DOE - Fossil Energy: How Fossil Fuels Were Formed You are here: Educational Activities > Energy Lessons > Coal-Introduction > Fossil Fuel Formation How Fossil Fuels were Formed Contrary to what many people believe, fossil fuels are not the remains of dead dinosaurs. In fact, most of the fossil fuels we find today were formed millions of years before the first dinosaurs. Formation of Coal Fossil fuels were formed from plants and animals that lived 300 million years ago in primordial swamps and oceans (top). Over time the plants and animals died and decomposed under tons of rock and ancient seas (middle). Eventually, many of the seas receded and left dry land with fossil fuels like coal buried underneath it (bottom). Ten feet of prehistoric plant debris was needed to make one foot of coal. Fossil fuels, however, were once alive! They were formed from prehistoric plants and animals that lived hundreds of millions of years ago. Think about what the Earth must have looked like 300 million years or so ago. The land masses we live on today were just forming. There were swamps and bogs everywhere. The climate was warmer. Ancient trees and plants grew everywhere.

Strange looking animals walked on the land, and just as weird looking fish swam in the rivers and seas. Tiny one-celled organisms called protoplankton floated in the ocean. When these ancient living things died, they decomposed and became buried under layers and layers of mud, rock, and sand. Eventually, hundreds and sometimes thousands of feet of earth covered them. In some areas, the decomposing materials were covered by ancient seas, then the seas dried up and receded. During the millions of years that passed, the dead plants and animals slowly decomposed into organic materials and formed fossil fuels. Different types of fossil fuels were formed depending on what combination of animal and plant debris was present, how long the material was buried, and what conditions of temperature and pressure existed when they were decomposing. For example, oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and "cook" the organic material under layers of silt. In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed.

Over time, some of this oil and natural gas began working its way upward through the earth's crust until they ran into rock formations called "caprocks" that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today. The same types of forces also created coal, but there are a few differences. Coal formed from the dead remains of trees, ferns and other plants that lived 300 to 400 million years ago. In some areas, such as portions of what-is-now the eastern United States, coal was formed from swamps covered by sea water. The sea water contained a large amount of sulfur, and as the seas dried up, the sulfur was left behind in the coal. Today, scientists are working on ways to take the sulfur out of coal because when coal burns, the sulfur can become an air pollutant. (To find out about these methods, see the section " Cleaning Up Coal .") Some coal deposits, however, were formed from freshwater swamps which had very little sulfur in them. These coal deposits, located largely in the western part of the United States, have much less sulfur in them.

All of these fossil fuels have played important roles in providing the energy that every man, woman, and child in the the United States uses. With better technology for finding and using fossil fuels, each can play an equally important role in the future. To read more about these fuels ï¿½ both past and present ï¿½ click on: The Energy Story - Chapter 8: Fossil Fuels - Coal, Oil and Natural Gas Fossil Fuels - Coal, Oil and Natural Gas Chapter 8: Fossil Fuels - Coal, Oil and Natural Gas Where Fossil Fuels Come From There are three major forms of fossil fuels: coal, oil and natural gas. All three were formed many hundreds of millions of years ago before the time of the dinosaurs – hence the name fossil fuels. The age they were formed is called the Carboniferous Period. It was part of the Paleozoic Era. "Carboniferous" gets its name from carbon, the basic element in coal and other fossil fuels. The Carboniferous Period occurred from about 360 to 286 million years ago.

At the time, the land was covered with swamps filled with huge trees, ferns and other large leafy plants, similar to the picture above. The water and seas were filled with algae – the green stuff that forms on a stagnant pool of water. Algae is actually millions of very small plants. Some deposits of coal can be found during the time of the dinosaurs. For example, thin carbon layers can be found during the late Cretaceous Period (65 million years ago) – the time of Tyrannosaurus Rex. But the main deposits of fossil fuels are from the Carboniferous Period. For more about the various geologic eras, go to As the trees and plants died, they sank to the bottom of the swamps of oceans. They formed layers of a spongy material called peat. Over many hundreds of years, the peat was covered by sand and clay and other minerals, which turned into a type of rock called sedimentary. More and more rock piled on top of more rock, and it weighed more and more. It began to press down on the peat.

