Landscapes

This page is about the shapes of the land and the forces that build them up and wear them down. Its sources are the US National Park Service (NPS) and the US Geological Survey (USGS), so most examples are American, and the Grand Canyon serves as a worked case that ties the ideas together.

Uplift against erosion

Mountains form because, over long spans of time, the land is pushed up faster than erosion can wear it away, and erosion then gradually cuts canyons into the high ground.^[1] Uplift comes from moving tectonic plates: where the Nazca Plate dives under South America, the overriding plate is lifted into the Andes.^[2] When erosion strips away a great deal of rock, the crust rises in response, a process called isostatic rebound.^[11]

Weathering breaks rock into fragments through chemical and physical processes such as freezing and thawing.^[3] Gravity, water and wind carry these fragments, called sediment, and drop them elsewhere, where compaction or cementing can turn them into sedimentary rock.^[3]

Rivers and valleys

The NPS calls stream erosion perhaps the most important force that shapes land: rivers erode landforms, transport sediment and deposit it to build new ones.^[4] The basic unit is the drainage basin, or watershed: a main river and its tributaries, separated from neighbouring basins by a high line called a divide.^[4]

When a river overflows its banks, water spreads across the floodplain, the fairly flat land beside the channel, and leaves sediment behind; ridges of sediment called natural levees may build up along the banks.^[4] Gentler streams fall into two broad types, meandering and braided, depending on the sediment they carry and drop.^[4]

A valley is low land surrounded by higher ground, carved by water or ice, or formed where the crust pulls apart.^[11] A canyon is an eroded valley with extremely steep, often near-vertical sides.^[11] Rivers deepen their beds by downcutting, as the rocks they carry act like chisels on the riverbed.^[11]

Ice and glaciers

Rivers normally cut valleys with a V-shaped cross-section, while mountain glaciers pluck and scour rock to make U-shaped valleys.^{[5, 11]} A USGS booklet from 1992 describes the Great Ice Age as a time of repeated, widespread glaciations in the Pleistocene, which that text says began about a million or more years ago.^[5] It says the last major ice sheet over the north-central United States peaked about 20,000 years ago and lingered in Canada until about 6,000 years ago.^[5]

Ice smoothed and scratched bare rock, and left till (an unsorted mix of clay, sand, gravel and boulders), ridges of till called moraines and rounded, elongated hills called drumlins.^[5] Swiss peasants concluded from boulders (erratics) in their pastures that Alpine glaciers had once been much larger, an idea the geologist Louis Agassiz later popularised.^[5]

The NPS says we have been in a warmer interglacial period for about 10,000 years.^[6] Glacier National Park’s present glaciers reached their largest size in the Little Ice Age, a cold period from roughly 1300 to 1850.^[6] By the 1990s, most of the park’s glacier retreat could be attributed to human-caused climate change.^[6]

Deserts and weathering

In dry climates, chemical breakdown of rock is extremely slow, but mechanical breakdown is relatively quick.^{[1, 11]} Daily heating and cooling, roots and ice in cracks, and wind and rain all break desert rock apart.^[1] Soil forms very slowly, so much bedrock stays exposed.^{[1, 11]} When rain does fall, large amounts of sediment rush downslope in flash floods or debris flows.^[1]

Plateaus, mesas and karst

Plateaus are large, raised, flat areas with a steep slope or cliffs on at least one side.^[7] Erosion can carve them into mesas (Spanish for tables): isolated, flat-topped hills with steep sides, capped by hard rock left behind after softer material is carried away.^[7]

Karst is a landscape where dissolving bedrock has made sinkholes, sinking streams, caves and springs, in soluble rocks such as limestone, marble and gypsum.^[8] About 20% of the United States is underlain by karst, and 40% of the groundwater used there for drinking comes from karst aquifers.^[8] Sinkhole collapse can cause serious damage (see Natural Hazards).^[8]

Volcanic islands

The Hawaiian Islands formed in the middle of the Pacific, more than 3,200 km from the nearest plate boundary.^[10] The classic explanation is a hot spot, a source of heat deep in the mantle that keeps making magma, with the Pacific Plate sliding over it.^{[9, 10]} Runny lava builds broad, gently sloping shield volcanoes from the ocean floor up.^[9] Kauai’s oldest rocks are about 5.5 million years old and deeply eroded, while the Big Island’s oldest exposed rocks are under 0.7 million years old.^[10] The USGS adds a caveat: since its text appeared in 1996, new studies suggest hot spots may be neither deep nor fixed in place.^[10]

The Grand Canyon: a worked example

The NPS sums up the canyon’s story as DUDE: deposition, uplift, downcutting and erosion.^[11]

Deposition. The rocks of the inner gorge formed almost two billion years ago, and layer upon layer of sedimentary rock was later laid on top.^[11] At the Great Unconformity, where flat layers rest on the Vishnu Schist, one location is missing 1.2 billion years of record.^[13]

Uplift. Plate tectonics lifted the region into the high, flat Colorado Plateau, beginning about 70 million years ago, during the Laramide Orogeny (about 70 to 40 million years ago).^[11] Scientists do not know exactly how this uplift happened, or whether the plateau was already high when the Laramide ended.^[11]

Downcutting. By the NPS’s estimate, the Colorado River has been cutting down for five to six million years.^[11] The older “ancestral Colorado River” upstream is at least 7 and perhaps 10 million years old, and how the river chose this course still awaits a clear answer.^{[11, 12]}

Erosion. Tributaries widened the canyon while the dry climate kept widening slow, giving a deep canyon instead of a broad valley.^{[11, 12]} Hard layers form bold cliffs and softer ones wear into slopes.^[12] The river and its tributaries are still slowly cutting deeper.^[14]

Common misconceptions

“Glaciers helped carve the Grand Canyon.” Glaciers never existed in or near the Grand Canyon.^[11]

“The canyon is as old as its rocks.” The river has been carving for five to six million years, through rocks up to 1.8 billion years old.^[11]

“Rock layers are a complete record.” In the Grand Canyon’s rock column, more time is missing than is preserved.^[13]

“Karst water is naturally filtered.” Water flows quickly through karst with little filtration, so pollutants spread fast.^[8]

What this page does not cover

Coasts and deltas, wind and dunes, soils, the world’s great rivers and the full stories of how mountain ranges were built are not covered here. Landslides, floods, sinkholes and eruptions as dangers belong to Natural Hazards.

Going further

See Geology for plates, rocks and geologic time, and Oceans for the seafloor.