How did Block Island form?

How is it changing now?

What will it look like in the future?

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)

Background Information

Wind Erosion

• Wind: erodes land by carrying away rock particles

• mostly smaller particles: sand and rock dust (silt and clay) are carried by wind

Wind Deposition

• deposition by wind: most common – dunes (mounds of wind-blown sand)

• gentler slope of a dune – faces wind

Slumping: erosion from groundwater and gravity

Large block falls off a cliff or bluffforms- sea cliffs/bluffs

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)

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

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??

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??

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:

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

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

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

Glacial Deposits

• glacial till: unsorted glacial materials

• stratified drift: sorted and deposited in layers by meltwater

Glacial erratic

Features from glacial deposition:

• Glacial moraines: landforms made from glacial till

• terminal (end) moraine: deposited at the end

More features from glacial deposition:• moraine

• erratics: large boulders transported by glaciers

More Glacial Deposition features

• drumlins: low tear-shaped mounds of till

• kettles: depressions left from melting ice

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

Your tasks in the field

(gathering evidence for your big 3 questions)

Task 1: create a topographic map of Block Island

Another look

Converting an island into a “topo” map

Hawaii topo

Creating a Contour/Elevation Map

• Use your elevation observations to determine the higher/lower points on BI

• Draw contour lines onto the map

Try a sample map

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

Soil sizes

Soil horizons, or strata

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

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

Task 3: create a beach profile

Beach profiling provides clues as to:• how the beach formed• how is it changing now

General beach anatomy

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).

More of the same

Task 4: Measuring wind speed and direction

• Wind speed: use an anemometer

• Wind direction: use the compass

Good luck geologists!

Review: Fill in the characteristics for each type of change

EROSION DEPOSITION