Geologic History

Front

What group of organisms survived mass extinctions at the end of the Paleozoic and Mesozoic Eras?

Back

Gastropods.

Front

What is the basis for subdividing geologic time into periods?

Back

Fossil evidence.

Front

If a layer of sandstone lies on top of a layer of limestone containing a 450-million-year-old fossil, what can be inferred about the sandstone layer?

Back

It is younger than 450 million years.

Front

How many years ago did a piece of timber containing one-fourth the amount of carbon-14 found in a living tree die?

Back

About 11,400 years ago.

Front

Which gas was absent in Earth's earliest atmosphere that is essential for life today?

Back

Oxygen.

Front

What is the significance of mass extinctions in geologic history?

Back

They mark significant changes in biodiversity and the evolution of life.

Front

What is the principle of superposition in geology?

Back

In undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest are at the top.

Front

What role do fossils play in understanding geologic time?

Back

Fossils provide evidence of past life and help date rock layers.

Front

What is radiometric dating and how is it used in geology?

Back

Radiometric dating measures the decay of radioactive isotopes to determine the age of rocks and fossils.

Front

What are the main eras of the geologic time scale?

Back

The main eras are the Paleozoic, Mesozoic, and Cenozoic.