Composition of the Earth

Earth, Composition of the            The Earth is composed of three zones characterized by distinct mechanical properties — crust, mantle, and core — separated by transition layers or discontinuities. Those zones and transitions will be discussed from the surface inward.

The rigid, outermost layer of the Earth, called the crust, is divided into two sections, oceanic (younger and denser) and continental (older and less dense). Oceanic crustal materials, which range in thickness from five to about ten kilometers, are typically composed of dark igneous rocks, particularly basalt (also known as sima — silica magnesium), which can also be found on the surface, especially in the form of extruded materials like lava. The continental crust, which ranges in thickness from 30 to over 65 kilometers, is composed of a variety of igneous rock types whose average composition is similar to granodiorite (known as sial — silica aluminum), but as we know also contains all the other rock types with which we are familiar, including sedimentary and metamorphic.

A strong seismic discontinuity (simply known to all geoscientists as the Mohoseparating the crust from the mantle is named in honor of its discoverer, the Croatian seismologist Andrija Mohorovičić. Although the rock above and below the Moho is solid, with increasing depth the upper mantle gradually becomes soft and pliable. It is that soft zone that allows tectonic plates at the Earth’s surface to slide about. In plate tectonics, the soft, ductile zone is the asthenosphere in which rocks experience plastic flow/plastic deformation owing to the heat and pressure and the hard rocks above it constitute the lithosphere, which is actually a combination of the crust and upper mantle extending to depths of about 80 kilometers.

The mantle extends from the base of the crust, whose boundary is marked by the Moho, to almost 1,800 miles below the surface (marked by a seismic boundary zone that separates the outer core and the mantle that is variously known as the core-mantle boundary, the Gutenberg Discontinuity, or the D’’ layer — D prime prime) and contains over 80 percent of the Earth by volume. It is divided into lower and upper mantle, thought to be composed of peridotite, an ultramafic magma primarily made up of the minerals olivine and pyroxene. The top layer of the upper mantle, 60-210 miles below surface, is called the asthenosphere and is made of silicon, oxygen, aluminum, iron, and magnesium. It is mostly a solid but very pliable type of silicate rock that flows plastically in response to heat and pressure. Separating the upper mantle from the lower mantle is the 670-km discontinuity, a boundary that may function as a natural barrier to whole-mantle convection.

The Earth’s internal structure changes abruptly about 1,800 miles beneath the surface, where the solid silicate rock of the mantle lies immediately adjacent to the molten iron of the outer core, at what is known as the core-mantle boundary (CMB). It is at this boundary that the two giant heat engines responsible for plate tectonics and the geodynamo interact. At one time scientists thought that that thermal and chemical boundary was sharply defined, smooth, and homogeneous. But, in the past two decades, by analyzing seismic waves researchers have found that the mantle-side portion of the CMB, known as the D’’ layer (the term D prime prime was established by the well-known New Zealand geophysicist and mathematician, Keith Bullen, as part of a larger classificatory system that is seldom used today) is the site of dynamic processes that may involve both thermal and chemical heterogeneity at various scales, seismic discontinuity, and substantial seismic anisotropy. That research evidence has demonstrated: 1) mushy regions, 2) a thin patchy zone of rigid material immediately below the CMB where the outermost core is more solid than fluid, 3) fuzzy patches, 4) an ultra low-velocity zone immediately above the CMB that has been interpreted as molten, 5) large dome-like structures, and 6) zones of avalanches. Today, scientists describe the region as being as dynamic as the Earth’s surface.

The nature of the CMB has been the subject of more scientific attention in the last several decades than during any period in the history of geophysics. The CMB is important because researchers now think it influences phenomena ranging from the behavior of Earth’s electromagnetic field to the magma plumes that rise through the mantle and erupt on the surface at volcanic hot spots such as Hawaii. Research in 2005-2006 demonstrated that the 660-km discontinuity may be a complicated structure characterized by single and double reflections at depths ranging from 64- to 720-km. The researchers conclusion was that that data almost required the existence of multiple mineral phase transitions at the base of the boundary zone that are consistent with a pyrolite mantle composition (pyrolite is hypothetical spinel-garnet facies and a term coined by the Australian geochemist Alfred E. Ringwood to represent a combination of basalt with dunite).

