The Earth is a terrestrial planet consisting of a crust, mantle, molten outer core, and solid inner core. The Earth’s crust is rich in silicon, aluminum, and other elements less massive than iron. Most of the Earth’s iron and heavier elements are located in the mantle and core. The crust extends about 40 miles below the continents but only 10 miles underneath the ocean basins. Strange as it may seem, the crust comprises only 1% of our planet’s mass; the mantle and core constitute the other 99%.
To put it in perspective, if the Earth were the size of a peach, the crust would correspond to the skin of the peach. The fruit and pit correspond to the mantle and core, respectively. The mantle consists of semisolid igneous rock that gradually becomes entirely liquid closer to the core. Small amounts of mantle are ejected during volcanic eruptions, given that the roots of volcanoes often extend 100 miles or more below the Earth’s surface. The outer core is thought to be made up of molten iron, nickel, and other metals. The inner core is an extremely dense metallic ball. Even though the inner core is hotter than the outer core, it remains solid due to its high density as well as the immense pressure exerted on it by the surrounding mantle and outer core.
All geological processes that occur below the Earth’s surface are driven, directly or indirectly, by heat released from the Earth’s core. According to geologists, most of the intense heat in the Earth’s interior was produced by the accretion of the planet over 4 billion years ago. The remainder is produced by the continuous decay of radioactive elements.
As with other bodies, the Earth’s heat is dissipated by three mechanisms: radiation, conduction, and, most importantly, convection. Briefly, radiation means the transmission of heat into surrounding space. This is the way Earth receives solar energy. Conduction means the transfer of heat from one object to another by direct contact. For example, a metal spoon placed in a cup of hot coffee will gradually become hot to the touch. Finally, convection refers to heat transfer by means of directly moving hot matter to a cooler region, as with a convection oven. The most well understood example of geologic convection is volcanism, discussed next.
Volcanism: Hot Spots and Subduction Zones
Hot spot volcanoes result from the upwelling of magma plumes from the Earth’s mantle, which penetrate through the crust and ultimately burst through the surface to form a volcano. These hot spots are scattered across the continents and ocean basins. They include Yellowstone National Park (located atop a caldera volcano), the Galapagos, the Hawaiian Islands, and other volcanic peaks in Africa, the Arabian peninsula, Indonesia, Siberia, Antarctica, and elsewhere.
Subduction zone volcanism occurs at regions where the ocean floor dives underneath an adjacent area of thick continental crust. As the oceanic crust melts into the underlying mantle, magma chambers are produced, resulting in chains of volcanoes. Examples of subduction zone volcanism include the Aleutian Islands off Alaska, Crater Lake, and Mt. St. Helens. Many volcanoes around the Pacific Ocean, the so called Ring of Fire, were produced by subduction zone volcanism.
In addition to volcanism, convection in the Earth’s mantle is responsible for earthquakes and tsunamis.
The Earth’s lithosphere (crust and upper mantle) is fractured into 15 major tectonic plates, as shown on the diagram. Heat circulating in the Earth’s mantle is believed to drive the movement of tectonic plates. Although the plates are moving at approximately one inch per year, this accounts for the rearrangement of Earth’s continents over vast spans of geologic time. At their boundaries, tectonic plates collide, spread apart, or move in parallel but opposite directions to each other. As a result of these movements, most earthquakes occur at tectonic plate boundaries. Seismic activity is a continuous phenomenon. Earthquakes strong enough to be felt, however, occur at intervals of months or years.
Tsunamis are caused by undersea earthquakes. When large areas of ocean floor are suddenly sink or are thrust up by tectonic plate movements, the result is a massive ripple effect below the ocean’s surface. In the open ocean, these waves may be undetectable without special monitoring equipment. As these waves approach islands and continental shelves, however, they gain amplitude quickly, with crests approaching 50 feet. When these waves crash onshore, they can devastate coastal regions, with death tolls in the hundreds of thousands, as happened in the December 2004 Indonesian tsunami.
Earth’s Magnetic Field
Although magnetism is traditionally the domain of physics and astronomy, Earth’s magnetic field is ultimately a product of geology. Many scientists think that the combination of a solid inner core surrounded by a molten outer core is necessary for the existence of Earth’s magnetic field. Planetary bodies lacking this core structure, including Earth’s moon, Venus, and Mars, have practically no magnetic fields. By studying the orientation of certain minerals in rock strata, geologists have determined that the direction of Earth’s magnetic field reverses every 300,000 years or so. The basis of magnetic field reversal remains poorly understood.