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Time Frame and Setting
Oil and coal take hundreds of millions of years to form, on the order of an estimated 300 to 400 million. Most of the world̵7;s coal and oil began to form during a time known as the Carboniferous period, which itself takes its name from the basic element of fossil fuels: carbon. During this era, the Earth̵7;s climate was hot, wet and junglelike. The world̵7;s land masses were in their infancy; water intermixed with the newly emerging land in countless swamps and bogs, which dotted the landscape. This environmental setting was crucial to the subsequent materialization of oil and coal.
Marine Environments Vs. Swamps
Whether oil or coal began to take shape depends on whether the living things needed for their formation inhabited land-based ecosystems or marine ecosystems. Oil began its existence in the basins of marine environments, such as oceans, seas and gulfs. Some of the organic matter contributing to the oil formation may have washed into these marine environments from rivers. Coal, on the other hand, began its formation at the bottom of shallow swamps and bogs, in the absence of oxygen.
Land Plants Vs. Marine Microorganisms
Oil and coal each formed from the decaying remains of once-living things, but different types of things. Coal is essentially composed of plant matter: ferns, parts of trees such as branches, leaves and twigs, and other plant debris. Oil formed from the remains of tiny marine organisms, namely plankton and bacteria. It is interesting to note that while they were alive, the plants contributing to coal formation as well as the sea creatures contributing to oil formation were able to convert sunlight directly into energy and to store that energy.
A Changing Climate and Geology
In the swamps and bogs, plants continued to die over time, and their remains piled up on top of the remains of long-dead plants. The freshly fallen plant matter continuously compressed the existing plant matter with its weight. Over time, as the Earth̵7;s climate changed, water and silt washed into the swamps, resulting in an even greater degree of compression. In oceans and other marine environments, dead organisms eventually became covered in layers upon layers of sand, mud, clay or rock as sea and land shifted and merged, due to the Earth̵7;s changeable geology. In both cases, the compression increased pressure, burying the materials progressively deeper within the Earth, where they became heated. Under these conditions, chemical and physical changes slowly occurred, eventually resulting in the hydrocarbon deposits of coal and oil that are used to power the modern world.