5.The Ice Age Overturn - ending the ice age

The Ice Age Overturn - ending the ice age

Overturn is a rare event in the earths history, but fundamental to explaining the ice ages.
Ice was building up at the poles, miles thick. Because the poles were on the edge of continental masses this buildup was eccentric.

The Ice Age South Pole

Yellow lines show the winds blowing the moist air onto the growing icecap.
Blue lines show the ocean currents.

The Ice Age North Pole

Notice that the winds in the ice age are blowing moist air onto the Antarctic continent, leading to a continuous build up of ice. Unlike the present day when the winds blow round Antarctica, and little new ice or snow is being delivered onto the ice cap. The saltiest ice age ice should be found onthe continental margins facing these prevailing winds.

The earths crust cannot sustain an eccentric mass on the axis of rotation, but the crust is plastic enough to deform as the ice accumulates - slowly, year by year, for thousands of years. However this will exert pressure on adjacent crustal plates, and these will drift in response to these eccentric pressures.
However, under the ice sheet are volcanoes and lakes. Should the glaciers start to flow more rapidly in response to an under ice erruption, the outflow can be dramatic and catastrophic. The collapse of an ice sheet two milesthick can throw the balance of mass of the earths crust around the axis ofrotation of the earth into a sudden imbalance. The crust cannot rotate outof balance - it must move - and once started it will move until frictionbetween the plates is greater than the imbalance of forces created by movementof the ice. This can at its least dramatic move the poles by a few hundredmiles, before sticking. The slow process of deformation and build up of icestarts again.

But at the end of the ice age the imbalance was far more dramatic, andthe result was complete overturn. All the crustal plates rotating roundan axis that runs from off the west coast of Africa to the middle of thePacific.

As the continental plates moved towards the equatorial bulge they were pulled part, and magma poured from these ocean rifts. Asthe plates moved over and away from the equatorial bulge plate margins were crushed togther again, and in some places one plate margin rode abovethe plate alongside.
The maximum uplift took place in the South American Andes, lifting salty coastal lagoons high into the mountains. Similar uplift took place on the opposite side of the world, raising the Tibetan plateau further.
Stretching of the plates caused sinking from the North Sea to the Caspian Sea, and again in China.

The evidence for these tumultuous events is clear. Great ocean trenches clear of sediment as they have been created too recently for the sediment to accumulate.
Banks of sea shells high in the Andes match those of the present day shores, and salt lakes still with marine life from the coastal lagoons up high in the mountains.
Mamoths frozen by an arctic winter, that were eating leafy plants in atemperate summer. The great barrier reef of Australia that only starteddeveloping when the ice age ended.
The planets bio-diversity hot spots remain in those climate zones where the climate before the overturn was close to the climate after the overturn. Where the climate change was greatest, many species have died, or remaintoday living at the limits of their ability to survive.

This information is copyright Peter Thomson 2001-2004