Describe Each of the Five Igneous Features

Underground Sheets

• A sill was formed by a layer of cooled magma. Typically, a sill runs horizontally through the earth. They vary dramatically in size. One can be less than an inch thick, or it can extend down for hundreds of feet. A sill may be localized, or it can run for hundreds of miles. Erosion may remove material surrounding a sill, and then it can be seen above the ground. It may appear as sheet-like layers in a rock wall or even a type of cliff.

Smaller Sheets

• Dikes were also formed by layers of magma, but they tend to be smaller than sills. The magma might have filled in existing openings in the earth's layers, or it could have forced its way through rock. If erosion has removed material surrounding the dike, a wall of igneous rock is exposed. It may run horizontally or vertically along the ground. If many dikes are in one area, they are called a swarm.

Under the Dome

• Laccoliths are dome-shaped features of igneous rock. They formed when magma pushed itself between layers of rock but did not spread out. The materials above the magma were forced up, creating a curved structure. Sedimentary rocks that often surround laccoliths erode easily, so the igneous domes are eventually exposed. Typically, the domes are less than 10 miles in diameter. Their thickness varies from hundreds to thousands of yards.

Saucer Rock

• Lapoliths form in the opposite direction of laccoliths. Magma pushes itself between layers of rock, but lower layers bend under the magma̵7;s weight and are packed together. This results in a shallow bowl or saucer shape. Lapoliths tend to be wide but not very deep. The diameters of the ̶0;saucers̶1; are 10 to 20 times greater than their thickness. The typical lapolith is far bigger than a laccolith.

Fire from Earth

• Perhaps the most familiar igneous features are volcanoes. These were formed when lava emerged from an opening and flowed onto the earth. Their shape can range from gentle slopes to steep cones. Although some active volcanoes are found in southern Europe, the Atlantic Ocean and Central Africa, most are located along the Pacific Ocean̵7;s Ring of Fire. This runs along the edges of the ocean, from the South and North American coastlines, across the Bering Strait and down to Japan, Indonesia and New Zealand.