The rock family tree begins with volcanic activity and the formation of ‘igneous’ rocks. These chemically rich rocks break down from weathering, producing gravels, sands, silts, and clays. As the finer sediments of clays, silts, and sands are deposited on the shallow ocean floor, they compact and become solidified into ‘sedimentary’ rocks. As sedimentary rocks are forced deeper into the Earth’s crust, pressure and heat alter their structure through metamorphosis. ‘Metamor-phic’ rocks melt through volcanic activity and the cycle begins again. Finer divisions are made within these three rock categories. For example, there are two basic types of igneous rocks, intrusive (such as diorite, granite) and extrusive (such as basalt, andesite, dacite, rhyolite, obsidian). Intrusive rocks are forced upward from molten rock beneath the surface and cool within the crust; some are later exposed through erosion. The cooling is slow, allowing crystals to form. These igneous rocks are coarse. Extrusive igneous rocks are ejected by volcanic activity and cool more rapidly, leading to smaller crystals and finer structures. All of the extrusive igneous rocks can be shaped by chipping. Silica content, however, increases from basalt, andesite and rhyolite to obsidian. Obsidian, a natural glass, has the highest silica content and is rapidly cooled magma, which cools so quickly that crystalline structure does not have time to form.
Sedimentary rocks are grouped into three classes: clastic, chemical, and biological. Clastic rocks include sandstone, shale, mudstone, siltstone, and conglomerate. Chemical rocks include limestone, dolostone (dolomite), and evaporates such as gypsum and salt. Biologic rocks include coal and chert. Chert and chalcedony are formed by the same process, but chalcedony has a more fibrous structure and is characteristically semi-translucent. A common term for chalcedony is agate. Both chert and chalcedony are replacement materials within fossil casts in sedimentary deposits. An example of replacement is silicified wood where chalcedony has replaced the cellular structure of the decayed wood.
World History
