A bay is a broad, curving inlet of the sea where the land naturally bends inward, creating a partially enclosed body of water. These coastal indentations vary significantly in size, shape, and depth around the world. Bay formation results from the long-term interaction of geological forces, including erosion, changes in global sea level, and movements within the Earth’s crust. Understanding a bay’s origin requires examining the specific geological history of the coastline.
Shaping by Erosional Forces
One primary way bays are carved into coastlines is through differential wave erosion, which occurs along shores composed of alternating bands of hard and soft rock. Resistant rock types, like granite or chalk, erode more slowly, projecting out into the sea as headlands. Conversely, softer rocks, such as sandstone or clay, are rapidly worn away, creating the concave shape of a bay between the headlands.
The concentration of wave energy influences this process. Incoming waves refract, or bend, around the headlands, focusing their power on these projections. This causes wave energy to disperse and decrease within the bay, leading to calmer waters where deposition of sand and sediment often occurs, forming a beach. Waves physically attack the coastline through hydraulic action (the force and pressure of water) and abrasion (where waves hurl rock fragments against the shore).
Fjords are a specific type of erosional bay formed by massive glaciers. During past ice ages, glaciers moved downslope, carving deep, U-shaped valleys using the immense weight and abrasive power of the ice. When the glaciers retreated, these steep-sided valleys were flooded by the rising sea, creating a long, narrow inlet. Fjords are often significantly deeper than the adjacent sea and frequently feature a shallow rock sill at their mouth, a remnant of the glacial moraine.
Creation by Sea Level Change
Another mechanism for bay formation involves the passive flooding of existing land features due to a rise in sea level, known as submergence. When global sea levels rise, often following the melting of continental glaciers, the lower parts of river valleys near the coast become inundated with seawater. These drowned river valleys are known as rias, forming wide, funnel-shaped estuaries that gradually deepen toward the sea.
The Chesapeake Bay in the United States is a prominent example of a ria, representing a massive river valley system flooded by the post-glacial rise in sea level. This submergence creates an intricate, branching pattern as the sea fills the main river channel and its tributaries. The original topography of the river landscape is preserved beneath the water, with hilltops often remaining as islands.
Bays can also be partially enclosed or separated from the open ocean by the deposition of sediment. Features like barrier islands and bay barriers are long, narrow accumulations of sand that run parallel to the coast. These depositional features form when sediment is transported by longshore currents and deposited in shallow water. The resulting barrier creates a sheltered body of water, such as a lagoon or bay, between the island and the mainland coast.
Structural Formation
Origins for bays are also linked to the movement and deformation of the Earth’s crust. Tectonic activity, such as faulting and subsidence, can create large depressions in the land surface that subsequently flood with seawater. Subsidence, the sinking of land, can occur gradually over geological time in a rift basin or as a localized event along active fault lines.
A structural formation involves flooded volcanic calderas. A caldera is a large, basin-shaped depression formed when a massive volcanic eruption empties the magma chamber, causing the overlying cone structure to collapse inward. If this depression is near the coast or below sea level, it fills with ocean water, creating a circular or semi-circular bay with unusually deep water. Kinkowan Bay in Japan, formed by the Aira and Ata calderas, is significantly deeper than typical coastal inlets.