The history of life on our planet is a long and winding path. Much of what we know about how life has evolved comes from fossils. The physical remains of organisms and the evidence of behaviors, such as footprints, tell us where, how, and who ancient life was. Uncovering the fossil record requires scientists with many different areas of expertise, including geology. Understanding geology is critical to studying paleontology because fossils are preserved in rock. The characteristics of the rocks that hold fossils can be as informative as the fossils themselves.
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There are three main types of rock: igneous rock, metamorphic rock, and sedimentary rock. Almost all fossils are preserved in sedimentary rock.
Organisms that live in topographically low places (such as lakes or ocean basins) have the best chance of being preserved. This is because they are already in locations where sediment is likely to bury them and shelter them from scavengers and decay. Mudstone, shale, and limestone are examples of sedimentary rock likely to contain fossils. As the layers of sediment build up on top of one another, they create a physical timeline. The oldest layers, along with the organisms that were fossilized as they formed, are deepest. The youngest layers are found at the top. Reading the layers is complicated by the fact that as continents move and mountains rise, the layers are often tipped sideways and altered in other ways.
Throughout the earth”s history, each of the three rock types has been continuously formed and recycled into other types of rock in a process called the “rock cycle.” This makes the rock record fragmentary and difficult to read, but enough rock has been produced over time to tell us a lot about the history of our planet.
Igneous rock is formed by the cooling of molten rock, either at the surface (lava), or underground (as granite or other similar rocks). The heat of molten rock usually incinerates organisms rather than preserving them. There are some exceptions, such as tree trunks surrounded by lava and preserved as casts, but these are rare.
Metamorphic rock is formed when existing rock is transformed under high heat and pressure, which generally destroys any fossils that were in the rock before it was metamorphosed. For example, marble is created when limestone is pressure-cooked within the earth, causing the minerals that compose the stone to re-form into new crystals.
Sedimentary rock is formed by the accumulation of minerals and other material. As rocks at the earth”s surface are broken down, or “weathered,” the sediment is moved by forces such as water and wind and deposited elsewhere in layers. Over time, these layers build up and solidify, becoming sedimentary rock. Organisms can be preserved as fossils if their bodies are buried within these layers.
Interactive Geology ProjectIGP is a group of scientists and animators based at the University of positiveeast.org Boulder whose mission is to illustrate basic principles of geology in entertaining and accessible ways.
A Brief History of positiveeast.org Through TimeThis 25-minute movie illustrates the geologic evolution of positiveeast.org through time.
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