The peat was squeezed and squeezed until the water came out of it and it eventually, over millions of years, it turned into coal, oil or petroleum, and natural gas. Coal Coal is a hard, black colored rock-like substance. It is made up of carbon, hydrogen, oxygen, nitrogen and varying amounts of sulphur. There are three main types of coal – anthracite, bituminous and lignite. Anthracite coal is the hardest and has more carbon, which gives it a higher energy content. Lignite is the softest and is low in carbon but high in hydrogen and oxygen content. Bituminous is in between. Today, the precursor to coal—peat—is still found in many countries and is also used as an energy source. The earliest known use of coal was in China. Coal from the Fu-shun mine in northeastern China may have been used to smelt copper as early as 3,000 years ago. The Chinese thought coal was a stone that could burn. Coal is found in many of the lower 48 states of U.S. and throughout the rest of the world. Coal is mined out of the ground using various methods.

Some coal mines are dug by sinking vertical or horizontal shafts deep under ground, and coal miners travel by elevators or trains deep under ground to dig the coal. Other coal is mined in strip mines where huge steam shovels strip away the top layers above the coal. The layers are then restored after the coal is taken away. The coal is then shipped by train and boats and even in pipelines. In pipelines, the coal is ground up and mixed with water to make what's called a slurry. This is then pumped many miles through pipelines. At the other end, the coal is used to fuel power plants and other factories. Oil or Petroleum Oil is another fossil fuel. It was also formed more than 300 million years ago. Some scientists say that tiny diatoms are the source of oil. Diatoms are sea creatures the size of a pin head. They do one thing just like plants; they can convert sunlight directly into stored energy. In the graphic on the left, as the diatoms died they fell to the sea floor (1). Here they were buried under sediment and other rock (2). The rock squeezed the diatoms and the energy in their bodies could not escape. The carbon eventually turned into oil under great pressure and heat.

As the earth changed and moved and folded, pockets where oil and natural gas can be found were formed (3). Oil has been used for more than 5,000-6,000 years. The ancient Sumerians, Assyrians and Babylonians used crude oil and asphalt ("pitch") collected from large seeps at Tuttul (modern-day Hit) on the Euphrates River. A seep is a place on the ground where the oil leaks up from below ground. The ancient Egyptians, used liquid oil as a medicine for wounds, and oil has been used in lamps to provide light. The Dead Sea, near the modern Country of Israel, used to be called Lake Asphaltites. The word asphalt was derived is from that term because of the lumps of gooey petroleum that were washed up on the lake shores from underwater seeps. In North America, Native Americans used blankets to skim oil off the surface of streams and lakes. They used oil as medicine and to make canoes water-proof. During the Revolutionary War, Native Americans taught George Washington's troops how to treat frostbite with oil. As our country grew, the demand for oil continued to increase as a fuel for lamps.

Petroleum oil began to replace whale oil in lamps because the price for whale oil was very high. During this time, most petroleum oil came from distilling coal into a liquid or by skimming it off of lakes – just as the Native Americans did. Then on August 27, 1859, Edwin L. Drake (the man standing on the right in the black and white picture to the right), struck liquid oil at his well near Titusville, Pennsylvania. He found oil under ground and a way that could pump it to the surface. The well pumped the oil into barrels made out of wood. This method of drilling for oil is still being used today all over the world in areas where oil can be found below the surface. Oil and natural gas are found under ground between folds of rock and in areas of rock that are porous and contain the oils within the rock itself. The folds of rock were formed as the earth shifts and moves. It's similar to how a small, throw carpet will bunch up in places on the floor. To find oil and natural gas, companies drill through the earth to the deposits deep below the surface. The oil and natural gas are then pumped from below the ground by oil rigs (like in the picture).

