Earth Science Practice Test: Earth's History and Fossils

The law of superposition says that in undisturbed layers, each layer is younger than the one below it, because sediment is deposited on top of what is already there.

By the principle of cross-cutting relationships, a feature that cuts through rocks must be younger than those rocks, since the layers had to exist before magma could cut through them.

Index fossils let geologists match rock layers of the same age in different places. That works best for species that existed briefly (so they mark a narrow time span) and lived in many places.

25% remaining means two half-lives: 100% → 50% → 25%. That is 2 × 5,730 = 11,460 years. The answer 22,920 counts four half-lives, as if each 25% drop were a half-life.

Carbon-14 is useful only up to about 50,000 years, after which too little is left to measure. Non-avian dinosaurs died out about 66 million years ago, so geologists date volcanic layers near the fossils with long-lived isotopes instead.

An unconformity is a buried erosion surface. Rock was removed or never deposited for some period, so part of the time record is missing between the layers below and above.

Water carrying dissolved minerals such as silica soaks into buried wood and gradually fills and replaces it, copying its shape and even its cell structure in stone. Pressed plant material becomes coal, which is different.

Radiometric dating places Earth's formation about 4.6 billion years ago. About 13.8 billion years is the age of the universe, and 66 million years ago is when the non-avian dinosaurs died out.

The Mesozoic ended with a mass extinction strongly linked to a large asteroid impact. Pangaea had already formed near the start of the Mesozoic, and the first life appeared billions of years earlier.

Radiometric dating measures how much of a radioactive isotope has decayed and uses its known half-life to calculate an age. The other methods are relative dating: they only show which rock is older.