Geoscience Reference
In-Depth Information
3. The carbon cycle. This simplified diagram shows estimates of the amount of carbon
(in billions of tonnes) stored in the atmosphere, oceans, and land. Figures by the ar-
rows show the annual fluxes between stores, figures in brackets the annual net rise.
Though small compared with most other fluxes, the input from burning fossil fuels
seems enough to upset the balance.
Almost an onion
The interior of the Earth is rather like an onion, made up of a series of concentric shells
or layers. On the top is a crust, averaging 7 kilometres thick under the ocean and 35 kilo-
metres thick in continents. That sits on the hard, rocky lithosphere at the top of the mantle,
and below it is the softer asthenosphere. The upper mantle extends to a depth of about 670
kilometres, the lower mantle goes down to 2,900 kilometres. Below a thin transition layer
comes the liquid outer core of molten iron and a solid iron inner core about the size of the
Moon. But it is not a perfect onion. There are horizontal differences within layers, vari-
