Chapter 4
Chapter 4: Up from the Inferno: Magma and Igneous Rocks
Animation
At this ancient volcano at Shiprock, New Mexico, ash and lava flows have eroded away, leaving a “volcanic neck”. Large dikes radiate outward from the center, like spokes of a wheel. The softer rocks that once surrounded the dikes have eroded away, leaving a wall-like remnant of the dike exposed. Shiprock was once in the interior of a volcano or below a volcano. For more information, see page 102 and Figure 4.9c in your textbook.
This dark sill, exposed on a cliff in Antarctica, is basalt; the white rock is sandstone. A geologist’s sketch shows the cliff face as viewed face on. For more information, see page 102 and Figure 4.9a in your textbook.
Torres del Paines is a spectacular group of mountains in southern Chile. The light rock is a granite pluton, and the dark rock is the remains of the country rock into which the pluton intruded. A screen of country rock (in the lower half) hides the front of the pluton. A geologist’s sketch labels the two major rock units. For more information, see page 102 and Figure 4.10 in your textbook.
Molten rock, or melt, develops only in special locations in the Earth: where a plume of hot mantle rock rises to the base of the lithosphere (a volcano above such a plume is a hot-spot volcano); in the asthenosphere above subducting oceanic lithosphere at a convergent plate boundary (the chain of volcanoes that results is a volcanic arc); in the asthenosphere beneath a mid-ocean ridge; and along a continental rift. For more information, see Geology at a Glance: Formation of Igneous Rock starting on p. 105 in your textbook.
Igneous magmas form at mid-ocean ridges because of decompression melting of the rising asthenosphere. Magma rises into the crust and pools in a magma chamber during sea-floor spreading. Some cools slowly along the margins of the magma chamber to form massive gabbro, while some intrudes upward to fill vertical cracks that appear as the newly formed crust splits apart. Magma that cools in the cracks creates basalt dikes, and magma that makes it to the sea floor extrudes as pillow basalt. For more information, see Geology at a Glance: Formation of Igneous Rock starting on p. 105 in your textbook.