Coastal Processes and Glaciers
Coastal Processes and Glaciers GEOL 101 001
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GEOL 101 001
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This 7 page Class Notes was uploaded by Madeline Wilson on Sunday April 17, 2016. The Class Notes belongs to GEOL 101 001 at University of South Carolina taught by Dr. Knapp in Spring 2016. Since its upload, it has received 9 views. For similar materials see Introduction to the Earth in Geology at University of South Carolina.
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Date Created: 04/17/16
, Coastlines, Oceans, and Ice The amount of windblown material that wind can carry does not depend on the temperature. Most volcanic activity on the seafloor takes place at mid-ocean ridges. In 1990, 50% of the US population lived within 75 km of the coast. In 2010, 75% of the US population lived within 75 km of the coast. Coastal Processes Wave motion: key to shoreline dynamics Wind waves: depend on wind speed, duration, and distance Wind Waves Are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing and can travel thousands of miles before reaching land. Wind waves range in size from small ripples to huge rogue waves. After the wind ceases to blow, wind waves are called swell. Wind waves in the ocean are called ocean surface waves. Coastal Landscapes are Highly Variable, depending on: Stability of the coastal region (e.g. uplifting, subsiding, stable) Nature of rocks or sediments at the shoreline Long-term changes in sea level Wave energy Tidal energy Variables controlling wave energy: key to shoreline dynamics wind velocity wind duration fetch (area over which the wind blows) Wind forces generate ocean waves! Wave motion is influenced by water depth and shape of the shoreline Wavelength, wave height. Waves travel but the water stays in the same place. Water particles move in circular orbits in the vertical plane. The radii of the orbits decrease gradually with depth below the surface. As waves approach the shore in the surf zone, the orbital motions of the water particles are restricted by the bottom and become elliptical. When the bottom shallows to about one-half the wavelength, the wave slows, its wavelength decreases, and its crest sharpens. The wave enters the zone of wave buildup. As waves approach the shore, they become too steep to support themselves and break in the surf zone, running up the beach in a swash (splash). Coastal Currents Longshore currents (parallel to shore): o caused by waves approaching the shore at an angle Longshore drift: o the transport of sediment (e.g. sand) parallel to the shoreline, mainly in the surf and swash zone Rip currents (perpendicular to shore) Ocean tides are the result of the gravitational attraction of the moon and sun on the ocean. The tides formed by the moon are the lunar tides, and those formed by the sun are the solar tides. The Moon’s gravitational attraction causes two bulges of water on the Earth’s oceans, one on the side nearest the Moon and the other on the side farthest from the Moon. As the Earth rotates, these bulges remain aligned and pass over Earth’s surface forming the high tides. Spring Tides The relative positions of the Earth, Moon, and Sun determine the heights of high tide during the lunar month. Spring tides occur when the solar and lunar tides are in phase. Neap Tides Neap tides occur when the solar and lunar tides are out of phase. These occur on a 28-day cycle. Coastal Erosion Rates in the US 30-50% of all the structures within 500 feet of the present Gulf shoreline will be lost due to erosion in the next 60 years. What Determines Whether a Beach is Eroding or Stable? The sand budget: o The rate at which sand is supplied (input) to the beach, versus the rate at which it is being removed (output). Sea-Level Change Sea-level change due to warming o melting of ice caps o expansion of water in oceans Effects on beaches: net erosion and loss of the beach Major Physiographic Features in the Ocean Continental margin o continental shelf o continental slope o continental rise Abyssal plain Seamounts Mid-ocean ridge o abyssal hills o central rift valley Continental Margins Types of continental margins o passive o active Components of continental margins o shorelines o shelves, slopes, and rises Continental Shelf A broad, flat platform extending from the shoreline to the continental slope Less than 200 m deep It may extend 100’s km offshore It is underlain by continental crust The more shallow areas are affected by waves and tidal currents, and is typically covered with sand and mud. Continental Slope A steeper (~4°) , typically mud-draped slope, marking the edge of the continental shelf Typically dissected by submarine canyons (created by the turbidity currents) Often formed by submarine landslides Tubidity Currents Slumps on slope, some triggered by earthquakes, generate turbidity currents that flow down slope and rise to abyssal plains, where they come to rest. Turbidity currents erode and deposit fine-grained sediments on the continental slope and rise Continental Rise A gently sloping apron of sediment formed by deposition of sands and muds at the base of the continental slope Typically at depths of 2-3 km May include large submarine fans underlain by several kilometers of sediment (like the alluvial fans on land). Abyssal Plain Extends beyond the continental