A deeper lookLurking beneath all the more recent layers of rocks that drape our continents are their
A deeper lookLurking beneath all the more recent layers of rocks that drape our continents are their roots, the oldest of which are the remnants of cratons, the first nuclei of the light buoyant surface rocks that float atop the underlying mantle. When they pop into visibility at the surface, the most amazing patterns reveal themselves. These rocks have been repeatedly heated, melted and compressed as the continents moved back and forth, colliding and separating into supercontinents in their Wilson cycles. Each mountain building event leaves its impression in the folding and melting of the rocks, forming a palimpsest of what geologists dryly term thermal and deformation events. Not a very poetic way, I know, to describe such tremendous forces that shaped the surface of long gone landscapes, but there you have it. The photo shows boudinage, from the French for blood sausage. The semi molten rocks reacted to stress by straining, sliding around in a plastic manner known as ductile. Large chunks of darker low silica volcanic rock have been broken apart and intermingled with others, revealing something of the pattern of the forces. These rocks were melting while this was happening, and the white veins are the resulting new born granite. As the dark chunks separated, lower pressures resulted on one side (abive the dark mass in the photo), and the grey gneiss flowed into the area, producing the folded structure. The white veins are even more intricately folded, revealing further moments of pressure while the rocks were soft. The photo was snapped in the Prince Charles mountains of Antarctica, 1 400 km long range whose main surface expression are nunataks (mountain tops poking out of an ice sheet). They are one of the few areas with extensive rock exposure in the whole continent, where a block of rock measuring 600x300km has sunk downwards between two faults forming a graben, a bit like Africa’s great rift valley. The event that shaped these rocks is thought to have occurred around a billion years back, with a second event around 500 million years ago with at least three recognised deformation events. LozImage credit: Adrian F Corvino/Outcropediahttp://www.antarctica.gov.au/living-and-working/stations/mawson/environment/prince-charles-mountainshttp://www.antarctica.gov.au/living-and-working/stations/mawson/environment/prince-charles-mountains/northern-prince-charles-mountains -- source link
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