They then usually travel through pipelines or by ship. Oil is found in 18 of the 58 counties in California. Kern County, the County where Bakersfield is found, is one of the largest oil production places in the country. But we only get one-half of our oil from California wells. The rest comes from Alaska, and an increasing amount comes from other countries. In the entire U.S., more than 50 percent of all the oil we use comes from outside the country... most of it from the Middle East. Oil is brought to California by large tanker ships. The petroleum or crude oil must be changed or refined into other products before it can be used. Refineries Oil is stored in large tanks until it is sent to various places to be used. At oil refineries, crude oil is split into various types of products by heating the thick black oil. Oil is made into many different products – fertilizers for farms, the clothes you wear, the toothbrush you use, the plastic bottle that holds your milk, the plastic pen that you write with. They all came from oil. There are thousands of other products that come from oil. Almost all plastic comes originally from oil.

Can you think of some other things made from oil? The products include gasoline, diesel fuel, aviation or jet fuel, home heating oil, oil for ships and oil to burn in power plants to make electricity. Here's what a barrel of crude oil can make. In California, 74 percent of our oil is used for transportation – cars, planes, trucks, buses and motorcycles. We'll learn more about transportation energy in Chapter 18. Natural Gas Sometime between 6,000 to 2,000 years BCE (Before the Common Era), the first discoveries of natural gas seeps were made in Iran. Many early writers described the natural petroleum seeps in the Middle East, especially in the Baku region of what is now Azerbaijan. The gas seeps, probably first ignited by lightning, provided the fuel for the "eternal fires" of the fire-worshiping religion of the ancient Persians. Natural gas is lighter than air. Natural gas is mostly made up of a gas called methane. Methane is a simple chemical compound that is made up of carbon and hydrogen atoms. It's chemical formula is CH4 – one atom of carbon along with four atoms hydrogen. This gas is highly flammable.

Natural gas is usually found near petroleum underground. It is pumped from below ground and travels in pipelines to storage areas. The next chapter looks at that pipeline system. Natural gas usually has no odor and you can't see it. Before it is sent to the pipelines and storage tanks, it is mixed with a chemical that gives a strong odor. The odor smells almost like rotten eggs. The odor makes it easy to smell if there is a leak. Energy Safety Note! If you smell that rotten egg smell in your house, tell your folks and get out of the house quickly. Don't turn on any lights or other electrical devices. A spark from a light switch can ignite the gas very easily. Go to a neighbor's house and call 9-1-1 for emergency help. Saving Fossil Fuels Fossil fuels take millions of years to make. We are using up the fuels that were made more than 300 million years ago before the time of the dinosaurs. Once they are gone they are gone. So, it's best to not waste fossil fuels. They are not renewable; they can't really be made again. We can save fossil fuels by conserving energy.

The history of natural gas Early human civilizations, while not understanding its cause, were certainly influenced by natural gas. In what was deemed a mysterious and otherworldly phenomenon, lightning is known to have ignited natural gas seeping from the earthï¿½s surface, where a continuous flame would persist. The most famous of these occurred near 1,000 B.C., on Mt. Parnassus whereï¿½ Near 500 B.C. the Chinese finally discovered its potential use. Seeping gas was corralled into bamboo pipelines, and the transported gas was used to boil sea water, thus separating out the salt and producing drinking water. They may have also drilled wells using bamboo poles and primitive bits. Natural gas was first commercialized in Britain in 1785, where gas produced from coal was used in gas lamps. In modern times, the first well drilled intentionally to obtain natural gas was drilled in 1821 in Fredonia, N.Y.ï¿½ It was 27 feet deep, and was drilled by William A. Hart to get a larger flow of gas than was currently seeping out of the ground. For more history, see .

