Geology

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include sandstone...

No! After the dinosaurs died out, nearly 65 million years passed before people appeared on Earth. However, small mammals (including shrew-sized primates) were alive at the time of the dinosaurs. Some scientists who study dinosaurs (vertebrate paleontologists) now think that birds are direct descendants of one line of carnivorous dinosaurs, and some consider that they in fact represent modern...

Dinosaur communities were separated by both time and geography. The 'Age of Dinosaurs' (the Mesozoic Era ) included three consecutive geologic time periods (the Triassic, Jurassic, and Cretaceous Periods). Different dinosaur species lived during each of these three periods. For example, the Jurassic dinosaur Stegosaurus had already been extinct for approximately 80 million years before the...

Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database . Some geologic maps can be purchased in hard copy through the USGS Store . Download digital geologic maps for entire states from the USGS Mineral Resources Online Geospatial...

Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website. The website has listings for regions of the country; selected parks are listed within each region...

There are many natural processes other than impacts that can create circular features and depressions on the surface of the Earth. Examples include glaciation, volcanism, sinkholes, atolls, salt domes, intrusions, and hydrothermal explosions (to name just a few). Prehistoric mines and quarries are also sometimes mistaken for impact craters. Although the USGS has been involved in impact crater...

Regulatory limits for safe levels of elements in water and foodstuffs are established by the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA). However, there are generally no regulatory limits that scientists can refer to when dealing with plants, soils, rocks, and sediments. Therefore, to determine whether a plant, soil, rock, or sediment contains a 'high...

Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Intrusive Igneous Rocks: Intrusive, or plutonic, igneous rock forms...

Some fossil photos can be viewed and downloaded from the USGS Photographic Library and our Multimedia Gallery . Fossil photos can also be viewed as published plates within many online USGS publications. Visit the USGS Publications Warehouse to search for publications. The best keywords for searches are author names, such as William Cobban, Norm Silberling, and Glenn Scott. The USGS fossil...

Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...