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Elements of Rocks

The earth, along with the rest of the solar system, condensed from the solar nebula about 4.6 billion years ago. Heavy elements, such as iron and nickel, sank to the center of the earth, and lighter elements moved toward the surface, because of their respective densities. As the earth cooled, it solidified to form the first rocks. Volcanism produced and still produces more rocks, which are called igneous rocks. At a much later date, water eroded the surface rocks to form particles, which formed the first sediments of the ocean floor. These were to become sedimentary rocks. Much later, metamorphic rocks were formed from the preexisting rocks.

Igneous Rocks
- Igneous rocks formed from hot molten rock beneath the earth's crust, and more than 99 percent of the chemical makeup of igneous rocks consists of nine elements. These are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium and titanium. Most minerals found in igneous rocks contain silicon and oxygen in the form of silicates. The earth's central core is composed of 90 percent free iron and 9 percent nickel. Lighter elements, such as oxygen and aluminum, do not exist at the core but are concentrated in the earth's crust.
Sedimentary Rocks
- Sedimentary rocks form with the weathering of either igneous or metamorphic rocks. This occurs through abrasion, cracking, chemical changes and biological means. Carbon is a major constituent of sedimentary rocks in the form of carbonates, and calcium is present, especially in marble and limestone, in the form of a residue from the decay of organic material. About 75 percent of the earth's land regions are composed of sedimentary rocks on top of an igneous base.
Metamorphic Rocks
- Metamorphic rocks are rocks that have been altered by pressure, heat or water acting on preexisting rocks, usually igneous and sedimentary rocks, but sometimes other metamorphic rocks. The major elements that make up these rocks are a mixture of those that make up igneous and sedimentary rocks. Metamorphic rocks include anthracite, gneiss, marble, slate, schist and serpentine, and the elements present are usually in the form of minerals, often in crystalline form.
Other Elements
- Although just nine elements comprise 90 percent of the igneous rocks, all of the elements found on earth can be found in some form or other in sedimentary and metamorphic rocks. Most elements occur as compounds or minerals. A few, such as gold, generally occur in natural form. Palladium, platinum and silver can also be found in natural form. Mercury is usually found in cinnabar, which is 86 percent mercury, but can occasionally be found in native form.

Rock Collecting