In the recent past, natural gas was recovered during the process of extracting petroleum and could not be sold profitably and so was simply burned at the well head (ï¿½flaringï¿½). It was dismissed as a useless byproduct of oil production until the second half of the 20th century. Its value now realized, gas is typically re-injected to the petroleum formation, aiding in petroleum recovery and making it available for future use. Natural gas now supplies 23% of all energy produced in the world. What is natural gas? Natural gas is principally methane, CH4, with some ethane (C2H6) and propane (C3H8), and impurities such as CO2, H2S, and N2. Natural gas is odorless and colorless; the slightly sour smell that we associate with the gas coming from a stovetop is due to an odorization process (for safety and leak detection) which adds mercaptan compounds to the end-use gas (more precisely, odorization compounds are mixtures of t-butyl mercaptan, isopropyl mercaptan, tetrahydrothiophene, dimethyl sulfide and other sulfur compounds).

ï¿½ Dry natural gas refers to a purified product that is almost entirely methane. ï¿½ Wet natural gas contains compounds other than methane and ethane. ï¿½ Sour natural gas contains larger amounts of hydrogen sulphide, which is highly undesirable due to corrosion, and results in SO2 formation upon combustion. When we say that natural gas is combustible, we are really just saying that it can be oxidized in a way that releases energy. The simplest form of combustion is for methane, which reacts with oxygen to form carbon dioxide, water, and energy by this reaction: This is what happens when methane ï¿½burnsï¿½. How is natural gas formed? As a fossil fuel, natural gas is formed from the decaying remains of pre-historic plant and animal life. As with petroleum, most natural gas formation is due to the breakdown of prehistoric marine zooplankton. Zooplankton subsist on a diet of phytoplankton, which, in turn, rely upon the energy of the sun to produce organic matter and energy through photosynthesis. Photosynthesis and the formation of organic matter are described in more detail on the PETROLEUM page.

We can think of gas (or oil or coal for that matter) as organic material that is prevented from complete decay.ï¿½ Typically, it will be found at the top of petroleum reservoirs, where it has been formed by the combined action of methanogenic bacteria (they produce methane while they decompose organic material) and through catagenesis (the thermal decomposition of kerogen). As marine sediments are buried deep within the earth, high temperatures and pressures lead to varying degrees of the completion of catagenesis, which is the process that produces both petroleum and natural gas. Higher temperatures and pressures favor the formation of lighter hydrocarbons (natural gas), and so oil/gas formations that are deeper in the earth tend to have a greater ratio of gas to petroleum. A more detailed explanation of the formation of natural gas and petroleum can be found on the PETROLEUM page. Following the production of natural gas through the processes of diagenesis and catagenesis, the newly formed natural gas will attempt to migrate to a new location.

Because the earth is filled entirely by layers of solid (or at significant depths) molten rock, the gas it contains cannot exist within a self-contained ï¿½lakeï¿½, but must decide to live within the small fraction of space (or pores) that exist in these rocks. Like the sponge in your kitchen sink (albeit, less spongy and a bit heavier) certain kinds of rock (mainly sandstone and limestone) contain pores large enough and with enough connections to serve as storage and migration sites for gas or oil or water or any other fluid wishing to call them home. Because the light hydrocarbons that comprise natural gas are lighter than water and rock, those that exist within the earth will tend to migrate upwards until they reach the surface, or are trapped by an impermeable layer of rock Methane is also produced by bacteria (methanogens) that decompose organic matter under anoxic conditions, referred to as biogenic methane. These microorganisms are active in the intestines of most animals, and are responsible for methane release from decomposing landfill waste. In the process of petroleum formation, methane may be formed in this manner during the early stages burial.

A large supply of methane is also present within coal seams, where it is found adsorbed to the structure of the coal; where it was formed by methanogenic bacteria during the decomposition process and also during catagenesis of the forming coal. ï¿½Coalbed methaneï¿½ is found to lack the presence of heavier hydrocarbons (as in hydrocarbon reservoirs) and also sulfur compounds (thus the name ï¿½sweet gasï¿½). This methane is in a near liquid state lining the pores of the coal, and is partially released when pressure in the reservoir declines due to the presence of a withdrawal well. What are methane hydrates? In the search for fuels that are alternative to petroleum and plentiful, a lot of recent talk has centered on what are called methane hydrates (also called methane clathrates or methane ice). Essentially, these are chunks of ice that are saturated with methane or natural gas (during formation, the gas becomes trapped in the structure of the freezing water). Because of their frozen structure, they are to be found in arctic permafrost regions or at the bottom of the ocean in places where conditions are favorable.

