Michigan Beaches,
Lakes: How they originate and why they Change
Objectives:
To Know:
- Know basic features of coastal erosion and deposition and how these features form.
- Describe and give examples of the dynamic nature of the coastal environment.
- Reconstruct the glacial lake history of Michigan.
Misconceptions:
1. Gravitational attraction between the moon and earth are responsible for
tidal waves.
2. Ocean and lake levels have changed little during the past two million years.
3. Sand on Michigan beaches is brown.
4. The shape and sorting of beach deposits are random.
5. Barrier islands are ideal locations for industry, ports, and residential development.
6. Bands of black sand on the Michigan beaches represent pollution.
7. Every river has a delta.
8. Coastal features are only found on present shorelines.
9. Water always gets deeper the farther one wades away from shore.
Objective 1. Know basic features of coastal erosion and deposition and how these features form.
The single most important process in shaping coastal topography is waves. Other
processes that shape the shoreline are long-term changes in water level, tides,
ice push (produced by an offshore ice shelf), and seismic waves.
Another process acting on the coastal environment is a longshore current. Think of this longshore current as carrying "a river of sand" parallel to the shoreline.
A tidal cycle, caused by the gravitational attraction between the earth and moon, goes from high to low and back to high again in about 12 hours. Tides become significant agents for erosion and deposition in narrow bays.
Seismic waves produce destructive tsunamis, incorrectly called "tidal waves, these waves would be hardly noticeable in the open ocean but pile up to great heights upon reaching the shallow coastal environment.
The cascading forward motion of breaking waves as they rush upon the shore is called the swash. A wave breaks because friction with the ocean bottom interferes with the wave's oscillatory motion. The retreat of waves is called the backwash.
Usually the work of waves is done is a slow, methodical fashion, waves can be catastrophic agents of change, however, when earthquake's generate seismic waves and produce huge waves called tsunamis
Some coastlines are actually living, like the Great Barrier Reef in Australia. This reef is composed of coral, a living creature that builds its skeleton with calcium carbonate. Reefs can take the form of barrier reefs, fringing reefs or atolls.
atolls are coastal or offshore landforms which almost always assume a circular shape around volcanic islands that have sunk beneath the surface. Most atolls begin as fringing reefs around volcanic islands which later sink beneath the sea.
Changes in sea level affect the location of erosional and depositional forms. Sea level can change from the submergence of landmasses, the emergence of landmasses, and increases/decreases in the volume of seawater
Melting glaciers within the last 15,000 years results in higher ocean levels
which drown river valleys. Nearly every coastline shows at least some
evidence of submergence. A coast embayed with numerous estuaries (bays) is
likely to be a shoreline of submergence.
The origin and
characteristics of erosional coastal landforms.
Processes that produce erosion:
- force of the waves
- air forced into joints, the air then expands violently when a wave breaks
- abrasion
- solution
- salt crystallization
Headlands and bays often represent rocks of alternating resistance. Headlands, which protrude into the ocean are attacked by waves changing direction and converging on the headlands, a process known as refraction.
Cliffs often become undermined when hydraulic pounding and abrasion attack the base of a cliff and produce a notch. The material eroded from a coastal cliff ends up as a terrace found just offshore.
Glaciers carve deep valleys. When these valleys become flooded by a rise in sea level a fjord is produced. Good examples of fjorded coasts occur in southern Chile, Norway, and Alaska.
The origin and
characteristics of depositional coastal landforms.
Where the ocean meets the land is a coastline. The most widespread marine deposit is the beach defined as: The beach environment can be broken into into several different zones:
The foreshore zone is regularly covered and uncovered by the rise and fall of the tides. The backshore zone is a site of wave deposition.
beach-the most widespread of all coastal depositional features, beaches form when
spit-a linear extension of sand from the shore into the ocean is a spit. Spits can take various forms such as tombolos, baymouth bars, hooks, and connecting bars
tombolo-an island connected to the coastline by a sandy spit.
barrier bars-offshore islands created by longshore drift. Some of these
islands, such as the ones off the east coast of the U.S. can extend for over 300
miles. Only after barrier islands have attained a large size do they begin to
migrate toward the shore.
Web Resources to Supplement this
Lecture
