GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Igneous intrusion. Cut-away illustration of the features produced when rising magma (molten rock) intrudes into existing rock. Clockwise from upper right: a volcano, formed when magma erupts through a conduit to the surface as lava; a stock or basolith, formed when rising magma replaces or forces aside existing rock; ring dykes, concentric vertical intrusions which cut through existing rock layers; sills, intrusions which run between the existing layers; and radial dykes, another form of vertical intrusion which may run for hundreds of miles. At bottom is a magma chamber.
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