Geomorphic features and landforms are of high importance in ASBOG tests. Specially in the FG (Fundamentals of Geology) section of the test, you would find lots of geomorphologic terms. This article introduces to some basic geomorphic features along the coast. Grab a general physical geology book for basic concepts. Review a geomorphology book in depth review. As a practicing geologist, you should be able to understand the feature by the name and start thinking about the origin of such features. Most of the coastal erosion in the United States is due to gradual rise in sea level as controlled by melting of glacial ice and expansion of water due to increased temperature. The average rate of sea level rise in the US is about 1 foot/100 years. Now with that introduction, lets get ourselves familiarize with some terms associated with coastal processes.
Waves are agent of erosion, transportation and deposition. The energy of the waves come from the wind. The energy moves sand along a beach. The size of wave is also directly related to the velocity of the prevailing wind.
Terms defining wave:
- Wavelength (L) = distance to complete one cycle
- Wave Period (P) = time required to complete on cycle.
- Wave Velocity (V) = wavelength/wave period (L/P).
What is Wave Refraction?
Waves generally never approach shoreline parallel to shore. Instead some parts of waves feel the bottom before other parts, which results in wave refraction or bending.
What is surf zone?
The zone between shoreline and breakers is known as the surf zone where rigorous erosion takes place.
What are the factors controlling marine erosion?
- Structural features, joints and faults
- Tidal range
- Depth of water offshore
- Shape of coastline
- Stability of sea level
- Strength of wave
What is the most effective process in marine erosion?
Corrosive or abrasive action (milling effect) of sand, gravel and pebble moved by waves along the shore causes the maximum erosion. Waves, tsunamies and storm waves in particular, are most important agent of marine erosion.
What is Wave base?
The lower limit of wave action is called the wave base. Water deeper than Wavelength (L)/2 does not move. Thus, waves cannot erode the bottom or move sediment in water deeper than L/2.
A rip current (often wrongly called Rip tide) is a strong channel of water flowing seaward from near the shore, typically through the surf line.
Longshore current / Littoral Current: Longshore current is generated when waves hit the coastline at an angle. Longshore currents move laterally along the coast. Longshore currents also transport sediments along a coast. in a lateral fashion (called the littoral drift). This process occurs in the littoral zone, and in or within close proximity to the surf zone. Longshore current is responsible for the shore drift that produces spits and bars along the coast.
Tidal currents are minor in transporting sediments and have insignificant effect on coastal erosion.
Tsunami: Tsunamies are waves produced by submarine earthquakes, volcanic eruptions, or landslide.
Here is the list of terms you should review for the ASBOG test:
Erosional Geomorphic features:
Half Moon Bay, CA
- Headlands: A headland is a point of land, usually high and often with a sheer drop, that extends out into a body of water.
- Sea Cliff: Sea cliffs mark the seaward limit of the coast, identified as a scarp.
- Wave cut bench: Wave cut bench extends seaward from the base of the sea cliff. A wave cut bench could be made of hard rock only. When the wave cut is covered with sand, gravel and pebbles it is called a beach. Marine terraces are elevated wave cut benches representing previous high sea level stand.
- Stack: A stack is a erosional geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast. Stacks are commonly founnd form in horizontally bedded sedimentary or volcanic strata particularly on limestone cliffs. Limestone has medium hardness and moderate resistance to erosion. Often a more resistant layer form a cap. Headlands and cliffs with weaker rocks are more prone to slump and erode too fast to form stacks. Harder rocks such as granite would erode without any particular manner and do not form stacks.
- Skerry: Skerry is a kind of stack that is only exposed during the low tide.
- Sea Arches and Cave: Wave erosion of the headland results the formation of caves and sea arches.
Swedish island of Fårö with Stacks and Arches
Ball's Pyramid, Australia, is the tallest volcanic stack in the world
The Google map below shows the Aguijan Island of Northern Marina. Blow it up and see if you can figure out the wave cut benches, some of them now covered by threes.
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Depositional Geomorphic features:
- Beach: Beach is a temporary accumulation of rock fragments on and along a wave cut bench. Beaches could be patchy or could extend for miles along the coastline. It is important to note that the most of the rock fragments on beach comes from land carried by various streams. Landslide, erosion and weathering of the sea cliff also contribute to beach fragments.A beach is defined as the gently sloping area of land between the high and low water marks.
- Bar: Various types of submerged or emergent embankments of sand and gravel built on the sea floor is known as a bar.
- Spit: Spit is a type of bar where one end of the embankment is attached to the land while the other end is terminating in the ocean.
- Hook: The open end of the spit may point landward if strong tides exist resulting hook or recurved spit.
- Winged headland: When both ends of a spit is attached to the headland, it is called winged headland.
- Barrier Island / Barrier beach / Offshore beach: Barrier islands are elongated sand ridges, mostly parallel to the shore, separated from the main beach by a lagoon. Bars are usually slightly submerged feature while barrier islands don’t submerge even during the high tide.
- Tombolo: Tombolo is a landform where island is attached to the mainland by a narrow piece of land such as spit or bar. Tombolo’s are created by longshore drift. A tombolo is formed when a spit continues to grow until it reaches an island, forming a link with the mainland.
Check out the SPIT at Spurn Head, Humberside.
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Types of Coasts:
- Neutral coastlines: not affected by emergence or submergence.
- Compound coast: include features of both submergence and emergence.
Geometry of a coastline and plate tectonics:
- The western margin of North America is tectonically active (active margin). So, the beaches along the western coast is characterized by cliffs above the water line, narrow continental shelf and relatively steep drop into the deep ocean.
- The eastern margin of North America is tectonically passive (passive margin). The beaches along the east coast is characterized by broad continental shelf, wide beaches and barrier islands.
Check out the following websites (Specially good for the PG part of ASBOG exam)
- St. Petersburg Coastal and Marine Science Center: Research Programs have a primary focus of investigating processes related to coastal and marine environments and societal implications related to natural hazards, resource sustainability, and environmental change. http://coastal.er.usgs.gov/
- California Coastal Records Project, an aerial photographic survey of the California Coastline: http://www.californiacoastline.org/
We will cover the engineering structures along the coast in another article.