As with natural gas, methane hydrates can be formed through biogenic or thermogenic processes: In biogenic formation, the methane is produced by biological activity as microorganisms attempt to decompose the remains of marine life (as above, primarily marine phytoplankton and zooplankton). In this case, methane is produced by the anoxic behaviors of methanogenic bacteria. In thermogenic formation, the gas is formed in the same manner as natural gasï¿½through catagenesis of kerogen. In fact, this may be the same natural gas that was formed above, it just migrates to a region (remember, gas is lighter than earth materials and wants to reach the surface) where the formation of hydrates is favorable. Through either method of formation (biogenic or thermogenic) the gases, once formed, are thought to migrate (perhaps through geologic faults) and, upon contact with cold sea water, to crystallize into ice-like structures. Historically, the U.S. has been able to produce roughly the same quantity of natural gas that it needs to supply energy to the country (about 23% of countrywide energy use).

However, estimates project an increase of 18% in the amount of natural gas that will be required in 2030. Therefore, it is becoming increasingly important to locate further sources of natural gas, and methane hydrates could fill this need. Hydrates are projected to exist in huge quantities (approximately the amount energy contained in all the worldï¿½s other fossil fuels combinedï¿½although, as we have yet to develop a successful extraction method, the fraction of this that is accessible remains an open question). The difficulty arising in the utilization of methane hydrates owes primarily to the development of an efficient extraction process. Beneath the sea floor, hydrates are stable (at these pressures, the ice chunks will remain in their structures up to about 18 degrees Celsius), but once removed, the ice become unstable as pressures decrease and the gas develops a strong desire to escape. Methods for efficient extraction are currently in development. Did you know? CATA (Centre Area Transportation Authority) has been operating their city buses on compressed natural gas (CNG) since 1993.

The fleet currently operates 44 buses and, in 1995, CATA, the Pennsylvania Department of Environmental Protection and Columbia Gas of Pennsylvania joined in a cooperative effort to build a CNG fueling station at CATAï¿½s administrative facility. In 2001 more than 3.8 million riders used CATA buses through Penn Stateï¿½s University Park campus. See . Fossil Fuel Facts Fossil Fuel Facts Fossil Fuel Facts Fossil fuels are essentially the remains of plants of animals. They provide us with a source of nonrenewable energy. Fossil fuels can either come in the form of an oil, coal, or natural gas. They are the result of the decomposition of dead plant and animal matter buried deep in the Earth's crust. These fossil fuels are then pumped from underground and used in a variety of ways. Interesting Fossil Fuel Facts: It takes millions of years for fossil fuels to develop. The fossil fuels we use today began forming during the Carboniferous Period which was before dinosaurs ruled the Earth. Oil, also called petroleum, is pumped from underground and can be turned into products such as gasoline and electricity.

If your house uses natural gas for cooking and heating, this is a form of a fossil fuel that lies underground usually above oil. Coal is another fossil fuel used to generate electricity and is found closer to the Earth's surface. Although fossil fuels are mainly used to make electricity, they are also used to power machines such as cars and planes. Fossil fuels are nonrenewable which means they cannot be made by humans. Fossil fuels in their natural form must first be burned in order to be used as electricity. When fossil fuels are burned, they release unhealthy toxins into the air we breathe. Around 90% of our energy comes from fossil fuels. The energy stored in fossil fuels comes from the sun. The main component of natural gas is methane which is highly flammable. Natural gas has no smell so a chemical called mercaptan is added so that it can easily be detected. Natural gas is pumped to houses by way of underground pipelines that connect directly to the natural gas source. Although renewable sources of energy are better than nonrenewable sources of energy, we continue to use nonrenewable sources because they are easier to obtain. Related Links: ENERGY RESOURCES ENERGY RESOURCES Energy is the force that makes things work.

