The Self Erasing Earth
April 27, 2001
Unraveling the geologic evolution of Earth's mountain belts includes the study of metamorphic rocks in the cores of those belts, and commonly involves searching for evidence of older events such as sedimentation that have been mostly erased by deformation and recrystallization during younger tectonic events, as the mountain belt grew. In this picture of a roadcut exposure of the Rangeley Formation in central New Hampshire, the delicate light and dark horizontal bands in the center of the picture reflect deposition of thin layers of mud and silt on a seafloor some 420 million years ago, prior to burial and metamorphism of the sediments during the Acadian orogeny, about 400 million years ago. Here they are frozen in the process of being destroyed by a tectonic event during mountain-building that produced the vertical foliation on the left and right sides of the picture. These rare glimpses into the distant past help geologists reconstruct the history of our earth.