Exchange Cycles - How We Find Out

The Making of Oil

Automobiles and industrial factories run on oil and coal. How are oil and coal made? Discovering the answer to this question will help to explain how we know about the recycling of carbon within ecosystems.

Oil and coal are hydrocarbons (made up of hydrogen, about 15% , and carbon, about 85%). To read about a specific example of a hydrocarbon, click here. The high-energy fat stored in the body is also made up of hydrocarbons. The excess carbon compounds come directly or indirectly from eating the carbon compounds that plants create during the process of photosynthesis.

How do we know that the carbon in oil and gas came from dead plants and animals? Couldn't the carbon come from volcanoes or hot springs? The main evidence is that volcanoes only produce simple arrangements of hydrogen and carbon. The hydrocarbons in oil are very complex and varied. To make large amounts of oil and gas requires high pressures and temperatures -- and lots of time (tens of millions of years). The pressure and temperature can be created when plants and animals become buried under deep layers of soil and rock. That is why people have to drill so deep into the earth to find oil.

Much of our oil and gas probably came from small animals and plants (plankton) in the ocean (remember that long ago much of the world was buried under water). Also, some of the carbon in oil could have washed into oceans from rivers.

We have discovered fossil remains of plants that existed around 300 million years ago during a time labeled the Carboniferous period. We can determine how long ago the plants existed by examining the carbon material of the plant fossils in a procedure known as radioactive dating. Essentially, fossils are mineralized organic materials. When oilmen look for oil, they look for earth layers that were formed during the Carboniferous period. When a well is drilled, samples of rock from different depths are examined under the microscope to see if fossils from small invertebrates and plants from that time period are present. If so, odds are that they might hit oil soon.

Fossils from the Carboniferous period indicate that dinosaurs flourished by eating land plants that grew in abundance because of the very warm and wet climate. The earth was most likely a huge jungle. Plants and dinosaurs even grew at the North Pole! And we think there is global warming today! This means that our automobiles get their carbon and their energy from the same source as dinosaurs did 300 million years ago.

Different drainage areas of the United States that create the Mississippi River. The "hypoxic zone" seen at the point where the river empties into the Gulf of Mexico contains so much dirt and plant matter that there is very little oxygen in the water.

Oil is found deep in the ground, sometimes several miles deep. How do these layers of plant and animal matter get buried so deep? As mountains push up from beneath the earth, rain erodes them, washing away soil and even cutting massive canyons. The sediment contains soil, sand, and plant and animal carbon, carrying it all away in rivers that dump into the ocean. As the sediment settles, it builds up layer upon layer of carbon-rich material on the ocean floor. The oil that forms eventually floats up into porous sand formations (which used to be beaches and river sand bars that have long since become buried).

The big problem with fossil fuels is that burning them pollutes our air and water with toxic compounds formed from the burning. All big cities have so much air pollution that on many days the air actually looks dirty. Another problem is that the earth's supply of fossil fuels may be used up in a few decades. We can get energy in other ways that are not harmful to the environment (such as windmills, systems for trapping sun energy, and fuel cells. However, these sources usually cost more than fossil fuels.