17.4: Erosional and Depositional Landforms
Erosional and Depositional Landforms
Some coastal areas are dominated by erosion, like California and the rest of the US Pacific Coast; other coastlines are dominated by deposition, like the Atlantic and Gulf Coasts (Figure 17.8). On almost all coasts, however, both deposition and erosion are happening to varying degrees in different places. For this introductory course, we will oversimplify the North American coastlines to illustrate basic concepts: the West Coast is emergent and therefore erosional, while the East and Gulf Coasts are submergent and therefore depositional. The real world is obviously more complex, and it is possible to find all landforms on both coastlines.
Figure 17.8: The Pacific Ocean is off the West Coast of North America, whereas the Atlantic Ocean is off the East Coast. The Gulf of Mexico is to the southeast. (CC-BY 4.0, Chloe Branciforte via Google Earth)
Erosional Landforms
A key factor in determining if a coast is dominated by erosion or deposition is its history of tectonic activity. An emergent coastline Links to an external site. has been exposed by the relative fall in sea level by either isostasy Links to an external site., eustasy Links to an external site., or tectonic uplift. In general, California’s coastline is relatively active tectonically; uplift for tens of millions of years has resulted in a local “fall” in sea-level. Our emergent coastlines are typically dominated by erosional features, with rocky shores, narrow beaches and continental shelves.
Figure 17.9: Point Reyes, a headland, and Drakes Bay, northwest of San Francisco. (Left, CC-BY 4.0, Chloe Branciforte via Google Earth; Right, CC-BY-SA 4.0; DickLyon Links to an external site.)
Several unique erosional features commonly form on these rocky shores. When waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted. Because of refraction, the energy of the waves, which moves perpendicular to the wave crests, is focused on the headlands Links to an external site. (Figure 17.9).
Figure 17.10: Marine terraces and wave-cut platforms in northern California. (CC-BY 4.0, Chloe Branciforte via Google Earth)
Wave erosion is greatest in the surf zone Links to an external site., where the wave base strongly affects the seafloor and waves break. The result is a narrow flat surface known as a wave-cut platform Links to an external site. or wave-cut terrace Links to an external site., found at the base of a sea cliff or along the shoreline (Figure 17.10). Read more about marine terraces from the USGS - Fact Sheet, Landscapes from the Waves - Marine Terraces of California Links to an external site..
More resistant rock that does not erode completely during the formation of a wave-cut platform often remains behind to form a sea stack Links to an external site.. These sea stacks are typically steep, vertical columns of rock formed over time. Sea arches Links to an external site. and sea caves Links to an external site., the results of sequential erosion of promontories and headlands, may eventually collapse and form stacks (Figure 17.11).
Figure 17.11: Point Arena-Stornetta situated along the rugged Mendocino County coastline in northern California. (Public Domain; Samantha Storms/BLM Links to an external site.)
Depositional Landforms
Recall that emergent coastlines result from a relative fall in sea level. Accordingly, a submergent coastline Links to an external site. has been covered or inundated by the sea as a result of a relative rise in sea level from either isostasy Links to an external site. or eustasy Links to an external site.. The US Atlantic and Gulf Coasts are passive margins and have not seen much tectonic activity during the last hundred million years. Generally, submergent coastlines are dominated by deposition, although erosion will still occur, and are characterized by sandy beaches and wide continental shelves.
The evolution of sandy depositional features on seacoasts is primarily influenced by waves and currents, especially longshore currents. As sediment is transported along a shore, it is either deposited on beaches Links to an external site., or creates another depositional feature. A beach Links to an external site. is a landform along a body of water that consists of loose sediments, which range in size and composition but are typically derived from the local bedrock, shells or coral. A spit Links to an external site., is an elongated sandy deposit, similar to a beach, that extends out into open water in the direction of a longshore current (Figure 17.12). Should the spit begin to grow backwards and curve, it is referred to as a hook. A spit that extends across a bay to the extent of closing, or almost closing it off, is known as a baymouth bar Links to an external site.. Most bays have rivers flowing into them, and since this water must get out, it is rare that a baymouth bar will completely close the entrance to a bay. In areas with near-shore islands and sufficient sediment transport, a tombolo Links to an external site. and tied-island Links to an external site. may develop. In areas where coastal sediments are abundant and coastal relief is low, it is also common for barrier islands Links to an external site. to develop. These islands are composed entirely of sand and have a distinct elongated shape; typically they develop only a few miles from the mainland (Figure 17.12).
Figure 17.12: Depositional coastal landforms. (Public Domain, Surachit Links to an external site.)