Mari Lynn Haley, age 11, of Ashland, Kansas, for her question:
How does gas form deep in the earth?
Our gorgeous planet seems to have a gassy tummy. Here and there, plumes of steamy smoke and gaseous vapor well up from cracks and vents in the earth's crust. These gaseous substances are related to the solid rocks in the same way that steam is related to solid ice.
Our entire earth is built from 90 or so different chemical elements. Atoms of assorted elements unite in molecules that form thousands of different substances. All these substances, however, must obey certain set rules of nature. They are changed by changing temperatures. But each substance obeys these rules in its own way. At normal earth temperatures, most of the crustal rocks are frozen solid. But with more heat, they become liquids and with still more heat certain substances in them may become gaseous vapors. And each substance has its own melting point and its own boiling point.
The crust of the earth is restless and changeable. Sometimes forces of heat and pressure mount up far below the surface. This happens around the deep roots of volcanoes where heat and pressure changes melt the solid rocks into molten pools of buried magma. The heat may reach several thousand degrees. This is hot enough to change certain of the molten minerals into gaseous vapors. A lot of the gases trapped deep in the earth are formed by this kind of volcanic activity.
An erupting volcano has been known to belch 16,000 tons of steamy gases a day along with 100,000 tons of molten lava. Most of this cloudy gas is steamy water vapor. Some of it was separated from the mixture of substances in the molten minerals. But earth scientists suspect that most of it was ground water from rain that seeped down through the rocks. Heat from the seething volcano boiled it and changed it to clouds of frothy steam.
Volcanic heat also creates carbon dioxide and sulphurous gases. Substances such as phosphorus and fluorine dissolve in the molten mixture and combine to form an assortment of other gases. When the eruption quiets down, these gases often are trapped in underground pockets below layers of solidified lava. They spout up with the water of hot springs and geysers and sometimes in fountains of plumy gases called fumeroles. Volcano made gases are mostly super heated ground water. But other, very different gases also become trapped deep within the earth's crust.
One of these is called natural gas. This useful product is mostly methane gas but actually it is a mixture including ethane and butane, propane and pentane. We find these valuable gases above underground reservoirs of petroleum trapped under rocky roofs. They are the lightest elements in the petroleum, light enough to become gases at normal temperatures. They rise to the top of the buried oil.
The two main reasons why gases form in the earth's tummy are volcanic activity and buried fossils, rich in hydrocarbon chemicals. These hydrocarbon chemicals were created by living plants and animals in ancient times. When buried, the complex chemicals may change into coal or oil. The lighter materials float to the top, become gases and usually escape through cracks to the surface. But when oil bearing rock is sealed below a dense rocky roof, these light gases may form buried pockets of natural gas.