1. Education
Send to a Friend via Email

Geological Sphere-words


Geologists use a lot of "-sphere" words in their work—well, mainly in the papers they write, and in textbooks. At their best, the words are good shorthand. At their worst, they're a Victorian fetish. And they don't always agree with what teachers tell children. So to help you keep things straight, here's a guide to the various -sphere words, starting from the largest and ending with the deepest.

Extraterrestrial -Spheres

The heliosphere, often thought of as the Sun's atmosphere, encloses the whole solar system by definition. It matters for Earth because the solar wind, the constant flow of electrically active gas from the Sun, interacts with the Earth's magnetic field in ways that affect the geologic record. For instance, radiocarbon and other cosmogenic isotopes are influenced by the state of the heliosphere.

The magnetosphere is the large area around the Earth where our planet's magnetic field controls the motion of charged particles. Think of it as a bubble in the heliosphere where Earth holds sway. It has a teardrop shape, the blunt end extending roughly 40,000 kilometers toward the Sun and the tail reaching a much greater distance in the other "downwind" direction. Changes in the geomagnetic field affect the magnetosphere, particularly reversals, during which the magnetosphere is very small.

The Soft -Spheres (and a Zone)

The atmosphere—the air layer—is familiar from school and everyday life. In school we're taught that the atmosphere is where weather happens. Later in school we're taught that weather happens in the bottom layer of the atmosphere, the troposphere, and that above it lie several other layers (stratosphere, mesosphere, ionosphere and exosphere). Geologists care about those layers and include them among their -sphere words.

The other -spheres in this category don't form clean, spherical layers. They're conceptual -spheres that are nevertheless as real as rocks to Earth scientists.

The biosphere or ecosphere is the only one of all these -spheres that is growing. It means the zone where life is possible, and it extends deep underground as well as high in the air. In the last several decades, we've documented living things—mostly microbes—in the air above storms, everywhere in the sea, in the driest deserts, and far beneath the seafloor in solid rock. Life persists inside glacier ice, in caustic mine acids, in beds of salt and at temperatures well above the boiling point. So we still don't really know the size of the biosphere, but everywhere life exists it strongly affects the chemistry of the environment, which in turn shows up in the geologic record. Because life is found everywhere on Earth, the biosphere is a truly global layer and is taught in school as a genuine -sphere.

The hydrosphere is everywhere that water moves and rests, whether in vapor, liquid or ice. Some people exclude the atmosphere, but groundwater and surface water of all kinds are included. Water vapor in the soil is part of the hydrosphere. So is hydrothermal water. Because water is present, somewhere, beneath every square meter of the Earth's surface, the hydrosphere too is presented in schools as a true -sphere.

The cryosphere is the part of the hydrosphere consisting of ice and frozen ground. This is an easy division to make because the various physical and chemical processes that take place in ice are very different from those in water. And life in the ice, while it's present and interesting, is far less active than it is in water and can usually be neglected.

The pedosphere is the smallest -sphere, yet the most important of all. It's the all-of-the-above part of the Earth where soil exists and soil-forming processes operate. The pedosphere is the arena where rock, sediment, water and life exist and interact in rich intimacy, and so do many different scientific specialists. Geologic processes in the pedosphere include weathering of all kinds and the formation of surface minerals.

Related to the pedosphere is the critical zone, a new concept that encompasses everything from the treetops to the bedrock. The Critical Zone Observatories project is a major research effort to study "Earth's outer skin—where water, atmosphere, ecosystems, soil and rock interact."

The Hard -Spheres

The lithosphere is the outermost layer of the solid Earth that is strong, hard rock (lithos is "rock" in scientific Greek). All of plate tectonics is based on this definition, so it's worth learning. However, schoolchildren are taught that "lithosphere" means the whole solid Earth. I think that's unfortunate. For one thing, not everything in that "lithosphere" is a rock, and for another, geologists don't use the word that way, just as physicists don't speak of "weight" the same way we do. But the big unfortunate thing is that it makes the solid planet sound like just an undifferentiated ball of rocks. That's not so: Only the outermost shell is rocky in the everyday sense. Fortunately, kids don't seem to remember the word at all.

The asthenosphere is the layer of hot, softer rock that lies beneath the lithosphere. Its yielding nature gives the lithosphere a way to move around in the system of plate tectonics. The asthenosphere ("strengthless layer") is limited by basic physics to a narrow depth range: at its base, high pressure overcomes the effect of high temperature to make the rock stiff again.

The tectosphere is a concept proposed in 1975 to mean the part of the solid Earth capable of plate motion. It differs from the lithosphere in including parts of the asthenosphere beneath the continents that appear to be attached to the overlying lithosphere. It's used most often to refer to continental "roots" that extend deep into the mantle.

The perisphere is a concept proposed in 1995 to mean the part of the uppermost asthenosphere that produces a major set of magmas that is enriched in trace elements. It's a key part of an Earth model that does not include mantle plumes. Like the tectosphere, the perisphere may gain or lose prominence as science moves on, but for now it's little known and not widely cited. If you search for the term on the web, be sure to add "geology" or all you will see is references to the Perisphere, the signature building of the 1939 New York World's Fair.

The mesosphere is the name for the part of the mantle beneath the asthenosphere. You won't see it used much, for two reasons. First, there's a part of the atmosphere with that name, the nondescript part above the stratosphere. Second, this part of the mantle has a lot going on in it—the transition zone with its boundaries at 410 and 660 kilometers depth, plus the whole lower mantle and the strange D˝ zone at its base—and there's little reason to dismiss all of that under a single name.

Other -Spheres

The geosphere is used by writers and teachers more than by scientists. If it means anything at all, it refers to the chemical Earth—"lithosphere," hydrosphere and atmosphere—without involving the biosphere.

The siderosphere is a fancy name formerly used for the Earth's solid inner core, from sidero- meaning iron. If you feel you must use this term in speech, be sure to pronounce it with a short "I."

Finally, the geoblogosphere and geotweetosphere are names used by the community of geologists and geoscience writers who maintain blogs and Twitter accounts, respectively. These names have just the right mixture of seriousness and self-mocking. Geology is a science whose lexicon was created by hoarders, ready to coin and steal words whenever they might be handy. On the other hand, too many words can get in the way of clear thinking—and nobody is certain which attitude is best.

©2014 About.com. All rights reserved.