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How did Block Island form?
How is it changing now?
What will it look like in the future?
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Block Island:a product of Erosion and Deposition
• weathering: the change/breakdown of rock
• erosion: movement of weathered materials
• agents of erosion: gravity, wind, glaciers, and water (ocean currents and waves, streams, groundwater)
• deposition: the leaving behind of materials carried by erosion
• agents of deposition: gravity, wind, glaciers and water (same as for erosion)
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Background Information
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Wind Erosion
• Wind: erodes land by carrying away rock particles
• mostly smaller particles: sand and rock dust (silt and clay) are carried by wind
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Wind Deposition
• deposition by wind: most common – dunes (mounds of wind-blown sand)
• gentler slope of a dune – faces wind
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Slumping: erosion from groundwater and gravity
Large block falls off a cliff or bluffforms- sea cliffs/bluffs
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Erosion by ocean waves
• physical weathering: force of the water weathers and erodes the shoreline affecting beaches
• Big waves erode, small waves deposit (ex: Narragansett)
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Ocean Wave Deposition• beaches: deposits of sand or larger particles on ocean
(or lake) front– beach changes seasonally
• berm: mini-bluff on a beach• sand bar: long underwater ridge of sand carried away
from beach
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Wave anatomy
• Wavelength: distance between 2 crests or 2 troughs
• Wave height (2 X amplitude): height from trough to crest
• How could you determine these??
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Wave characteristics• Wave period: the
time is takes for a wave to pass a fixed point
• Wave frequency: the number of waves that pass by in a given time.
• How could you determine these??
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Wave characteristics 2
• Wave speed: the distance traveled by a given point on the wave (such as a crest) in a given interval of time. In equation form:
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Longshore Currents• general movement of
sand along beach: in same direction that waves hit shore
• longshore current: movement of water and sediment parallel to, and near shoreline
• animation: http://www.learnnc.org/lp/multimedia/14706
• Real video: http://www.youtube.com/watch?v=l70ioFTD6TY&safe=active&safety_mode=true
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Features from longshore current deposition:
• Headland: land surrounded by water on three sides
• spit: long narrow deposit of sand connected at one end of shore
• tombolos: ridge of sand deposits connecting islands to mainland
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Glacial Erosion
• Glaciers are “rivers of ice” slowly flowing, moving sediments and changing the surface and shape of the ground beneath it
• Last glacial ice age in NE ended about 18,000 years ago
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Glacial Deposits
• glacial till: unsorted glacial materials
• stratified drift: sorted and deposited in layers by meltwater
Glacial erratic
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Features from glacial deposition:
• Glacial moraines: landforms made from glacial till
• terminal (end) moraine: deposited at the end
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More features from glacial deposition:• moraine
• erratics: large boulders transported by glaciers
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More Glacial Deposition features
• drumlins: low tear-shaped mounds of till
• kettles: depressions left from melting ice
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Stratigraphy
• Stratigraphy is the study of strata, or layers in the ground
• Stratigraphy can tell us about how the soil was deposited, they are clues to the land’s origin
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Your tasks in the field
(gathering evidence for your big 3 questions)
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Task 1: create a topographic map of Block Island
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Another look
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Converting an island into a “topo” map
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Hawaii topo
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Creating a Contour/Elevation Map
• Use your elevation observations to determine the higher/lower points on BI
• Draw contour lines onto the map
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Try a sample map
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Task 2: Sieving soil, to tell particle size
• You will collect soil samples to sift with “sieves”
• These will separate the soil into different sized particles – gravel, fine gravel, coarse sand, fine sand and silt and clay
• Sieving Purpose: this tells how the soil got there
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Soil sizes
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Soil horizons, or strata
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Sand, different sizes deposit by different ways
• How is sand classified? If you classify sand by size, you look at the diameter of each sand particle. Very coarse sand, like you might see in a sand box, has the largest particles. The diagram below shows the actual size of sand particles from 0.5 to 2 mm in diameter.
• very fine 0.05 - 0.01mm • fine 0.1 - 0.25mm • medium 0.25 - 0.5 mm • coarse 0.5 - 1 mm • very coarse 1 - 2 mm
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Conclusions from sieving and soil profiling:
• The bigger the sand particle size, the steeper the beach typically is:– Steep, big particle beaches are formed from
strong deposition forces and erosion• This makes them less stable
– Flat, small particle beaches are formed from gentle deposition forces, and are more stable
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Task 3: create a beach profile
Beach profiling provides clues as to:• how the beach formed• how is it changing now
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General beach anatomy
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How does the beach change over time?
• The shape of the beach is in continual change due to the weather and tides.
• In calmer weather with regular tides, sand is gradually deposited onto the beach from sand bars out in the ocean, dunes and berms may form.
• In stormy weather, the beach erodes and sand is brought from the beach to sand bars, which run parallel to the shore (provides better surfing).
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More of the same
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Task 4: Measuring wind speed and direction
• Wind speed: use an anemometer
• Wind direction: use the compass
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Good luck geologists!
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Review: Fill in the characteristics for each type of change
EROSION DEPOSITION