribution of plants and animal fossils is consistent with the idea that the continents were once joined. This explains why identical plants and animal fossil are found on different continents.  The plants and animals were originally on a single huge continent and then later split into smaller continents.  Fossil data also incudes that Antarctica was once farther north and away from the South pole. 3.2 How Did Continential Drift Explain Glacial Deposits in Unusual Places?  Geologists found that ancient glaciers had once covered places that are now close to the equator  South Africa has a policed scratch surface identical to the bases of modern glaciers  Sedimentary rocks in South Africa similar to those near glaciers  The scratch mark on the surface tells that the glaciers moved across land as they gouged the bedrock  The direction of glacial movement inferred from the scratch marks made it seem that it came from the oceans-however based on the hypothesis there was a polar ice cap centered over Africa and Antarctica. Old and New Ideas about Continental Drift:  Geologist working on the southern hemisphere found similarities in rock fossils and geological structure. However the geologists in northern hemisphere were more skeptical.  the weakness of the hypothesis was: he couldn’t explained how or why the continents moved.  He believed that the continents plowed through or over the oceanic crust. However continental crust is not strong enough t survive the forces needed to move a large mass across such a great distance while pushing aside oceanic crust  By World War 2 there was new technology and that gave rise to magnetic data. This data showed that ocean floor had long submarine mountain belts (e.g. mid-Atlantic ridge).  Hess and Dietz proposed that oceanic crust was spreading at underwater mountain belts carrying the continents apart. This process was called seafloor spreading 3.3 Where do Earthquakes and Volcanos Occur?  the distribution of earthquakes and volcanoes are not random but instead define clear patterns and show a close association with mountain belts and other regional features  earthquakes are mostly concentrated in discrete belts Where do Most Earthquakes Occur?  most earthquakes in the ocean floor along the winding crest of mid-ocean ridges  earthquakes are sparse in some continental interiors but are abundant in others  large areas of sea floor, especially the abyssal plains have few earthquakes  some continental edges experience earthquakes  ocean trenches and associated island arc have numerous earthquakes Which areas have volcanoes?  Volcanoes are widely spread but commonly occur in belts  Some volcanoes occur in the centre of oceans  Volcanoes occur along the western edge of the pacific ocean, extending from north of Australia through the Philippines and Japan  Volcanic eruptions occur beneath the ocean  Some volcanoes form in the middle of the continents 3.4 What Causes Tectonic Activity to Occur in Belts: What do earthquake and volcanic activity tell us about the lithosphere?  Earthquake, volcanoes and other process that deform the crust and mantel are called tectonic activity/tectonics  Earths strong outer layer, lithosphere, is broken into a dozen or so fairly dozen pieces called tectonic plates  Some earthquakes occur in the middle of plates, becomes some plates are weaker than others. Forces can be transmitted through the strong parts, causing the weaker party so break generating an earthquake in the middle. How do plates move relative to one other?  Plate boundaries have tectonic activity because plates are moving relative to one another  Divergent boundary: two plates move apart relative to one another-magma fills in the space between the plates  Convergent boundary: two plates move toward one another-one late slide under the other  Transform boundary: two plate move horizontally past one another. 3.5 What Happens at Divergent Boundaries?  At mid-ocean ridges, Earth’s tectonic plates diverge.  Ridges are the sites of many small earthquakes and volcanism  On the continents, divergent motion can spit a continent into two pieces, forming a ocean basin What Happens at Mid-Ocean Ridges? 1. A trough or rift runs along the mid-ocean ridges. The rift is formed because large crust slips down as spreading occurs. The movement causes earthquakes 2. As the plates move apart the mantle in the asthenosphere rises towards the surface along with magma. The magma chambers beneath the rift eventually become a part of the oceanic lithosphere 3. Much of the magma solidifies, but some of it erupts into the seafloor. The eruptions can create new ocean crust 4. Mid ocean ridges are elevated above the seafloor because they consist of hotter, less dense materials (magma). Also because the underlying lithosphere is thinner beneath the ridges. The lower denser materials and thin lithospheres allow the plate to float higher than the asthenosphere. The elevation of seafloor decreases as the rocks cool and contract. What happens when divergence splits a continent apart?  Most divergent plate boundaries are beneath oceans, but a divergent boundary may also form within a continent. This process is called continental drifting, creating a continental rift. 1. Initial stage: continental rifting includes broad uplift of the land surface as mantle magma ascends and pushes up the crust. The magma heats and melts the continental crust- produces more magma. Heating causes expansion-results in further uplift 2. Stretching of the crust causes large crustal block to drop down along fault-forms continental rift. The down-dropped blocks may form basins that and trap sediment and water. Deep rifting causes solid mantel material to flow upward and then melt. The crust begins to thin; as a result the elevation of the central rift begins to lower in elevation. 3. If rifting continues, the continent splits
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