How Scott and I plan
lectures
Coastal Processes
Geology 115
Ocean properties
• Seawater -- 3.5% by weight various
salts in water, denser than freshwater
• Ocean water represents 97% of all
water on the Earth’s surface
Ocean floor topography
Submarine canyons
Sources of ocean motion
• Waves -- generally wind-driven, maybe
due to sea floor movement in
earthquakes or landslides
• Tides -- driven by the gravitational pull
of the Sun and the Moon
• Currents -- driven by the Earth’s
rotation (Coriolis effect)
Wind-driven waves
Waves break as they shallow
Waves slow as they approach shore
Waves distribute their energy evenly
Sea floor movement waves
Tides
Currents
Coasts
Types of coasts
There are also two types of coasts,
depending on what they are made of –
carbonate versus siliciclastic
Carbonate = made of calcium carbonate (calcite or aragonite)
Siliciclastic = made of silica-based minerals (e.g., quartz)
Carbonate coasts are analyzed by the type of
organisms that create the reef and how the
sediment is generated
Carbonate sediment grains typically grow in grain size
The United States has an awful lot of coast
(= land bordering ocean). When sea level
Dry Tortugas NP
changes, as it does
when glacial and
interglacial times
alternate, the position
of the coast changes.
The area of ocean
floor that is below sea
level now but would
be exposed if sea
level dropped 100
meters is called the
continental shelf
(see Florida).
Dry Tortugas (Spanish tortugas = tortoises) NP (1935)
Clustered with Everglades
NP for administration;
exists as a series of
seven keys (islands)
about seventy miles west
of Key West in Florida.
Fort Jefferson is the
structure on Garden Key;
the key is 16 acres in
extent, the fort takes up
10.
Reefs are submarine structures made by living
organisms
Mostly made of coral
these days, but can be
made by algae, clams and
other organisms
Carbonate platforms may be uplifted and preserved
Guadalupe Mountains NP (Texas) and Carlsbad Caverns NP (New Mexico)
Siliciclastic coast is analyzed
principally by sediment size
Siliciclastic sediment grain size decrease as weathering occurs
Process: Sediment movement
by longshore drift
Results for coasts
• Aggradation (building)
• Erosion
• No change
Some coastal terminology
Process: growing spits
(aggradation)
Boston
Cape Cod National Seashore (1961)
Sand dunes at Cape Cod NS
Cape Cod sediment source: Laurentide Ice Sheet
18,000 years ago
More coastal terminology
Process: making a marine terrace
A series of marine terraces
How do multiple terraces form?
Continual
uplift
Process: making the shape of a bay
• Bays along a coast
with a strong lateral
(longshore) current
have a characteristic
shape.
• Given an erosionresistant headland,
this shape emerges
every time.
Point Reyes National Seashore (1962)
Fault-controlled coast
Log spiral bay
Logarithmic spiral nature of the shape of
Drake’s Bay
All you need is an erosion-resistant headland and
a current!
The log spiral
• The shape of the
bay conforms to the
“log spiral” equation.
• In polar coordinates,
r = exp (-/4)
• Due to distributing
wave energy evenly
around the bay
Human effects - Half Moon Bay