Center of the Earth, Hot Spots and Plate Theory

WE TEND TO think of the atmosphere and oceans as fluids in constant motion, and of the Earth as a solid. But study of the earth has shown the structure of our planet is anything but static. It is a constantly moving system. Rocks roll and land slides on the surface and great plates that form the planet’s crust shift, causing earthquakes and tsunami. The mantle of the Earth sags under the weight of the highest mountains, and recovers as those mountains erode away. Solid rock melts, its lighter elements rise and heavier ones sink. The fluid material of lava moves in currents deep under the Earth, and if it can find a way to the surface, the planet erupts.

Internal Heat

Some of the planet’s incredibly intense heat is left over from its formation. Radioactive material in the heart of the planet also creates extremely hot temperatures. But much of Hawai‘i’s 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. This is primarily iron and has more other elements than the inner core. The temperature of the molten outer core registers some 7,000 degrees Fahrenheit. The diameter of both the inner and outer cores is about 4,000 miles. In 1996 scientists discovered that together the cores spin independently of the mantle, generating a magnetic field. They appear to turn in the same direction as the rest of the Earth, but slightly faster, gaining a quarter turn every century. This discovery came about at Columbia University’s Lamont-Doherty Earth Observatory when the seismic waves from natural earthquakes were studied. The combine core 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 heart is required to change rock from its solid to liquid state. Although the outer core of the Earth already has a very high pressure, the 7,000-degree-plus temperature is so high that the surrounding rock has liquefied. Deeper down, the pressure increases so dramatically that even higher temperature has no effect and the iron core is solid.

Tectonic Plate Theory

Most volcanoes are found in areas where the planet’s tectonic plates meet. The Earth’s surface is divided into these plates. They are often described as similar in appearance to the sections on the back of a turtle. Or imagine the pattern on a soccer ball.
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 that allowed the dating of lava flows 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. The theory developed from this information was that each chain was developing over a “hot spot,” where magma melts through the crust to create volcanoes. The theory further concluded that the entire floor of the Pacific Ocean was moving westward at a steady rate. As the Pacific plate moved, the hot spot remained relatively fixed. The line of volcanoes left behind gives an indication of the direction of the plane’s movement.

Major Tectonic Plates

The Pacific Plate is just one of several that cover the Earth’s surface. Some geologists count seven large plates and two dozen or more 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. The small Nazca Plate is under the eastern South Pacific, between the Pacific and South American plates, and bounded on the south by the Antarctica Plate.

Ring of Fire

All the 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, with the intervening space filled with magma rising 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. And there are active volcanoes like those found at the Hawaiian hot spot.