Geoscience Reference
In-Depth Information
and sufficiently abundant in the solar system to make up the bulk of the core is iron. Al-
though we do not have samples of the Earth's core, we do have pieces of something that's
likely to be similar, in iron meteorites. Though not as common as stony meteorites, they
are easier to spot. They are believed to come from large asteroids in which an iron core
separated out before they were smashed by bombardment early in the history of the solar
system. They are mostly made of iron metal but contain between 7% and 15% of nickel.
Often, they have a structure of intergrown crystals of two alloys, one containing 5% nickel,
the other about 40% nickel, in proportions that give the bulk composition.
An iron core must have formed in the Earth by gravitational separation from the silicate
mantle when the new Earth was at least partially molten. As the layers separated, so-called
siderophile elements such as nickel, sulphur, tungsten, platinum, and gold that are soluble
in molten iron would have separated with them. Lithophile elements would have been held
back by the silicate mantle. Radioactive elements such as uranium and hafnium are litho-
phile, whereas their decay products, or daughters, are isotopes of lead and tungsten so
would have been separated out into the core at its formation. That consequently reset the
radioactive clock in the mantle at the time the core formed. Estimates of the age of mantle
rock put that separation at 4.5 billion years ago, about 50 to 100 million years after the ages
of the oldest meteorites which seem to date from the formation of the solar system as a
whole.
The inner core
The centre of the Earth is frozen. Frozen at least from the viewpoint of molten iron at the
incredible pressures down there. As the planet cools, solid iron crystallizes out from the
molten core. Present understanding of the electrical dynamo that generates the Earth's mag-
netic field requires a solid iron core, but the planet may not have had one for its entire his-
tory. There is evidence of the Earth's past magnetic field locked into rocks throughout the
Phanerozoic. But most Pre-Cambrian rocks have been so altered that it is difficult to meas-
ure any original magnetism. So the only estimate of the age of the inner core comes from
models of thermal evolution of the core as the Earth slowly cools. It's the same sort of cal-
culation that Lord Kelvin performed in the late 19th century to estimate the age of the Earth
from its rate of cooling. But now we know there is additional heat from radioactive decay.
The latest analysis suggests that the inner core began solidifying somewhere between 2.5
and 1 billion years ago, depending on its radioactive content. That may seem a long time,
but it implies that for billions of years of its early history, the Earth was without an inner
core and perhaps without a magnetic field.
Today, the inner core is about 2,440 kilometres across, 1,000 kilometres smaller than the
Moon. But it is still growing. The iron is crystallizing at a rate of about 800 tonnes a second.
