DAVID HARDY / SCIENCE PHOTO LIBRARY DAVID HARDY / SCIENCE PHOTO LIBRARY
Illustration showing types of volcanic intrusion. At left is a large magma chamber, above which is a ring dyke (converging toward the surface) and cone sheets (diverging toward the surface). The dark brown 'T' shape to the right of the chamber is a sill, where lava has been injected between sheets of rock. If there is a bulge in the rock, then a laccolith forms (mushroom-shaped feature). The vertical sheet intrusion is a normal dyke, exploiting a fault or crack. When found together, several dykes form a swarm. Next to the dykes is a pluton, a rising bubble of magma - a collection of these forms a batholith (far right). The small intrusions right of centre are laccoliths.
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