How rock helps the days roll by
1 December 2005
Huge chunks of the Earth's crust crashed into the edge of the core 2.
Crust made on the ocean floor slides into the Earth's mantle. But David Dobson and co-researchers at University College London have come up with a theory to explain some unusual patterns in seismic waves travelling through the Earth. Dr Dobson will present their findings at the William Smith meeting of the Geological Society of London today.
Dr Dobson came up with the theory while reading a book by Richard Fortey, the Natural History Museum palaeontologist. The Earth: An Intimate History mentions a type of rock found in crust known as banded iron formations. These appeared between 1.8bn and 2.5bn years ago and their structure is thought to be influenced by the emergence of life around that time, when algae began to appear. Before then, the atmosphere had been oxygen-free, but when algae evolved, their breathing introduced oxygen which reacted with iron in the oceans and deposited iron oxide on the ocean floor. For more than 700m years, layers of iron oxide, interspersed with layers of dead algae, built up on the ocean floor.
The high density of banded iron formations means that the rock sinks relatively quickly to the Earth's core, taking around 100m years, and then sits there, unlike other types of crust which melt into the mantle, or sink and rise on buoyant plumes. But Dr Dobson was interested in regions of the crust that were swallowed by the Earth's interior. Because the iron formations are so dense, and assuming they did not melt, they should stand out seismically. More than a 10th of the edge of the Earth's core appears to be covered with the lumps, each measuring a few hundred miles wide and not more than six miles thick. "These bits of crust appear to float on the core, a bit like a football with leaves stuck on it," Dr Dobson said.
Support for the theory comes from computer calculations of the likely minerals found in the rocks. Dr Dobson and his team have shown that it would be possible for the rocks to be stable at the extreme temperatures and pressures found at the edge of the Earth's core.
Even now these chunks of crust could still be making their presence felt. Because they are rich in iron, they are more conductive than other rocks and could alter the Earth's magnetic field.
Kate Ravilious, 'The Guardian', 24 November 2005