Sound Science Bite: December 13. Two items: Melting Antarctic Ice Shelves and Where Does Hawaiian Lava Come From?

Melting Antarctic Ice Shelves. Published scientific studies show that the West Antarctic Ice Sheet is melting, with possible consequences for future generations. Ice sheets are glaciers covering large areas of a continent and are often referred to as "continental glaciers". All glaciers tend to flow like rivers of ice. Ice sheets flowing into the sea are slowed by ice shelves, which are extensions of the ice sheets made of floating ice. The breakup of ice shelves may allow an ice sheet to flow faster into the sea, possibly leading to its ultimate breakup. The problem is, although floating ice doesn't raise sea levels when it melts, the ice entering the sea from ice sheets does, because it is ice that was originally grounded.

A recent study (Science 346:1227 (2014)) finds evidence that the diminishing of the West Antarctic Ice Sheet is being abetted by warmer water flowing up under Antarctic ice shelves. One component of the two that constitute this water, the Circumpolar Deep Water, has warmed all around the Antarctic continent. Since it has recently be shown that by far most of "global warming" is in the oceans, this situation may accelerate and enhance rising sea levels.

Where Does Hawaiian Lava Come From? A long-lived controversy in geophysics is the origin of volcanism at so-called "hot spots", such as the one under the Hawaiian Islands. There are many others, including the one under Yellowstone National Park. One view believes these hot spots are due to magma rising from deep within the Earth - from the mantle-core boundary. Others think the volcanism is from a thin layer of the upper mantle just below the Earth's crust (of which the continents and sea floor are made) that appears to consist of "partial melting": analogous to water in a bucket of sand where the water is the magma and the sand the unmelted minerals. This layer is called the asthenosphere, and much of the Earth's volcanism away from hot spots is attributed to it. The controversy hasn't been decided, but there is growing evidence in favor of the asthenosphere being the source of magma that fuels Kilauea, among other volcanoes. If this is the case, then there is a problem. Hot spots have long been thought by many to be pretty much immobile due their origin deep in the Earth. Motion of the crust has been measured by its movement over these hot spots. Some re-evaluation may be necessary if the hot spots originate close to the Earth's surface.