Lakes are large bodies of standing water contained within a depression or basin on the Earth’s surface. While some lakes are saltwater, the majority are freshwater reservoirs, representing a tiny fraction of the planet’s total water. Lakes form when a geological low point, or basin, fills with water from precipitation, rivers, or groundwater sources. The variety in the size, depth, and shape of the world’s lakes reflects the powerful geological forces that created these depressions.
Lakes Formed by Earth’s Tectonic Shifts
The movement of the Earth’s tectonic plates creates some of the world’s deepest and oldest lakes. These bodies of water form where the crust stretches, pulls apart, or shifts along fault lines, initiating the formation of enormous basins.
Rift valley lakes are the primary example of this process, occurring at divergent plate boundaries where the crust is actively being pulled apart. As the continental plates separate, the central block of land drops down along parallel faults, forming an elongated, deep trough known as a graben. Water collects in these depressions, creating lakes like the African Great Lakes and Lake Baikal in Siberia, the world’s deepest and oldest lake.
Other tectonic lakes form through faulting that causes blocks of the Earth’s crust to tilt or drop. In fault basins, one side of a fault block might be uplifted while the other side subsides, trapping water in the resulting depression, such as Abert Lake in Oregon. A third type, known as a pull-apart basin, develops where two plates slide past each other horizontally along a transform fault, creating localized stretching and subsidence that fills with water, exemplified by the Dead Sea.
Lakes Sculpted by Glacial Activity
Glacial activity is the most common mechanism for lake formation globally, as ice sheets and alpine glaciers sculpted the landscape during past ice ages. The weight and grinding motion of moving ice create depressions and leave behind debris that acts as natural dams.
Glacial erosional lakes, also known as ice scour lakes, form when the glacier scrapes and plucks away bedrock. The Great Lakes, for instance, were formed and deepened as the Laurentide Ice Sheet flowed across the continent, carving out the basins. These lakes often occupy U-shaped valleys that have been over-deepened by the erosive power of the ice and embedded rock fragments.
When a glacier retreats, it deposits the unsorted mix of rock and sediment it carried, called till, into ridges known as moraines. Moraine-dammed lakes form when these ridges block the natural drainage of a valley, causing meltwater to accumulate behind the debris dam. Since these dams are made of unconsolidated material, they can be unstable and susceptible to failure.
In mountainous terrain, small, bowl-shaped depressions called cirques are carved into the mountainsides by localized glaciers through a process of plucking and abrasion. When the ice melts, the depression often fills with water to create a small, deep lake known as a tarn.
Kettle lakes form in flat plains when large, stagnant chunks of glacial ice break off and become buried by outwash sediment carried by meltwater streams. As the buried ice slowly melts, the overlying sediment collapses, leaving a circular or oval depression that fills with water. These kettle holes are shallow, rarely exceeding ten meters in depth, and are common in the glaciated regions of the northern United States and Canada.
Lakes Created by Volcanic Action and Water Erosion
Volcanic activity and water erosion also create distinct types of lake basins. Volcanic lakes can form rapidly when a large eruption empties the underlying magma chamber, causing the volcano’s summit to collapse inward.
This collapse forms a massive, cauldron-like depression called a caldera, which subsequently fills with rain and snowmelt to create a caldera lake, such as Oregon’s Crater Lake. Lava flows can also create lakes by acting as a natural dam. When molten rock flows into a river valley and solidifies, it creates a barrier that backs up the existing river or drainage system, forming a new lake upstream of the lava dam.
Water erosion creates lakes in two primary ways. Oxbow lakes are crescent-shaped bodies of water formed when a meandering river cuts across the narrow neck of an extreme bend. The river takes the new, straighter path, and sediment deposition seals off the abandoned channel.
Solution lakes, also known as karst lakes, form in areas with highly soluble rock, such as limestone. Slightly acidic groundwater dissolves the bedrock, creating an underground cave system. When the roof of a large cavity collapses, it forms a sinkhole that fills with water. All lakes are temporary features on a geological timescale, destined to eventually fill with sediment or drain away.