Over many years, people have learned to produce energy from various sources, such as by burning FOSSIL FUELS and using NUCLEAR and RENEWABLE ENERGY technology. WHAT IS FUEL? Fuel is something that can be burned to produce energy in the form of heat or power. Fuels include coal, oil, natural gas, and wood. Humans have been burning wood for warmth, and to light homes and cook food, since civilization began. However, in some parts of the world, so many trees have been cut down for fuel that firewood is now scarce. FOSSIL FUELS Fossil fuels are made of the fossilized remains of living things that died millions of years ago. Coal, oil, and gas are the most important fossil fuels. Today, they provide most of the world’s energy. Plant remains decay to make brown, crumbly peat when they are buried and squashed. Over many thousands of years, more layers build up on top, and heat and pressure convert the remains first to lignite, then to bituminous coal, and finally to anthracite, the highest grade of coal. HOW DID OIL AND NATURAL GAS FORM?

Oil is formed from the remains of tiny plants and animals that were buried and squashed on the seabed. Natural gas forms in a way similar to oil and is found trapped in underground reservoirs. HOW OIL FORMS 1. Decaying plants and animals sink to the sea floor and are buried by layers of sediment. 2. Heat and pressure increase as the sediments sink. The organic remains become oil and gas. 3. Gas and oil rise through porous (holey) rock and collect below a layer of solid rock. HOW ARE FOSSIL FUELS USED TO PRODUCE ENERGY? Power plants burn fossil fuels to heat water and produce steam. The steam pushes around turbines in a generator, which then spin a magnet, creating an electric current. Oil refineries process oil to make fuel to run cars, trains, ships, and aircraft. NUCLEAR ENERGY In the 1930s, scientists discovered that huge amounts of energy could be produced by splitting atoms (tiny particles) of a rare mineral called uranium. The technology was first used to make nuclear bombs. Later, it was used to generate electricity. WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF NUCLEAR ENERGY?

Nuclear fuel does not release the same polluting gases as fossil fuels, and does not use up the Earth’s natural resources. However, uranium and other nuclear materials are radioactive—they give off radiation that can harm living things. This means that great care must be taken when they are processed and transported. Nuclear fuel is also difficult to dispose of safely once used. RENEWABLE ENERGY Renewable energy is generated using the power of natural forces such as sunlight. Fossil fuels release harmful waste gases when burned, and they cannot be replaced when their limited supplies are used up. Renewable energy, however, will not run out and produces little pollution. WHAT ARE THE SOURCES OF RENEWABLE ENERGY? Most of the energy on Earth comes from the Sun. Solar power uses the energy in sunlight directly. Moving water (in the form of waves, tides, and flowing rivers) and the wind also contain energy that can be used to spin turbines to generate electricity. Geothermal energy taps heat from inside Earth. WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF RENEWABLE ENERGY? Renewable energy will last as long as the Sun shines, winds blow, waves crash, and rivers flow.

These technologies are safe to use and do not pollute the environment. However, renewable energy plants can be expensive to build and may not produce enough energy to meet local requirements. FIND OUT MORE Fossil | Department of Energy Fossil Ever wonder how carbon capture works? This infographic breaks it down. | Graphic by Carly Wilkins, Energy Department. Fossil energy sources, including oil , coal and natural gas , are non-renewable resources that formed when prehistoric plants and animals died and were gradually buried by layers of rock. Over millions of years, different types of fossil fuels formed -- depending on what combination of organic matter was present, how long it was buried and what temperature and pressure conditions existed as time passed. Today, fossil fuel industries drill or mine for these energy sources, burn them to produce electricity, or refine them for use as fuel for heating or transportation. Over the past 20 years, nearly three-fourths of human-caused emissions came from the burning of fossil fuels. The Energy Department maintains emergency petroleum reserves , ensures responsible development of America’s oil and gas resources and executes natural gas regulatory responsibilities.

