Subsequent to final crystallization and solidification, such rocks can be exhumed by erosion many thousands or millions of years later and be exposed as large bodies of so-called granitic rocks, as, for example, those spectacularly displayed in Yosemite National Park and other parts of the majestic Sierra Nevada mountains of California.
Two Polynesian terms are used to identify the surface character of Hawaiian lava flows. Aa , a basalt with a rough, blocky appearance, much like furnace slag, is shown at the left. Pahoehoe , a more fluid variety with a smooth, satiny and sometimes glassy appearance, is shown at the right. L ava is red hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies.
Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty, blocky masses.
A ll magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid not viscous , the gases may escape easily. But if the lava is thick and pasty highly viscous , the gases will not move freely but will build up tremendous pressure, and ultimately escape with explosive violence.
Gases in lava may be compared with the gas in a bottle of a carbonated soft drink. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake.
A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents.
Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.
Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface. The Evolution of a Composite Volcano A. Magma, rising upward through a conduit, erupts at the Earth's surface to form a volcanic cone. Lava flows spread over the surrounding area.
As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepend their valleys.
When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief.
All that remains is a projecting plug or "volcanic neck," a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau. Shield volcanoes The internal structure of a typical shield volcano S hield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Steppes usually do not receive enough rain for tall grasses and trees to grow.
Tropical grasslands are called savannas. Savannas exist in places that are warm throughout the year. They often have scattered trees. Savannas such as the Serengeti plains stretch across much of central Africa. Not all plains are grasslands. Forested plains have different types of trees, shrubs, and other vegetation. Deserts can also be plains. Parts of the Sahara, a great desert in North Africa, are plains. In the Arctic , where the ground is frozen, plains are called tundra.
Despite the cold, many plants survive here, including shrubs and moss. Plain Formation Plains form in many different ways. Some plains form as ice and water erodes, or wears away, the dirt and rock on higher land. Water and ice carry the bits of dirt, rock, and other material, called sediment , down hillsides to be deposited elsewhere. As layer upon layer of this sediment is laid down, plains form. Volcanic activity can also form plains. Lava plains form when lava pushes up from below ground and flows across the land.
The earth in a lava plain is often much darker than the surrounding soil. The dark earth is a result of the lava, mostly a dark-colored mineral called basalt , broken down into tiny particles over millions of years.
The movement of rivers sometimes forms plains. Many rivers run through valleys. As rivers move from side to side, they gradually erode the valley , creating broad plains. As a river floods, it overflows its bank. The flood carries mud , sand , and other sediment out over the land. After the water withdraws, the sediment remains. If a river floods repeatedly, over time this sediment will build up into a flood plain. Flood plains are often rich in nutrients and create fertile farmland.
Around the Pacific Rim is Indonesia, a nation built from the dotted volcanoes of an island arc. Indonesia is distinctive for its rich volcanic soil, tropical climate, tremendous biodiversity, and volcanoes. These volcanoes are in Java, Indonesia. The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes.
Eruptions also take place through other types of vents, commonly from fissures Figure 1. The eruptions that created the entire ocean floor are essentially fissure eruptions. Figure 1. Viscous lava flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome.
Because it is so thick, the lava does not flow far from the vent.
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