Index fossils are used to identify geologic periods or faunal stages in rock layers. These fossils must have a wide geographic distribution and manifest obvious evolutionary trends to help geologists and others who need this geological data to better understand the time scale and to pinpoint a time period. For example, ammonites were common during the Mesozoic Era, but they were extinct by the Cretaceous period. Geologists would use ammonites to help determine this time frame if present.
A hot spot develops above the plume. Magma generated by the hot spot rises through the rigid plates of the lithosphere and produces active volcanoes at the Earth's surface. And as oceanic volcanoes move away from the hot spot, they cool and subside. As a result they produce older islands, atolls, and seamounts.
Which aspect of government does the United States have in common with the Roman Republic? Two consuls run the executive branch. Senators are elected into office for life. There are three branches of government.Oct 22, 2014
