GEOL-1010 Lecture Notes - Lecture 16: Longshore Drift, Refraction, Continental Crust

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5 Feb 2018

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Lecture 16: Coastlines
Geology in the News
River in Alaska has changed course and flowing in reverse due to glacial melting
Drain backwards out of its own channel and form a different pathway
We’ve only seen evidence of this happening in prehistoric times
This is the first time in modern history
Why do We Care?
About 53% (>3.5 billion) of the world population lives within 120 miles of the coastline
Part One: Coastal Processes and Features
Remember- almost everything we’re talking about today is still on continental crust
o The ocean doesn’t begin right on the beach
o The continental crust stretches more offshore before it becomes oceanic
Several forces acting on these environments
o They’re very dynamic environments that can change very rapidly
o Don’t worry about memorizing the pic on this slide: it represents the fact that you
have a lot of different factors acting on coastlines
Coastal Processes
Tides- sea level rising and lowering twice a day. Process repeats over and over
Tidal Flats- some areas are exposed and then not exposed throughout the day. These are
tidal flats (I think maybe the area exposed idk)
High tide-sides facing towards and away from the moon
Low Tide-the in-between sides
Tide Height
How much will the tide rise and fall
o How connected is it to the rest of the ocean
Bay/ lagoon vs right on the beach
if it is well connected to the ocean the tidal range is very small
Hawaii has a tidal range of 1-2 feet
o Since it is well connected to the Ocean
Bay of Fundy has a tidal range of ca 40 ft!
Wavelength-distance between two adjacent waves
o main term to focus on
o How far apart will the wave lengths be
Waves change as they approach shore
o First change: They get shorter and shorter as they get closer to shore, they start to
bunch up
o Second change: they slow down as they get closer to shore
As you get into shallower water the wave drags on the seafloor, causing
greater amount of friction
About half the wave length is the key depth where you really start to see the
waves slow down
o Third change: the waves are getting taller when they get closer to shore
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Waves getting closer together, the extra water has to go someplace so the
waves start getting taller
o Fourth change: wave refraction
Starts to change the angle of approach as it gets closer to shore
Farther out- seems like they’ll hit at an angle but as you get closer waves
start refracting and they hit the shore pretty much head on
Some waves travel faster than others, the waves on the outside have to move
faster and they catch up with the slower moving waves in shallower water.
Similar to meanders
Longshore current- a current of water that runs along the shore
It doesn’t follow a straight path, it follows a zig zag pattern
Wave pushes, then it retreats back, another wave pushes it forward, then it retreats back,
Important because it moves a lot of sediment on the shore for us
Longshore drift-movement of the sediment that is being moved by the current
Shoreline Features
Shoreline features depend on
o Tectonics- are you close to a boundary or far away from one
o Rock type- some rocks are harder to break down than others
o Sea level fluctuations
o Storm size/ strength- what kind of storms are you dealing with? Lots of hurricanes?
Types of Coastlines
Emergent Coastlines- coastlines that are being uplifted
o Move in elevation quickly coming away from the coat (think California)
o Stacks- very small islands that have very steep sides to them. Usually represent the
uppermost parts of things that are still mostly below water
o Terraces- big flat stair step like surfaces. Represent older beaches. Where the beach
used to be before the area got uplifted. Show how many uplifts you’ve had over
time. Give you history of the emergent pattern of that coast
Submergent Coastlines- coastlines are flooded. They’re sinking for some reason so the
coastlines are flooded
o Lots of inlets and straights, before the area was submerged they would’ve been hills
and valleys
o Lots of sediment deposition that causes subsidence
o Long, wide beaches and coastal planes
o Spit- narrow body of sand, deposit all of this sediment and it piles up.
Long skinny bodies of sand that are still attached to the mainland
o Barrier Islands- same idea but not connected to the mainland anymore.
Size can change a bit which can be problematic because people love to build
on them
Part 2: Offshore Features
Continental margin- section of continent that is flooded, but it is still technically part of the
o Depends on how close you are to a plate boundary
Types of Continental Margins
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