When molten rock (magma) rises to the surface and flows out onto the land or into the oceans, it is called lava . After lava cools and hardens, it becomes rock. Figure 11-1 shows the path that magma takes to the surface to become a volcano. By forcing its way out of the magma chamber deep within the earth, it makes its way upward. Heat and increasing pressure keep pushing and melting the rock in the way, until finally magma erupts at the surface as lava or tephra or under the sea in a boiling gush.
Fig. 11-1. Magma surges to the surface in a volcanic eruption.
When Mount St. Helens in Oregon, Mount Vesuvius in Italy, and Mount Pinatubo in the Philippines erupted at different times in history, human kind realized how powerless it was when compared to the tremendous forces of nature.
Basalt is the most common volcanic igneous rock in the Solar System. The majority of the crusts of Mercury, Venus, Earth, the Moon, and Mars are made up of basalt.
Basaltic rock is an extrusive rock found over much of the earth’s crust. It is composed mostly of flood , pillow , pahoehoe , and Aa lava. The different types and textures are formed according to how fluid the different lava types are during an eruption and how quickly they cool to form rock.
Nearly all of the Earth’s ocean floor is made up of basalt. It is a mafic rock with only about 50% silica by weight.
Flood lava is pretty much what it sounds like, a flood of molten rock. It is made up of running basaltic magma that erupts on flat land and spreads out in thin sheets as a flood of lava. When there are a bunch of different lava floods over a period of time, they pile up on top of each other forming thick basaltic lava plateaus, called flood basalts . The Columbia River plateau of Oregon, Idaho, and Washington in the United States is a huge flood basalt covering millions of square kilometers. The Deccan and Siberian Traps (flood basalts) of India and Asia are even larger.
Pahoehoe and Aa lava types are made up of cooling basaltic lava that has flowed downhill. Pahoehoe is the Hawaiian word for “ropy” and it looks like long, thick, stringy strands of rope. It forms when fluid lava flows into sheets and forms a cooled glassy skin. As the molten lava beneath the surface moves along, it pulls and twists the surface skin into thin, sinewy, coiled folds.
Aa lava is very different in form from pahoehoe lava. It looks like freshly dug earth, after it has been turned over in preparation for spring planting. However, it is really hot lava! It is said that someone trying to cross Aa lava in bare feet yells, “Aa! Aa!” when discovering the mistake. Aa lava has lost most of its gases and flows much more slowly and steadily than pahoehoe lava. A thick skin forms at the cooler surface and as the lava keeps moving, it cracks and breaks into jagged chunks and blocks. The moving flow piles up boulders in front of it like a bulldozer.
Pillow lava is formed from slow, gurgling underwater eruptions. These oval, blobs of lava look a bit like rounded sand bags of 20–100 cm in width. When geologists find pillow lava on land, they know that the area was once underwater. Divers have seen pillow lava formed on the ocean floor off the coast of Hawaii. Flows of molten basaltic lava form tough skins when they hit cold ocean water, but the lava on the inside is a different story. It cools much more slowly forming a crystalline structure, while the outside skin hardens to a smooth (non-crystalline) glass. As the pillow lava continues to flow, it pushes the hot inner lava through the cooled glass shell and cracks it. When this happens, a new oval, lava lobe is formed. As the flow continues, these lobes break off and cool as individual masses (pillows).
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