The Earth is not the fixed, solid
mass that we usually envision. It is actually a
sphere of solids and molten rock fluids that are
gradually but continuously moving and changing. For
example, South America is drifting away from Africa
at about the speed your fingernails grow.
Earthquakes and volcanoes are reminders of the
Earth's instability and changing face.
The planet's crust is divided into numerous tectonic
plates. These push against each other, rise and
fall, tilt and slide, buckle and crumple, break
apart and merge together. As a result, sediments
from the bottom of ancient seas can today be found
in rocks on the tops of mountains. In fact, the
summit of Mount Everest is marine limestone.
For more than half a billion years, photosynthesis
has made life possible on Earth. Plants absorb solar
energy and use it to convert carbon dioxide and
water into oxygen and carbohydrates, such as sugar,
starch, and cellulose. These carbohydrates and other
organic materials eventually settle on the ground
and in stream, lake and sea beds.
As these organic materials become more deeply
buried, heat and pressure transform them into solid,
liquid, or gaseous hydrocarbons known as fossil
fuels—coal, crude oil, or natural gas. Coal is
generally formed from the remains of terrestrial
(land-based) plants. Oil is typically derived from
marine (water-based) plants and animals, mainly
algae, that have been gently "cooked" for at least
one million years at a temperature between fifty
degrees Celsius and
150 degrees Celsius. Natural gas can be formed from almost
any marine or terrestrial organic materials, under a
wide variety of temperatures and pressures.
Due to the force of gravity and the pressure created
by the overlying rock layers, oil and natural gas
seldom stay in the source rock in which they are
formed. Instead, they move through the underground
layers of sedimentary rocks until they either escape
at the surface or are trapped by a barrier of less
Most of the world's petroleum has been found trapped
in porous rocks under relatively impermeable
formations. These reservoirs are often long
distances away from the original source.
A seep occurs when hydrocarbons migrate to the
Earth's surface. Over time, huge amounts of these
hydrocarbons have escaped into the atmosphere.
Flowing water can also wash away hydrocarbons.
Sometimes only the lighter, and more volatile compounds
are removed, thus leaving behind reservoirs of heavier
types of crude oil.
The Athabasca oil sands in northeastern Alberta are
one example of a petroleum resource that has lost
its lighter components or fractions. The tar-like
bitumen in the oil sands was formed largely by the
effects of bacterial processes, water flows and
oxidation on the petroleum in the reservoir.