Many geologic processes are almost unenduringly slow in human terms. Our experience is so riveted in processes and activities that take place in seconds, days, and months that it is difficult to imagine an event lasting millions of years. We can experience a wave crashing on a seashore, but how do we "experience" one continent crashing into another? Mountains stand as a cultural icon of permanence, yet in Earth's history, enormous ranges have risen and been worn down not once, but many times. Continents move with a slowness imperceptible to all but our most sensitive instruments, but the pieces that make up continents have danced around the circumference of the globe a dozen times since their formation.

Paleogeography, or the geography of Earth in ancient time, vividly shows how much Earth has changed through time. Not only have the location and outlines of land masses changed, but Earth's climate has changed from tropical to glacial to temperate and back again many, many times. The very composition of Earth's atmosphere has since dramatically changed as well. And, of course, the location, type, and variety of life forms have ebbed and flowed across the ephemeral landscape. It is interesting to realize, for example, that for most of Earth's history, only simple, single-celled life forms existed. Multi-celled life appeared only in the last 15 percent of geologic time, land plants and animals during the last 10 percent, and human beings during the last 0.01 percent!

Paleographic maps are provided in several places in the Middle School Earth Science Explorer. These maps reveal the changes Earth has undergone--how some places that are now high and dry were once low and wet, and vice versa. Every place on Earth has undergone a long series of environmental changes. Seas, lakes, ancient mountains, and deserts might all in turn have occupied the place where your school now stands. Paleogeographic maps can be reconstructed with any accuracy only back about 600 million years because the dynamic nature of Earth's surface has progressively destroyed rocks containing evidence of past life and climate. The very earliest maps in the "Geologic Time" section, such as the Hadean Earth map, are conjectural but based on our current understanding.

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