Sand Dune Formation

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Information about Sand Dune Formation

Published on February 13, 2008

Author: Savin


Sand Dune Formation:  Sand Dune Formation By: Cynthia Pancake Great Lakes Sand Dunes:  Great Lakes Sand Dunes Largest assemblage of freshwater coastal dunes in the world Geologically young Only 3,000 to 4,000 years old Sand dunes are divided into different ecological zones:  Sand dunes are divided into different ecological zones Beaches Foredunes Interdunal wetlands or toughs Back Dunes or Dune Forests Critical factors for sand dune creation::  Critical factors for sand dune creation: Source of abundant sand Relatively consistent wind Water level fluctuation dune growth accelerated by high water levels Vegetation to foster sand accumulation traps and and stabilizes sand Factors within the Great Lakes basin:  Factors within the Great Lakes basin Source of Abundant Sand Continental glacier provided the major source for sand and other sediments Glacial drift (a mix of boulders, cobbles, sand, and clay that are deposited after glacier melts) Factors within Great Lakes basin (continued):  Factors within Great Lakes basin (continued) Relatively consistent wind Prevailing winds typically from the southwest resulting in the greatest concentration of large dunes along the eastern and northern shorelines 10 miles/hour winds are enough to put fine sand in motion Sand on the Move!:  Sand on the Move! There are three ways wind can move sand: Suspension Moves the finest grains 1% Impact creep Moves the heaviest grains 4% Saltation Moves intermediate grains 95% Factors within Great Lakes basin (continued):  Factors within Great Lakes basin (continued) Vegetation to foster sand accumulation Pioneer grasses’ fibrous root systems grow rapidly, binding sand together and stabilizing the dune Factors within Great Lakes basin (continued):  Factors within Great Lakes basin (continued) Water level fluctuation Nipissing period 4,000 to 6,000 years ago 25 to 30 feet higher Created vast embayments at the openings of rivers Rivers entering bay carried abundant sand Sand bars migrated shoreward with wind as water receded Pile up! Dune Formation:  Pile up! Dune Formation The sand moves until it meets a slight obstruction, such as a clump of grass, which deflects the wind and allows the sand grains to drop Thus a slight mound or hummock is created Wind moves sand up to the top of the pile until the pile is so steep it collapses under its own weight and falls down the leeward side called the slip face The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable This angles is called the angle of repose and is usually 30-34 degrees Migrating dunes:  Migrating dunes The repeating cycle of sand inching up the windward side to the dune crest and falling down the leeward side allows the dune to inch forward in the direction the wind blows Coastal Dunes of Michigan:  Coastal Dunes of Michigan General Shape of Dunes:  General Shape of Dunes Barchan Dune Formed under “perfect” conditions: Flat landscape One wind direction Limited sand Windward face is gentle, usually not over 15 degrees Slope of the lee side much steeper, sometimes approaching the “angle of repose” Four Distinctive types of dunes in Michigan:  Four Distinctive types of dunes in Michigan Parabolic Perched Linear Transverse Transverse Dunes:  Transverse Dunes Associated with large bays of Glacial Lake Algonquin 11000 years ago Linear to scalloped in shape Formed in shallow bays along the edge of glacier 30 to 60 feet high Linear Dunes:  Linear Dunes Ridges parallel to the shoreline Rarely higher than 30 to 50 feet Most common dune near water Usually formed when wind comes from two directions Parabolic Dunes:  Parabolic Dunes U-shaped Found only in moist environments, where extensive vegetation cover often stabilizes dunes Grasses allow dunes to reach a heights between 250 to 300 feet Parabolic Dunes Continued:  Parabolic Dunes Continued Created by “Blow Outs” Erosion of vegetated sand causing U-shaped depression Natural Causes of Blow outs: Fires Windstorms Plant disease Other causes of Blow Outs: Human traffic Clearing to build Human Traffic:  Human Traffic Before After Parabolic Dunes Continued:  Parabolic Dunes Continued Leeward motion occurs if sand from blowout is deposited on the opposite slope Vegetation holds the “arms” in place Leeward “nose” migrates forward Perched Dunes:  Perched Dunes “perched” atop glacial moraines that have bluffs 90 to 360 feet above present lake level Sand moved by waves and long shore currents was blown up the steep faces of the moraines by shore winds Composed of sand and other loose material We’ll see these at Sleeping Bear Perched Dunes Continued:  Perched Dunes Continued

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