The Inside of Earth: Deep-Earth Science from the Top Down

Abstract

Earth is really several planets. Which planet you see depends on where you view it from. Looking at it from outside, from space, stripped of clouds, you can see that Earth has two quite different hemispheres—a continent hemisphere and an ocean hemisphere. The latter, the Pacific hemisphere, is underlain almost entirely by one gigantic tectonic plate—a continuous chunk of Earth’s crust— which is diving under what is called the ring of fire because of the volcanoes that line the plate boundary along Oregon, Washington State, British Columbia, the Aleutians, the Kuriles, Japan, the Marianas, Tonga-Fiji, South America, and Central America. There are also volcanoes in other places: along other plate boundaries on the sea-floor and in island chains throughout the Pacific. One of the unanswered questions in geology is: why are there volcanoes in some places? In my own work, I turn the question around and ask: why aren’t there volcanoes everywhere? For seismology tells us that there is a semimolten layer underneath the plate almost everywhere. Something is keeping it down.