We tend to think of the atmosphere and ocean as fluids in constant motion, and of the Earth as a solid. Yet, studies of the earth have shown the structure of our planet is in fact anything but static. It is a constantly moving system. Rocks roll, land slides on the surface, and plates that form the planet’s crust shift, all causing earthquakes and tsunamis. The mantle of the Earth sags under the weight of the highest mountains, and recovers as those mountains erode. Solid rock melts, it’s lighter elements rise and heavier ones sink. The fluid material of lava moves in currents deep under the Earth.
Some of the planet’s incredibly intense heat is left over from its original formation. Radioactive material in the heart of the planet also creates extremely hot temperatures. In Hawaii, heat comes from mantle rock that has melted, forming magma--molten rock that is still underground. Once magma erupts to the surface, it is called lava. When lava hardens, or magma hardens underground, it is called igneous rock. Our planet has a solid inner core, believed to be made up of high percentages of iron and nickel. The inner core, about 1,500 miles in diameter, is surrounded by a fluid outer core. The temperature of the molten outer core registers some 7,000 degrees Fahrenheit. The combined core: inner and outer is hotter than the surface of the sun. And it is under incredible pressure from all the layers of material around it, which are pressing down under the influence of gravity. Molten rock heats up deep in the Earth, but its melting temperature changes with pressure. The higher the pressure, the more heat is required to change rock from a solid to liquid state.
Tectonic Plate Theory
Most volcanoes are found at meeting points of the Earth’s tectonic plates. Imagine the pattern on a soccer ball-that is how the earth is divided. Tectonic plate theory is fairly recent, having been developed in the 1960s. The Hawaiian Island chain played a major role in the formation of the theory. Geologists noted that Hawai‘i is a long line of peaks, and originally they believed all the islands appeared at roughly the same time, arising from some kind of geologic rift. But more recent geological work has indicated that the oldest islands are at the far northwest end of the chain, and the youngest are at the southeast, near the Island of Hawai‘i. They also noticed the same pattern on two other island groups on the Pacific plate. All three chains, though hundreds or thousands of miles apart, form lines that run parallel to each other. As the theory goes, each chain was developing over a “hot spot,” where magma melts through the crust to create volcanoes.
Major Tectonic Plates
The Pacific Plate is just one of several that cover the Earth’s surface. Some geologists count seven large plates and at least two dozen smaller ones. Among the major ones is the North American Plate, which includes all of North America and about half of the North Atlantic Ocean. The South American Plate has most of South America and approximately half the South Atlantic. A single plate accounts for most of Asia and Europe, there is one for Africa, and one that forms Antarctica. Australia lies on the last major plate, which includes parts of the Indian Ocean and the western South Pacific.
Ring of Fire
Earth’s tectonic plates are separated by cracks in the planet’s crust, and they move toward or away from each other. The points where the plates meet are some of the most geologically active places on Earth. The circumference of the Pacific Plate is sometimes called the Ring of Fire, for the number of active volcanoes that exist along the boundary. Tectonic plates slide laterally in some areas, as at the San Andreas Fault in California. In other areas they spread apart, leaving the intervening space to fill with the magma that rises from the Earth’s mantle. There is also subduction zones where two plates are pushed into each other, one sliding up over another while the lower plate is driven back into the mantle. The Hawaiian hot spot contains zones such as these.