Geology of Cape Hatteras National Seashore
Let's explore how Cape Hatteras formed, and the geologic processes shaping it today.
What is a barrier island?
To understand the geologic history and forces that shaped the Outer Banks and Cape Hatteras, let’s first explore the form of a barrier island. Barrier islands, in general, consist of beaches, inlets, washover deposits, sounds or estuaries, dunes, and capes (like Cape Hatteras!).
Beaches consist of the land from the point of mean low water landward to the nearest permanent vegetation or dune. Beaches are formed in processes that include forces from waves, currents, storm surges, and tides. As ocean waves approach shore, they agitate sand on the seafloor and grow taller. Once the waves break on the shore, they drop the sand they stirred up, helping to create the beach. Beaches protect the rest of the barrier island from storms.
Inlets are a break in the barrier island chain that connect the ocean to the sound behind the islands. Between the Virginia-North Carolina border and the southern tip of Cape Hatteras the only break in the Outer Banks is the Oregon Inlet, south of Cape Hatteras the Hatteras inlet separate Cape Hatteras from Ocracoke Island. Inlets allow release of fresh water from the mainland into the ocean beyond the barrier island as well as providing a return path for saltwater that floods the sound during storm events.
Washover deposits are the result of waves and highwater washing over the tops of dunes during storms and deposit sand and water on the other side. Over time and repeat storms events, these depositional areas can grow in size creating washover flats.
The sounds behind the Outer Banks are an area of water protected from the ocean by the barrier islands. The Pamlico Sound, which separates Cape Hatteras from the mainland, is the second-largest estuary in the United States smaller only than the Chesapeake Bay. Estuaries are places where freshwater from rivers like the Roanoke and the Pamlico mix with the salty ocean water. Estuaries are biologically rich habitats, supporting all kinds of marine life.
Dunes are formed as wind transports sand from the across the beach landwards onto overwash flats and vegetated areas. Mature dunes are characterized by a cover of maritime forest and other vegetation. North of Cape Hatteras at Kitty Hawk lies Jockey’s Ridge, which is the largest active dunefield on the Atlantic Coast. Jockey’s Ridge was formed when mature, stable dunes were broken down and the sand was redistributed. Dune heights at Jockey’s Ridge can surpass 100 feet but are in constant flux as the sand is moved by aeolian transport.
Capes are areas of barrier islands that jut out into the ocean in contrast to the greater coastline. On the Outer Banks, Capes are believed to be the product of long-term currents as well as underlying-geology. Cape Hatteras is associated with a large, shifting sand deposit known as Diamond Shoals, which affect coastal circulation patterns and can represent navigational hazards. USGS scientists are currently involved in research to better understand the morphology of and physical forces that form Diamond Shoals in order to better predict coastal change.
What shapes barrier islands?
Cape Hatteras and the rest of the barrier islands of the Outer Banks are the product of intricate, interconnected forces dictated by the amount of sediment present, the strength of storms and other natural events, and the change in sea levels. Ultimately, oceanic and atmospheric forces (wind, waves, tides, storms, and sea-level rise or fall) drive sediment transport which controls the shape and location of barrier islands. Aeolian (wind) distribution, inlet dynamics, longshore transport, and overwash are the four most dominant sedimentary processes on the Outer Banks.
How have storms affected the Outer Banks and Cape Hatteras?
Barrier islands are in constant flux—generally changing gradually each day. When storms hit, however, islands can be reconfigured in mere hours. The length and intensity of storms dictate how profound the change. Storms mean higher waves and stronger winds, but they can also cause storm surge, which multiplies the power of wave action. Storm surge is the rise of water associated with a storm (normally from changes in atmospheric pressure and winds) beyond the tide. Storm surges are the largest cause of destruction during storm events. USGS, in collaboration with other organizations, has developed the Surge, Wave, and Tide Hydrodynamics (SWaTH) Network along the Northeastern Atlantic Coast to gather scientific information that will help to build more resilient communities before storms hit.
The two types of storms that most profoundly affect the Outer Banks are tropical cyclones (hurricanes) and northeastern storms (“nor’easters”). Hurricanes, which originate in warm, tropical waters south of Cape Hatteras, are classified by atmospheric pressure, rainfall, and wind speed. Over the years, hurricanes have caused catastrophic destruction of property and even the loss of life on the Outer Banks. Hurricane Isabel in 2003 cut two inlets through Hatteras Island, generated the largest wave ever recorded at the Field Research Facility in Duck, NC (39.7 feet), and resulted in millions of dollars of damages. Hurricane Dorian brought destruction to parts of the Outer Banks in 2019, after hitting the Bahamas as a powerful Category 5 storm.
Nor’easter storms are not as well as known as hurricanes, but they actually cause greater change to the Outer Banks as they are longer and more frequent events. Nor’easters develop in midlatitudes and are named after their northeasterly winds. Whereas hurricanes normally pass over fairly quickly, nor’easters can linger for days. Nor’easter occur most frequently between October and April and are closely tied to changes in the seasons. The Halloween Nor’easter of 1991 is an example of a particularly powerful northeast storm event. The storm produced a two-foot storm surge in the Outer Banks and generated waves in excessive of 19 feet.
What is the geologic history of the Outer Banks and Cape Hatteras?
The Outer Banks are located along a passive margin (not at a tectonic plate boundary) and are the result of the global fall and rise of sea levels and surplus sediment. Some 110,000 years ago sea levels fell over 400 feet during the Wisconsin glacial episode resulting in shoreline of North Carolina extending more than 50 miles seaward of its present position. Wind and waves would have formed dunes at this coastal edge.
Then about 14,000 to 18,000 years ago glaciers began to melt and sea-levels rose rapidly, which initiated the Holocene marine transgression. As sea-levels rose, water would break through the dunes, creating sounds and isolating the dune line as a string of islands. Eventually, sea-level rise slowed allowing sediment deposited by longshore transport or from coastal river systems to grow the islands. Longshore transport leads to the development of spits, which can eventually connect formerly separate islands.
This process likely produced the long chain that is the Outer Banks. As sea-levels continue to rise in the present day, most of the Outer Banks are moving landward through washover and inlet processes. As islands move landwards, the remnants of maritime forests that used to be on the backside of the island are being exposed on the beach face.
How are the Outer Banks changing today?
Barrier islands are in constant flux. Dramatic change can occur in a matter of hours, as happens with powerful storms, or much more slowly, such as is the case with sea-level rise. Coastal change can be costly and potentially poses risks to coastal communities like those along Cape Hatteras National Seashore. USGS is currently conducting research to understand physical processes that cause coastal change, and ultimately improve our capability to predict the processes and their impacts.
Through the USGS Coastal and Marine Hazards and Resources Program, studies on historical shoreline change and the geologic structure and history of coastal regions, sediment supply and transport, sea-level rise, and how extreme storm events affect rates and impacts of coastal change are conducted. At the USGS’s Coastal Change Hazards Portal you view predictions of the effect of extreme storms and sea-level rise on Cape Hatteras and the rest of the coastline of the United States.