rise typically 4-6 km below sea level It is the flattest surface on the earth May include submerged volcanoes called seamounts. Most of the sediment consists of very fine clay, windblown dust, and the shells of microscopic organisms. Carbonate sediments are rare. Carbonate Compensation Depth The depth below which carbonate tends to dissolve. Only siliceous shells can be found below the CCD. The deep waters are (1) colder, (2) contain more CO2, and (3) are under higher pressure. The deepest part of the ocean is ~10 km deep. Glaciers Glacier – moving body of ice that forms from accumulation and compaction of snow and erodes the land as it moves. ~ 10% of Earth’s land surface is covered by glacial ice In order to form a glacier, need to have more snow fall during the winter than snow melt during the summer In the relatively short geologic time span of the recent ice ages, glaciers carved far more topography than rivers and wind. Glacial erosion creates enormous amounts of debris. Ice transports huge tonnages of sediments that are either deposited or carried away by meltwater streams. Glacial Erosion and Sedimentation Affect: Water discharge and sediment loads of rivers Quantity of sediment delivered to the oceans Erosion and sedimentation in coastal areas Ice as a Rock Ice is much less dense than most other rocks and it melts at a much lower temperature. Like igneous rocks, it is a frozen liquid. Like sedimentary rocks, it is deposited in layers at the surface of the Earth. Like metamorphic rocks, it is transformed by recrystallization under pressure in glacial ice. Glacier Types Two main types of glaciers: o Valley (alpine): form in the cold heights of the mountains, where snow accumulates usually in preexisting valleys, and they flow down the bedrock valleys. o Continental: extremely slow moving, thick sheet of ice that covers a large part of a continent. Valley (Alpine) Glaciers form in the cold heights of the mountains, where snow accumulates usually in preexisting valleys, and they flow down the bedrock valleys Continental Glaciers Extremely slow-moving thick sheet of ice that covers vast land areas. Today, the world’s largest ice sheets cover much of Greenland and Antarctica. The glacial ice of Greenland and Antarctica is not confined to mountain valleys, but covers virtually the entire land surface. In Greenland, ~80 % of the island’s total land area is covered by ice. 90% of the Antarctica is covered by ice. Glacier Formation Requires Low temperatures to keep snow on the ground year-round: o High altitudes (top of mountains) o High latitudes Large amounts of snow Near the equator, glaciers form only on mountains that are higher than ~5,500 m. In arid climates, glaciers are unlikely to form at all, unless the temperature is so frigid all year round that virtually no snow melts and all is preserved, like in Antarctica. Transformation of Snow to Glacial Ice With Burial Icebergs Calving from Glaciers – Ablation Ablation: all processes that remove snow, ice, or water from a glacier or snowfield Ablation Varies with: Ice melting Iceberg calving Sublimation (transformation of solid to gas) Wind erosion Glacial Ice Budget Depends on the: o the rate of glacial accumulation at the upper end of the glacier versus o the rate of glacial wastage (loss) at the lower end of the glacier The difference between accumulation and ablation, called the glacial budget, results in the growth or shrinkage of a glacier. Accumulation of a glacier takes place mainly by snowfall over the colder upper regions. Ablation takes place mainly in the warmer lower regions by sublimation, melting, or iceberg calving. Principle of Isostasy If all Earth’s ice shelves were to break off into the ocean over the next few years, with no additional complications, sea level would not change. Glacial Erosion: Striations Striations are grooves created by rocks scratching against bedrock at the base of a glacier. Are evidence of the direction of ice movement. Ice is a far more efficient agent of erosion than water or wind. Glacial Erosion: U-shaped Glaciated Valley: steep upper walls grading to a nearly flat floor Glacial Erosion: Cirque: an amphitheater-shaped hollow formed at the head of a valley glacier Glacial Erosion: Fjord: a glacial valley flooded with seawater Glacial Deposits: Moraines: Accumulation of rocky, sandy, and clayey material carried by the ice and deposited Permafrost: Permafrost covers ~25% of the Earth’s total land area. Permafrost is defined solely by temperature. Any rock or soil remaining at or below 0 deg C for more years is permafrost. Permafrost can be as thick as 500 m. The periodicity of glacial and integral cycles is best explained by cyclic variations in solar energy, governed by periodic variations in the Earth’s: Eccentricity of Earth’s orbit around sun (100,000 years) Tilt of Earth’s rotation axis (41,000 years) Precession or “wobble” of the axis of rotation; 23,000 years) The most recent Ice Age was in the Pleistocene (~18,000), ended ~10,000 years ago Snowball Earth Hypothesis There is evidence that the Late Precambrian glaciation (750-600 million years ago) extended to the equatorial regions. Some scientists suggest the entire Earth was covered in glacial ice, and melted only when volcanic eruptions raised the CO2 content in the atmosphere, facilitating Global Warming.
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