Earth

Unlike the other three terrestrial planets, Earth's crust is divided into eight large and 20 small, solid "plates" which float around independently on top of the semi-fluid mantle below. Under the modern the theory of plate tectonics, some plates move away from each other and new crust is created by upwelling magma from below in a process called "spreading." Other plates are being destroyed through the process of "subduction," where plates collide and the edge of one dives beneath the other to be melted in the hot mantle below.

NASA

Earth is a geologically active planet. Some geological processes such as
plate tectonics are always slowly moving the planet's mass around (more),
other processes such as water -- or ice -- flows may shift masses faster (more).

Once in a while, a truly massive plume of hot rock from the Earth's mantle can erupt through the crust for centuries or even millenia, producing acid rains, destruction of the ozone layer from emissions of chlorine-bearing compounds, and a chill down in climate from the resulting increase in atmospheric dust and sulphur-based aerosols. The last such eruption occurred some 65 million years ago (press release, Basu et al, 1993), created India's Deccan Traps, and -- in combination with the Chicxulub meteorite impact -- contributed to the extinction of the dinosaurs. An earlier and even larger event in Siberia in combination with an impact by a meteorite that may have been slightly larger than the one associated with the Dinosaur extinction event -- that is, 3.7 to 7.4 miles wide (six to 12 km) and equivalent to a magnitude-12 earthquake -- may have caused the Permian-Triassic extinction 251 million years ago, when 90 to 95 percent of all species were wiped out. The researchers (U.R. and U.W. press releases) suggest that perhaps in both extinctions, a plume of lava may have already risen close to the surface, but a massive meteorite impact turned a small eruption into a colossal one. As a result, within less than a million years, enough lava oozed out of the ground in Siberia to cover the entire planet 10 feet (three meters) deep. (One Permian-Triassic crater candidate is the 125-mile/200-km or more diameter Bedout Structure off the coast of northwestern Australia. For more discussion of geologic evidence, see: Asteroid and Comet Impact Craters and Mass Extinctions; and Luann Becker, Scientific American, March 2002.) In 2002, physicist Richard Muller suggested that large asteroidal or cometary impacts could trigger unusually widespread "avalanches" of lighter minerals (such as sulfur and silicate that drift up to to collect at inverted mounds at the outer core/mantle boundary) and cause large plumes of magma to rise to the surface (Richard Muller, 2002; in pdf).

NASA Total Ozone Mapping
Spectrometer (TOMS)

More images.

Mostly Human-made, record-sized
ozone hole over Antarctica on
September 9-10, 2000 allowed
intense ultraviolet radiation to
damage tissues and DNA of surface
lifeforms on land and in water,
leading to severe sunburns,
blindness, skin cancers, and death.
For the first time, the hole extended
over a major Human population --
the 120,000 residents of Punta
Arenas, a city in southern Chile.

By mass, the Earth is composed of mostly iron (35 percent), oxygen (30 percent), silicon (15 percent), and magnesium (13 percent). It is made of distinct layers: a thin crust, upper mantle, lower mantle, outer core, and inner core, as well as transition zones. The crust is thinner under the oceans but thicker under the continents. While the inner core and crust are solid, the outer core and mantle layers are semi-fluid.

NASA -- Larger image

Gravitational bumpiness exaggerated.


While Earth is a fairly uniform sphere,
its gravitational field does vary by small
fractions of a percent from place to place.
Not only is the planet bumpy as the result
of slow geological processes such as
tectonic plate movements or polar ice cap
changes, but these processes also create
mass density variations that affect the
local gravitational field (more from NASA
and Astronomy Picture of the Day).

Most of the mass of the Earth is in the mantle, which is mostly made of the minerals olivene and pyroxene (iron and magnesium silicates). The elements calcium and aluminum are abundant in the upper mantle and while silicon, magnesium, and oxygen are major components of the lower mantle. The core is probably composed mostly of iron (perhaps with some nickel), where the temperature may be as high as 7227 °C (13,040 °F) at the center -- which is hotter than the surface of the Sun (See further discussion of the Earth's interior structure ).

