Earth has a magnetic field that is thought to be caused by the circulation of interior
fluids combined with Earth’s rotation producing a “dynamo” effect.
The deepest borehole ever drilled wasto a depth of about 12.3 km and this
leaves humans without direct sampling of most of Earth’s interior. Of crucial value
to us are the interior motions of the planet we detect as earthquakes. There is a
global system of seismometers measuring location and strength of the thousands
of major and minor earthquakes occurring each year. Through careful study of
the timing of the passage of various energy wave types through the interior, the
basic nature of the interior has been revealed. Near the surface, explosives are used
to create small crustal motions to discern rock configuration that could point to the
presence of exploitable oil and natural gas.
The inner core extends outward 1,450 km from Earth’s center. It is solid and
dominated by iron and nickel or iron and silicate. At temperatures between
5,000°C and 6,000°C, the core is kept hot by nuclear reactions under tremendous
pressures. Although the core is cooling, it is cooling exceedingly slowly. The outer
core is probably made of the same materials and extends from 1,450 to 3,500 km
from the center, but it is liquid and the source of Earth’s magnetism.
Above the outer core is the mantle extending to the crust within a few tens of
kilometers of the surface. It is less dense than the deeper layers. Yet, because it
is farther from the center, it has the greatest volume and mass of any layer. It is
composed mainly of iron and magnesium and is hotter with depth. The top of the
mantle down to 100 km under continents seems to be fairly rigid. Below, down
to 350 km, the rocks are heated so much that they are capable of great deformation
and slow, plastic flow. This zone is the asthenosphere and forms the basis for the
motion of crustal plates collectively known asplate tectonics.
The solid surface of Earth is known as the crust and it has been likened to a thin,
brittle eggshell surrounding a more pliable interior. The crust contains an impres-
sive mixture of rock types with increasing density of materials with depth. The
boundary of the crust with the upper mantle is at markedly different depths
beneath continents and oceans. When studying the respective suites of rocks it
has become clear that continental crust reaches greater depths (40 km) than oce-
anic crust (8 km). This configuration makes for continents and ocean basins and
was imparted to the planet early on as lighter-density liquids were extracted from
the deepest parts of the mantle and cooled. The resulting continental rocks are less
dense than oceanic rocks and, so, do not press as much on the asthenosphere and
maintain higher elevations. The upper part of continental crust is sometimes
referred to as “sial” from the relative abundance of silica and aluminum; a typical
rock type is granite. Oceanic crust is referred to as “sima” from the combination of
silica and magnesium; the signature rock type is basalt.
Earth 107