Nearly all the world's oil and every scrap of coal sit in sedimentary rock
Over millions of years, sagging patches of Earth's crust fill with layer upon layer of sediment, sometimes almost 20 kilometres thick. These sedimentary basins hold virtually all the planet's petroleum and natural gas, all of its coal, and a long, readable diary of the planet's past.
More than six hundred basins have been mapped worldwide, spanning anything from tens of square kilometres to over a million. Each begins when the lithosphere subsides and creates room for sediment, mostly material washed in by water. As layers pile up, burial pressure compacts them and cements them into rock. Geologists keep calling the result a basin even after it stops being a hollow in the landscape; the Williston, Molasse and Magallanes basins are examples.
Several mechanisms make the crust sink. Stretching thins it like a rubber sheet pulled at both ends: the hot lower lithosphere flows away while the brittle upper crust cracks into faults. The North Sea, rich in hydrocarbons, formed this way, as did Nevada's Basin and Range Province with its alternating blocks. Taken to completion, stretching opens an ocean; the Red Sea is an infant one, and its mouth is the only spot on Earth where a triple junction in oceanic crust lies above sea level.
Weight also bends the lithosphere like an elastic plate. New mountain ranges press down to create foreland basins, and volcanic arcs or accretionary wedges do similar work. Cooling adds another push: young oceanic crust contracts and grows denser as it ages, sinking lower over tens of millions of years, which is why ocean depth tracks the age of the seafloor. Whatever starts the process, the incoming water and sediment then add their own load and deepen the basin further through isostasy.
Survival over geological time depends on setting. Basins in stable continental interiors are likely to endure, whereas those on oceanic crust usually vanish into subduction zones. Margins created when continents split apart, as the Atlantic did, can last hundreds of millions of years but may be partly destroyed when oceans close again. About a dozen basin types are widely recognised, though no single classification has become standard, and one basin can evolve from one type into another over its life.
Source: Sedimentary basin
