The heart of South Africa’s shabby Vredefort crater lurk curious black and green rocks. This is all that remains of the magma sea, once filled the hole in the ground, say Desmond Moser Western University of Ontario city of Canada. Vredefort crater ten-formed 2.02 billion years ago, and since then, of course, it did not spare the time.
Education called Vredefort was once very different. Its original diameter is estimated at 300 km. Asteroid or meteoroid, please Africa had probably 10 km in diameter and dug a hole, the depth of which is ten times larger than the Grand Canyon, says Mr. Moser. The Earth’s crust is melted and the point of impact on the lake was formed magma. Roughly the same thing happened later in Ontario, Sudbury crater appeared. He group Mr. Moser also
What’s left of it melt? It was believed that little or nothing. Observed chaotic breccia layers formed layers of the cortex, which fell into the crater immediately after impact. Layers slid so fast that the resulting friction melted stone and turned it into a glassy rock called psevdotahilit. There are also dikes, ie cracks in the surrounding material, which were filled with lava.
But in 1990, Mr. Moser found in the center of the crater is something else. He was trying to determine the age of the Vredefort and stumbled on virgin zircon age 2.02 billion years – tiny pieces of minerals without any traces of impact was. They just hid in the dikes. Dikes pass through the ancient crust, which once was at a depth of 20 km. Mr. Moser believes that podostyvshaya magma penetrated the crust and rocks as if clogged crater. Bark with arched dome: remember video demonstration of drop downs on the water surface.
Mr. Moser published his arguments in 1997, and immediately a discussion about whether you can take a dike filled gabbronorites, residue impact melts. The battle has not ended to this day. Some scholars reject this view, as the magma has an unusual layering serving a sign that the breed has changed. Perhaps this is another example psevdotahilita or initial portion of the crust. Others believe that young zircons could crystallize under the influence of heat from the blow.
Zircon age of 3 billion years old, a survivor of a blow.
Therefore, Mr. Moser and his colleagues returned to South Africa and looked for evidence that the dikes and crater have the same age. It turned out that the zircons are randomly distributed and interspersed with surrounding minerals, ie, they could not appear due to the heat from the blow later than neighbors.
Finally, the concentration of hafnium suggests that magma is molten rock age 3 billion years, which was before the impact on the Earth’s surface (similar sedimentary and igneous rocks can be found in the nearby Witwatersrand), and not the deep crust that is now exposed due to two billions of years of erosion.
If the scientists are right, then it makes sense to look for signs and other craters. Indeed, there are more ancient rocks with similar composition and texture.
The study is published in the journal Geology. In the same issue, Matthew Hubert of the University of Brussels (Belgium) and his colleagues report the detection of the ejected material hit Vredefort in Karelia. Vaporized rock fragments up into the atmosphere and fell in 2500 km from the crater on protokontinent, who later became the northwestern part of Russia and Scandinavia.