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A sheet intrusion is a rock formation generally created by the rising of molten rock, or magma, in between the top layers of the Earth’s crust. The top layers can expand as a result, and the underlying material is sometimes exposed because of surface erosion. Rock sheets are usually somewhat flat in shape, but can take on an undulating or dome-like structure; geological processes can alter a relatively flat sheet intrusion into a more vertical position. The intrusion also usually appears different from material that originated as flowing lava, by incorporating components of the surrounding rock into the top and bottom layers.
Rising magma typically causes heated material to crystallize as it cools, forming igneous rock. It can form dikes, or sheet-like formations that cut across layers; a sheet intrusion, however, is generally oriented across, over, or under the landscape. Both variations are often seen in volcanically active areas, as well as places known to have been so in the past.
One type of sheet intrusion is called a laccolith. The molten material is typically pushed up between layers of overlying sedimentary rock, which in turn rises because of the force. A laccolith’s base is usually horizontal and the rock normally cools slowly. By the time these are discovered, the overlying layers have usually eroded away and it can be difficult to determine the original shape of the intrusion. Some of these structures or dome-shaped or they can be vertical in nature; wind and water often remove many layers of rock over time.
A sill is generally a kind of sheet intrusion that sits flat in between two other layers of rock, or even solidified lava. The material that is part of this structure usually forms deep underground, so pressure typically prevents gases from escaping while it hardens. Bubble-like formations that are common in lava are generally not seen in sills, which are protected for much of their life by the layers of rock surrounding them.
Computerized seismic mapping is often used to see where a sheet intrusion is. Sills and laccoliths can be mapped fully and the flow pattern of an underground sheet intrusion can be determined as well. The pressure during formation of the structure in the host rock, and how much space there was in the sediment when it formed, can be determined as well. Molten rock movement and erosion have led to the formation of many different features of landscapes throughout the world.