Lecture 22 Flashcards
Tides
Sea level rises and falls twice in just over 24 hours; monthly variation too.
Causes of tides
Caused by rotation of earth and gravitational attraction of Moon and Sun.
Moon ore important (less mass, but closer to Sun).
Sun can add or subtract from tides.
Spring tide
Sun and moon align and make highest tide of the moon - full and new moon.
Neap tide
Sun, Earth, and Moon make right angle, so lowest tides of the month - quarter moon.
Ice ages and sea level
Longer term effect of sea level changes is ice ages when more water is stored on land, so less in oceans (sea level drops). Alters location of coast.
Tectonic uplift
Like West Coast of NA - relative drop in sea level.
Subsidence
Like Gulf Coast - relative rise in sea level.
Wind waves
Only extend to depth of about one-half wavelength.
Size of waves
Determined by wind speed, wind duration, and fetch (distance wind blows over water). largest waves about 30 meters high.
Tsunamis
Different kind of wave, caused by earthquake, landslide, volcano. Low height over deep water, very high at coast; often highly destructive.
Wave breaking
In shallow water, wave deforms bc it is in contact with sea floor.
Breaking; wave falls over bc too steep. Water runs up the beach, runs out of energy, and gravity pulls it back into ocean.
Wave refraction
Bending of waves as they approach the shore. Generally approach at an angle to beach, so one part of waves touches sea floor before rest. Refracted bc one side slowed before other.
Littoral drift
beach drift + longshore drift.
Beach drift
Water and sand rush up beach at angle, return straight down; net movement down beach.
Longshore drift
Longshore current (buildup of water at beach, current of water parallel to shore) acts like stream, carrying sand.
Wave cut platforms
Base of cliff erodes, so cliff falls down and cliff moves back over time. Wave cut platform develops at sea level where cliff used to be.
Marine terraces
Platforms elevated above sea level.
Headlands and their erosion
Headlands are bedrock jutting out from coast.
Refraction causes waves to hit sides.
Erosion, especially along cracks, leads to distinctive landforms. (Caves, arches, stacks).
Winter beach
Season of big waves. Sand eroded off beach and deposited offshore - small beach.
Summer beach
Season of small waves. Sand pushed back onto beach - broad beach.
Spit
At indention in coast, sand grows out into the water.
Tombolo
Spit connects mainland and island.
Barrier beach
Drifting sand blocks stream mouth.
Humans modify littoral drift
To keep a spit from blocking a bay, a dredge may be used to move the sand.
Groins and jetties are used to build up beaches, but often cause erosion elsewhere.
Seawalls built to absorb energy of waves, but may cause erosion on seaward side.
Beach nourishment; artificially bring sand to beach. (expensive and temporary).
Barrier islands
Narrow islands parallel to mainland.
Common on Gulf and Atlantic Coasts. Protects mainland form big waves.
CHECK DIAGRAM.
TX beaches
Net erosion since at least mid 1800s.
Sea level rise and subsidence.
Damming of streams too.
1-2 m of erosion per year.
Coral reefs
Formed by accumulation of calcium carbonate from shells of coral. Requires warm and shallows waters, so found in tropical areas. For reefs to get big, the relative sea level must rise, so subsiding coasts.
Fringing reef; connected to land.
Barrier reef; separated from land (land sinking).
Atoll; land underwater, just reef at surface.
Submergent coasts
Relative sea level rise (water rises above land).
Ria coasts
Drowned stream valleys.
Fiord coasts
Sea fills glacial valleys
Emergent coasts
Relative sea level drop (land rising above water).
