GeologyOfDevilsTowerNationalMonument sedimentation

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Ngày đăng: 30/11/2016, 18:07

GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT Sedimentation Pat Frolander John Aloisio Marlon Poole Lee McDowell Devils Tower used to be under water in a shallow sea about 145 to 65 million years ago This was North America during the late Cretaceous time thelivingmoon.com This was the United States Devils Tower USGS pubs.usgs.gov The bottoms of the ocean were covered in deep sand or mud called sediments The Mississippi pouring sediment into the Atlantic Ocean (ESEA esa.int) There are categories of sedimentary rocks: • Clastic • Organic • Chemical Clastic:Big rocks break up into small And become rocks sand, mud or silt that settle to the bottom of an ocean Which become rock again Organic : LIMESTONE Sedimentary rock made from the shells of billions and billions of microscopic ocean organisms called plankton (foraminifera) over millions of years Limestone can also be chemical, when the calcium from the shells becomes rock This is what it looks like today Coal is an Organic rock made from ancient forests From this To this Organic Sedimentary rock Sediments can be Chemical As an ocean dried up, different sediments called salts were left behind Chemical rocks include • Table salt • Gypsum • Some Limestone These are the sedimentary rocks you see today at Devils Tower Which rocks are the oldest? The Spearfish formation The sea disappeared as the Rocky Mountains, with the Black Hills, began to form, about 66 – 45 Million Years Ago (MYA) The land covering Devils Tower was no longer under water This mountain building was called the Laramide Orogeny Devils Tower Rocky Mountains magma Magma Future Devils Tower Future Little Missouri Buttes magma Molten rock that cools and becomes hardened is called Igneous Rock Igneous rock that cools quickly above ground is called Extrusive Igneous rock that cools slowly underground is called Intrusive This magma caused bulges to form, making dome mountains such as Bear Butte near Sturgis, S.D Today we see layers 18, 19 and 20 Where are layers – 17?? The Sedimentary layers 1-17 on top of Devils Tower eroded away, leaving the harder, igneous rock standing Layers 18, 19 and 20 Are now being eroded At the end of the last Ice Age (10,000 years ago) the outwash from the melting glaciers increased the flow of rivers throughout the North American continent This is when geologists assume the majority of the sedimentary rocks in this area were eroded away Today, the Belle Fourche River does not seem large enough to wash away the vast amount of sedimentary rock eroded from above and around the Tower Erosion Have WE changed the amount of erosion at Devils Tower? Yes Humans have dammed the Belle Fourche River, creating Keyhole Reservoir, and preventing the floods that cause the greatest amount of erosion Erosion (cont.) It is important to note the upper portion of the tower appears more weathered with numerous vertical and horizontal cracks Why you think this is? Some Geologists think that when the vertical cracks formed, the horizontal cracks also formed Maybe the top was exposed to weathering longer than the bottom was Devils Tower Today
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