In addition, scientists at the Energy Department’s National Labs are developing technologies to reduce carbon emissions and ensure fossil energy sources play a role in America’s clean energy future. Featured Power Plants—The Origin of Fossil Fuels - Brooklyn Botanic Garden Power Plants—The Origin of Fossil Fuels By Janet Marinelli | June 1, 2003 Tree-size mosses of the carboniferous era As we see daily proof, fossil fuels make the world go round. For the past century, and especially the past 50 years, they've created vast fortunes for individuals and entire nations, powered the global economy, and provoked geopolitical tensions and sometimes wars. No news there. But did you know that current events on this petroleum-addled planet are closely linked to weird and wonderful plants that lived hundreds of millions of years ago? Recent international developments propelled me to do some paleobotanical detective work. Many people think that fossil fuels—oil, coal, and natural gas—come from the bodies of dinosaurs that ruled the earth during the mid to late Mesozoic era, 213 to 65 million years ago. Not so. The origin of oil is still a matter of scientific controversy.

However, it is generally believed that the world's great oil deposits were formed from diatoms. These tiny, mostly single-celled, photosynthetic organisms were, and still are, extremely important components of phytoplankton, the basis of the food chain for all marine and freshwater animals. When viewed with a microscope, they look like little jewels in a splendid variety of forms—round, triangular, fan- and barrel-shaped, twisted like rotini pasta—and in brilliant shades of emerald and peridot. Ancient diatoms eventually became not only petroleum but also diatomaceous earth, the nontoxic pest control so popular with modern-day organic gardeners. An estimated 5,600 species of diatoms are alive today, and many more species have gone extinct; scientists surmise that at least 40,000 species have been around at one time or another since life began on this planet, about 3.5 billion years ago.

The diatoms that became the crude oil so coveted today lived during the Jurassic period, 213 to 144 million years ago, the age when Stegosaurus, Apastosaurus (better known as Brontosaurus), and other giant reptiles dominated the earth's terrestrial environments. The diatoms were deposited on the beds of inland seas, lagoons, lakes, and river deltas, then transformed into liquid gold by forces like high temperature and geologic pressure. The origin of natural gas, like that of oil, is not entirely clear. The leading theory is that it also came from ages-old diatoms, but under different degrees of heat and pressure. Gigantic Club Mosses A slightly different alchemy resulted in the planet's vast coal deposits, many of them in the United States. But the plants that produced them were a lot bigger, and even more bizarre and beautiful than diatoms. Coal formed from the dead remains of magnificent trees, ferns, and other primitive plants that lived in lush swamp forests during the Carboniferous period, 360 to 286 million years ago, long before a single dinosaur roamed the land. Back then, the various continents we know today were positioned very differently.

For most of the period, Laurentia (ancient North America) and Baltica (ancient Europe) were pretty much a single landmass located near the equator. The climate was warm and humid year-round—in other words, perfect for the proliferation of plant life, especially pteridophytes, or plants that reproduce by spores. Not surprisingly, under such ideal conditions, size was a major factor in the evolution of Carboniferous plants. Some plants reacted to competitive pressures by becoming taller, while others adapted to life beneath these giants. Great swamp forests grew in lowland areas, dominated by gigantic arborescent plants that were very different from the flowering trees and conifers found in today's tropical, temperate, and boreal forests. The most imposing plants in these wet forests were colossal lycopods, the ancient relatives of modern club mosses such as Lycopodium, which today generally grow just a few inches tall. The tree-sized lycopods of the Carboniferous soared to 130 feet tall. These were truly primitive-looking creatures. The evidence suggests that the largest lycopods had a main trunk with a shaggy crown of dichotomous, or occasionally bifurcating, branches.

