#changjiang

The mountain is composed of igneous rocks surrounded by a much more ancient igneous and sedimentary province. Much of Asia as we know it today is built of ancient continental fragments formed in the Precambrian, over 500 million years ago. Those continental fragments moved around the globe as various plates subducted and occasionally collided with each other. Finally, many of them were scooped up and accreted to Asia as the Tethys Ocean and the Pacific Plate (and its predecessors) subducted beneath Asia over the past few hundred million years. Huangshan lies at the border of two of these ancient continental fragments. Two pieces of crust collided over 700 million years ago and rode together, occasionally submerging beneath the waves to allow sedimentary rocks to form. During the Jurassic, around 125 million years ago, subduction took over and began forming new rocks and thrusting up mountains throughout this area. Large magma chambers filled with granitic magma rose into the old continental fragments, fusing them together. The rocks of Huangshan are mostly two granite bodies; a body known as the Shizilin is surrounded by a larger granite known as the Huangshan granite. Both of these granites and many of the other rocks in the area were born as magma bodies formed during this pulse of subduction. Granite magma chambers solidify slowly while they’re buried beneath several kilometers of rock. When they’re brought to the surface, either by erosion or by faulting, they tend to split apart since they’re no longer feeling pressure on all sides. This process forms sets of fractures called “joints” that break the granite into giant boulders with planes of open space between them. These joints dominate the erosion pattern of the granite. Water moving along these joints has, over time, carved this landscape. Some of the incised valleys have a U-shape as seen in glaciated areas, leading scientists to speculate that this area had once been glaciated, but further work has found little evidence of glaciers. Instead, the U-shapes are controlled by the joint sets in the granite – vertical fractures allow the peaks and valleys to form steep sides. Lucky visitors will occasionally catch an inversion like this one, where a layer of moist, cloudy air is trapped beneath a layer of dryer air and the valleys between the peaks are filled with what is called a sea of clouds. -JBB Image credit: Haifeng Zhou http://imaggeo.egu.eu/view/3070/ Read more http://bit.ly/1JE6qwW http://whc.unesco.org/en/list/547 http://bit.ly/1DhmBiK