The outer core, about 2,050 miles below the Earth’s surface, consists of superheated, molten materials that are mostly iron that flow plastically, with lesser amounts of nickel and silicon that combined with small amounts of other elements, most likely sulfur or oxygen. The inner core is thought to be composed largely of a mixture of solid iron and nickel (owing to the enormous confining pressure of the thousands of miles of overlying rock) and has a radius that has been estimated at around 800 miles and is stronger than the liquid outer core. Until recently some theorists argued that electromagnetic forces inside Earth cause the core to spin separately from the planet’s outer layers while others hypothesized that the core spins in synchrony with the mantle and the crust. But evidence compiled since 1996 and most recently in 2005 that seems to demonstrate the core actually spins faster than the rest of the planet. Geophysical data collected and analyzed by scientists working independently at Lamont-Doherty Earth Observatory of Columbia University and Harvard University indicated that the inner core appears to make one full extra spin relative to the Earth over a period that has been estimated at a minimum of 120 years.

The inner core was only discovered in 1936. The Danish geophysicist Inge Lehmann was the first to demonstrate the existence of a change in composition midway through the core at 5,150 kilometers, dividing it into an inner core and an outer core, a division now known as the Lehmann Discontinuity or the inner-core boundary. In 1980, Adam Dziewonski (Harvard University) and Freeman Gilbert (University of California — San Diego) proved the inner core was solid, rather than liquid, postulating it to have formed by the freezing of iron. In 1986, Andrea Morelli, John Woodhouse, and Dziewonski, working at Harvard University, found that the inner core exhibited anisotropy. Shock waves from earthquakes travel through it in a north-south direction faster than in other directions, a situation attributed to the crystalline structure that iron, the major ingredient, assumes under the intense pressure near Earth’s center, more than a million pounds on every square inch. In 2003, Dziewonski and research associate Wei-jia Su showed that the axis of symmetry of the inner core tilts about ten to twelve degrees from the north-south axis of its rotation by analyzing records from 15,722 earthquakes that sent shock waves though the inner core. Working independently of the researchers at Harvard, Xiaodong Song and Paul G. Richards of the Lamont-Doherty Earth Observatory found that the inner core rotates in the same direction as the rest of the planet but about one degree per year faster. The Lamont-Doherty and Harvard scientists and other teams of researchers are now examining different seismic records to test the discovery and to more precisely measure the core’s rotation. A critical set of measurements was collected at the Japanese High-Sensitivity Array from a magnitude-7 earthquake that rocked Mozambique in 2006 and analyzed by James Wookey and George Helffrich, geophysicists at the University of Bristol, and reported in Nature, (August 14, 2008). Wookey and Helffrich were the first geoscientists to detect two sets of the ground motions associated with core-crossing shear waves — one set triggered by left-to-right vibrations and the other by up-and-down shear waves. The seven-second disparity in travel time recorded between the two sets of ground motions indicated that the crystal structure of the inner core is anisotropic and likely composed of solid, hexagonal, close-packed iron.

In the fall of 2002 Harvard geophysicist Dziewonski and graduate student Miaki Ishii announced the discovery of an inner inner core at the center of the Earth. That previously unknown sphere, about 360 miles in diameter, was detected by examination of 325,000 records of earthquake waves that passed through the Earth’s center in the past 30 years. When they looked closely at more than 3,000 records of earthquake waves that traveled closest to Earth's center they found an obvious change in wave speed with direction — a phenomena known as anisotropy — in an area 360 miles in diameter at the center of the inner core.

Naturally, many geoscientists expressed some degree of skepticism at their discovery. But in 2008, Xiaodong Song and Xinlei Sun, geologists at the University of Illinois, confirmed the existence of an inner, inner core and have created a three-dimensional model that describes the seismic anisotropy and texturing of iron crystals within the inner core. In their analysis, Sun and Song used three-dimensional tomography to invert the anisotropy of the inner core and parameterized the anisotropy of the inner core in both radial and longitudinal directions. They then used a three-dimensional ray method to trace and retrace the seismic waves through the inner core iteratively. They discovered a distinct change in the inner core anisotropy, clearly marking the presence of an inner inner core with a diameter of about 1,180 kilometers, slightly less than half the diameter of the inner core, confirming the earlier discovery of Ishii and Dziewonski.