National Academy of Sciences


Larger image.


The inner core appears
to have an unexpectedly
complicated subcore
structure of its own (more
from BBC On-Line News).

Discovered in the 1930s, the inner core appears to be a solid sphere of mostly iron and nickel. In 2002, its diameter was finally measured to be about 1,520 miles (2,440 km) across based on shock wave data generated by earthquakes from more than 300,000 seismic events between 1964 and 1994. In a phenomenon called anisotropy, those shock waves were found to travel faster moving from north to south than from east to west, probably because the inner core formed in crystalline manner, where the iron and nickel atoms are line up in the same direction and so change the speed at which the waves pass through depending on their initial direction.

The shockwaves reveal that the inner core appears to harbor an "inner inner" core that may be about 370 miles (600 km) in diameter. The anisotropy effect was much stronger in this region, which suggests that its composition is even more crystalline composition with a different angle of most resistance (arising from different crystalline alignment) from the rest of the inner core. The scientists analyzing the anisotropy believe that this difference may be the result of changes in the environment of the core during its formation. Professor Guy Masters of the Scripps Institute of Oceanography believes that the "core within a core" could be left over from an early stage of the planet's formation.

According to one interesting hypothesis, the Earth (and possibly all of the planets and many of their larger moons) may have an innermost core of fully crystallized nickel silicide around an inner sub-shell of nuclear decay and fission products surrounding a five-mile (8 km) wide sub-core of uranium and plutonium. Acting like a natural, fast-breeder nuclear fission reactor, the sub-core generates heat that propels charged particles to ultimately produce the Earth's magnetic field in a non-linear process that periodically weakens and strengthens, and even reverses polarity every 200,000 years. Lighter than the uranium-plutonium sub-core, fission by-products absorb neutrons and gradually slow core fission until they slowly float out to join the sub-shell, and fission resumes and strengthens. (See: J. Marvin Herndon, 1998; Brad Lemley, "Nuclear Planet," Discover, August 2002; and www.NuclearPlanet.com.)

NASA -- larger image

Many charged particles from the Sun's Solar Wind are trapped by Earth's magnetic field
to form its magnetosphere, deflecting the Wind from a head-on collision with its atmosphere.

Earth has a modest magnetic field produced by electric currents generated in its metallic core by its rapid spin, which is distorted into a tear-drop shape by the solar wind. Along with the atmosphere, the magnetic field shields life on Earth's surface from most harmful solar and cosmic radiation. This field traps radiation in a pair of doughnut-shaped rings of ionized gas (plasma) in orbit around the Earth called the Van Allen radiation belts. The outer belt stretches from 19,000 to 41,000 km (11,800 to 25,500 miles) in altitude, while the inner belt lies between 7,600 to 13,000 km (4,700 to 8,100 miles) in altitude.

NASA



Larger ultraviolet image



Although the relatively cold plasma
captured by the Earth's magnetic field
is usually transparent, its ultraviolet
glow can be seen with NASA's Imager
for Magnetopause to Aurora Global
Exploration (IMAGE) spacecraft.

According to the geological record, Earth's magnetic field dwindles down to nothing for about a hundred years every few hundred thousand years. Then, it gradually reappears but with the north and south poles flipped. The last reversal of the poles occurred about 780,000 years ago, and the strength of the magnetic field has diminished by about five percent over the past century. Hence, the Earth may be overdue for this cyclical event. Given the anticipated loss of the magnetic field's shielding against the Sun's energetic subatiomic radiation and more ultraviolet radiation from the consequent erosion of the ozone layer, however, a magnetic reversal may cause serious ecological disruption to all surface lifeforms on Earth as well as threaten human well-being.

NASA




Larger false color image




Neutral atoms glowing from relatively hot
plasma within the Earth's magnetosphere
can be seen with IMAGE's High Energy
Neutral Atom (HENA) instrument.