Exam 3 Flashcards
What is happening in South Carolina? (Edisto Beach area)
“Hammock” island (sand island in front of main barrier island)
Sand from the offshore overtaking soils - due to sea level rise
Dead trees in the sand - why? saltwater, eroding & unhealthy soils from sand encroachment
Sea level rise x increased storm surge activity
Waves x time
(The landscapes along a coast are very sensitive to multiple geological processes, rapidly)
Coastal Geomorphology (ALL of the factors that will affect any coastline)
Coastal morphology (shape and form) is controlled by many processes…
1. Sediment input from rivers (e.g. deltas)
2. Erosional processes (e.g. waves and sometimes tides)
3. Transport processes (e.g. waves, longshore currents, tides)
4. Pre-existing shoreline topography (e.g. continental shelf, submarine canyons).
5. Relative sea level change (e.g. tectonics (active vs. passive), isostasy, climate)
6. Human activity (e.g. jetties, groins, sea walls, fabrics, etc.)
Primary Coastal Processes That Influence Coastal Morphology
Rivers
Waves
Tides
Relative sea level change
Sediment Routing System
“a highway of sediment” how it moves in the landscape
Sediment supply to the coast primarily comes from river input.
Sediment emerges at discrete points along coastlines, sometimes forming protruding deltas.
(collecting land and sediment and funneling it into the river)
Delta
a fan-shaped feature formed by clastic sediments that build into standing water
Deltas are produced where river flow decelerates as it enters a body of water. The deceleration results in rapid deposition and the construction of a sedimentary fan.
(named after Nile River Delta)
Characteristic Fan Shape? - of a delta
Avulsion
Mouth bar
Creates distributary channel morphology and classic delta shape (Δ)
(deposits a broader fan of sediment)
Avulsion
channel switching caused by intra-channel deposition
Mouth bar (middle ground bar)
An intra-channel sediment bar that forms when a channel reaches its base level (due to flow deceleration)
Distributary Morphology/pattern
large river branches out into smaller rivers (this only occurs at the mouth of a river)
How waves are generated & measured
Waves are generated by friction between water and wind
We measure waves by their amplitude (height), wavelength (crest to crest distance), and frequency (how many waves pass a single point over time)
Factors that influence wave size/energy…
(1) duration of wind event
(2) velocity of wind
(3) fetch (distance that the wind blows across the surface)
Wave Types
Oscillating
Translational
Oscillating waves
waves that move in circular orbitals (deeper sea)
Translational waves
waves that interact with the seabed
(translate = to move horizontally, in this case towards the coast)
As waves approach shore, the wave base hits the seabed, what happens here?
(translational wave action)
- Friction slows wave motion near the sea floor
- Near the surface, waves continue moving fast
- The wave over-steepens and crashes onto the beach
- The wave energy also runs out of space, so the waves grow taller
Backwash
the current that pulls the water back to sea, this is entirely related to gravity & seaward transport
(The beach is a slope that gets deeper as you move out to sea. Gravity will pull some of the finer particles back out to the ocean.)
Swash
driven by translational wave energy which pushes sand up on the beach, shoreward & wave driven transport
(The coarser sand (or larger particles) gets left behind by this process)
Foreshore =
Beach face
Dunes are…
stabilized by vegetation
Waves as an Erosive Agent
Wave action can erode loose sand and erode bedrock cliffs (more on this later). Sediment may get transported away from beach.
Longshore Current
current that flows parallel to coastline
oblique attack angle of waves leads to a longshore current
Longshore Transport
Longshore currents move sediment along beaches
Builds beaches, sand bars, sand spits, barrier islands
Sand Spit Formation & Evolution
Spits are beach landforms that are entirely formed by longshore currents. Spits are made of sand that has been transported along the shore across bays, estuaries, and other inlets to land. The video here demonstrates spit formation and evolution.
What are our beaches composed of?
