how did block island form? how is it changing now? what will it look like in the future?

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