Their leaves were long and grasslike, and their reproductive structures were spore-bearing cones. Their trunks contained very little secondary wood; instead, the plants were supported by a thick, barklike periderm that enclosed soft, spongy tissue. Without enough water to keep the internal cells fully expanded, the plants would have collapsed under their own weight. As the climate of Euramerica began to change toward the end of the Carboniferous, the swamplands began to dry up, and the large lycopods disappeared, in geological terms, almost overnight. Second in height only to the great lycopods was another group of pteridophytes, magnificent tree ferns. Growing to 65 feet tall, with large, compound fronds, these were even more impressive than their descendants, the cyatheas and dicksonias, tree ferns that survive today. Unlike the lycopods, ferns diversified and increased in numbers when the Carboniferous came to a close.

In fact, they apparently were so abundant in at least some later Jurassic floras that they might well have been the dominant herbaceous plants on land—imagine a brontosaur straining its enormous neck to graze on the tasty frond of a 60-foot-tall fern. Today, ferns continue to flourish, numbering just over 10,000 species worldwide. Huge Horsetails and Unlikely-Looking Conifers Tremendous horsetails were another important spore-bearing component of Carboniferous plant communities. Called calamites, they resembled gigantic versions of modern-day Equisetum and often grew in dense clumps like bamboos. Unlike today's horsetails, which top out at 3 to 6 feet, the calamites reached a height of about 50 feet. Like modern horsetails, however, they consisted of a branched aerial portion growing from an underground rhizome, and both their leaves and branches grew in circles at nodes along the stems.

(Some artists' renditions look a little like chubby asparagus spears cloaked with whorls of branches cloaked in turn with whorls of lance-shaped leaves.) According to paleobotanists, even the stems were remarkably similar to those of still-existing horsetails, except that they had some secondary xylem, or vascular tissue, that accounted for the plants' considerable height and girth. Calamite spores were contained, like those of the giant club mosses, in cones. Calamites made it through the climatic transitions at the end of the Carboniferous, but subsequently declined. Today, horsetails exist as a single genus, Equisetum, consisting of about 20 species distributed around the globe. Carboniferous swamps also provided habitat for two groups of early seed plants. Many Cordaites, odd-looking early conifers with long, straplike leaves, were tall (50 to 100 feet), highly branched trees that formed extensive forests. Their leaves, up to three feet long, were arranged spirally at the tips of the youngest branches. The Cordaites produced male pollen-bearing cones and female seed-bearing conelike structures on separate branches.

The other large group of primitive seed-bearing plants were the "seed ferns." They had large, pinnately compound fronds so fernlike that they were long regarded as ferns. However, instead of reproducing via spores as true ferns do, they produced walnut-size seeds directly on the foliage. Some were tall, woody trees. Both the Cordaites and the seed ferns eventually disappeared, although more advanced seed-bearing plants were soon to take over the world. Herbaceous lycopods, herbaceous trailing and scrambling horsetails, small, shrubby, or scrambling seed ferns, and a variety of small and medium-size true ferns covered the floor of Carboniferous forests. Stalking these ancient swamps were early reptiles, cockroaches as big as house cats, and dragonflies with the wingspans of modern hawks. Many of the plants and animals of the Carboniferous swamps were dependent on abundant supplies of water. In order for spore-bearing plants to reproduce, the sperm cells must swim through water on plant surfaces to reach the eggs.

When tropical areas became drier and huge glaciers began to cool temperate as well as polar areas during the succeeding period, the Permian, most of the plants that flourished in the great swamp forests became extinct. Conifers, cycads, and other seed-bearing plants were better adapted to the drier conditions that came to dominate the next great era of life on earth, the Mesozoic. Yet even today, at the dawn of the 21st century, we continue to grapple with the legacy of diatoms and pteridophytes of eons past. Janet Marinelli is the former director of publishing at BBG. Her book, Plant, published by Dorling Kindersley, showcases 2,000 species worldwide that are threatened in the wild but alive in cultivation. Share This Article

Question: Which fuel is formed by the fossilization of plants?
Answer: Coal