Quartz (sand)
(Continental crust source (granite = quartz, feldspar, mica) –>
Far from continental crust source (long transport distance) –>
Quartz is all that’s left – mineralogically mature beach
Most dominant rock on our continental crust:
Granite (made of quartz, feldspar, mica)
(as the three travel, quartz is what stays/is left)
Characteristics of a Feldspar, Quartz, Mica Beach:
- Close to source – short transport distance on cont. crust
- No time to convert feldspar, mica to clay – immature
Characteristics of Gray and Black Sand Beaches (Intermediate to Mafic Beaches):
- Short river transport distance near mafic/intermediate volcanic systems
- Oceanic crust or volcanic arcs
- No time to convert feldspar, pyroxene, olivine to clay
Why would wind/waves leave olivine on a beach?
the wind and waves sort them out because they are denser than other mafic minerals in basalts (like pyroxene)
Characteristics of a Carbonate Sand Beaches (White Sand Beach):
- Beach formed in proximity to a limestone coral reef in the tropics
- Sand comprised of broken shell and reef fragments (calcite)
Tides
periodic change in water level as a result of lunar (moon) (and solar, which is weaker, bc not as close even though larger) gravitational force
[essentially the moon tugs on our ocean, creates two bulges, one on either side of the earth]
The result of this high and low tide sequence…
the water moves in and out twice per day
Tidal range
difference in sea level between high and low tide
High Tide and Low Tide Cycle
High tide = every 12 hours
Low tide = every 12 hours
6 hour high to low cycles
Earth rotates once every 24 hours while Moon stays relatively fixed in its location during that 24 hours
6 switch 6 switch 6 switch 6
Tidal Range & Geomorphology
- Tides can expose or inundate landforms (rise and fall of sea by several meters)
- Expose different parts of the beach to wave action
- Tidal currents can erode and transport sediment (tidal channels)
- Tidal energy is typically slow, so it usually only transports muds (silt & clay) not strong enough for anything larger
Minerals found in tidal/mud flats:
clay & silt (no sand)
Tidal Flats & Estuaries
Form in intertidal zones lacking strong waves!
- Cut off from stronger ocean waves
- Common behind barrier islands or in estuaries
- Consist of some sand, but mostly “mud” (low energy)
Estuaries:
submerged channel networks/submerged river valley
3 Types of Coastlines
- static
- submergent
- emergent
Static coastline:
sea level is stable/not changing
Submergent coastline:
sea level is rising relative to the coastline
Emergent coastline:
sea level is falling relative to the coastline
What kind of coastline is Chesapeake Bay, VA? (east coast)
Submergent
can tell that the sea is rising, encroaching into the land by the river channels, used to be a river valley but brought salt water to
(tide-dominated estuaries, wave-dominated barrier islands with limited sand supply from east coast rivers/streams)
A sign of sea level rising, an example,
barrier island forming a lagoon
Barrier beaches
may be partially submerged beaches – associated with sea level rise (de-glaciation)
may start as offshore sediment bars (piles of sand)
bars migrate landward and build upward with (relatively) slow rise in sea-level to become barrier beaches
barrier beaches grow, erode, and generally migrate due to changes in sea level, wave action, & longshore currents
Barrier Islands by Longshore Transport
Barrier islands may also be generated by longshore transport of sand (making a spit go all the way across the bay)
Lagoons
relatively shallow submerged region landward of a barrier beach island (low energy = muds & wetlands)
Sea level has risen globally since the last ice age…
“Recent” sea-level changes.
Sea level has been rising due to melting of the Pleistocene-age glaciers (~20,000 years ago) due to natural global warming (see upcoming lectures).
Modern ice caps are melting at an even faster rate today than can be explained by natural processes due to human-related global warming.
What is a Glacier?
form where snow accumulates faster than it melts through the seasons and longer-term climate cycles
(not just fluffy snow)
gets to be ice when deep enough
Glacial Ice
- Weight compresses snow into ice
- This pressure squeezes out air
- Ice melts and re-freezes under pressure causing crystals to fuse/merge into larger crystals (glacial ice)
- Makes denser ice particles
= Snow/ice metamorphism !
Glacial Advance
If accumulation (snowfall) is greater, ablation (removal of ice by melting, wind, etc.) the glacier gets bigger (gains mass/volume)!
This makes it appear like the front of the glacier is “advancing” (getting bigger / coming towards you)
Glacial Retreat
If accumulation is decreasing ablation the glacier is losing mass/volume
This makes it appear like the glacial front is “retreating” (smaller / going away from you)
(but ice is still sliding down hill!)
Why is Glacial Advance not Glacial Flow?
Glaciers always flow downhill, regardless of whether they advance or retreat
How does ice flow?
Glacial Flow
Wet-Based Glaciers!
- Weight of glacier & heat of the surface can cause basal melting
- Glacier can more easily slide along this film of liquid water
