Vents, Hot Spots, and Domes Help

Vents

From the deepest levels of the mantle, cracks or openings to the surface from the magma chamber are called volcanic vents . Magma is forced upward through these paths, then ejected as lava or pyroclastic ejecta. A vent located at the top of a volcano and at the end of a vertical magma feeder pipe is known as a central vent .

Vents mark the beginnings of a volcano and can have different shapes, but are mostly rounded. When a vent opens as a long slit in the ground, it is called a fissure . Sometimes lava is ejected forcibly from these fissures, but they can also ooze lava out regularly.

Fire fountains are the amazing jet-like sprays of liquid lava that spurt from central vent Hawaiian type eruptions. These incandescent jets spray hundreds of meters into the air. They shoot in brief spurts or sometimes provide fantastic fiery shows for hours at a time.

In 1959, the greatest fire fountain ever recorded in Hawaiian history shot lava 580 meters into the air from the Kilauea Iki vent. Then in 1986, this record was nearly tripled when an eruption on the Japanese Island of Oshima sent a lava fountain 1600 meters into the air. When fire fountain sprays are blown away downwind, they often produce an airborne curtain of glowing fragments that showers downward in a fiery breeze.

When small pyroclasts are carried downwind in a fire fountain, they cool quickly into black, volcanic glass beads, teardrop shapes, and oblong forms, known as Pele ’ s tears . When the wind is really blowing, the beads are sometimes pulled into long glass fibers. Then, when the fibers are broken off from the main bead, they are called Pele ’ s hair . I wonder if these glass fibers gave engineers the idea for fiber optics.

Sometimes after all the magma is used up, volcanic vents keep releasing gas fumes and steam through small vents called fumaroles .

Circulating groundwater that seeps down to hot magma is super heated and sent back up to the surface as hot springs and geysers . A geyser is a hot-water fountain that shoots a spray high into the air. Old Faithful geyser in Yellowstone Park in the northern United States sends a jet of water over 60 meters in the air roughly every hour.

Sometimes when magma and pyroclasts explode from vents deep in the interior, the vents and feeder channels are then filled with coarse fragments called breccia . The structure that is formed from all this rock debris is known as a diatreme . The minerals and rock found in some diatremes are formed at extreme depths in the upper mantle and then forced under pressure to the surface to be ejected explosively.

The surrounding softer soil often erodes from around a hard diatreme and leaves the rock standing like a lone guard. This is the case of a towering structure called, Shiprock, standing in New Mexico in the southwestern United States.

Hot Spots

Volcanoes formed singly, away from plate boundaries, are usually a result of hot spots .

A hot spot on the earth’s surface is generally found over a mantle chamber of high pressure and temperature magma. For example, the temperature of basalt lava at Kilauea reaches 1160°C.

Volcanoes are formed from vents that rise to the surface from the magma chamber. Island chains like the Hawaiian Islands were formed over a hot spot in the mantle. As the plate goes through the process of new magma upwelling within deep ocean rifts, the original volcano overhead moves on past the hot spot. The hot spot then begins forming a new volcano. Repeated eruptions from the hot spot create a string of closely spaced chain volcanoes.

Currently, a new island called Loihi is being formed at the end of the Hawaiian Island chain over the hot spot. Its peak is 1000 meters below the water’s surface and will break the surface in the next 10,000 to 100,000 years. Figure 11-2 shows the formation of hot spot volcanic islands.

Fig. 11-2. As the crust moves over a hot spot, a volcano is formed.

Rocks from a string of volcanoes that have passed over a hot spot can be studied and their age calculated. In this way, the speed and direction of a plate’s movement over that spot can be figured out.

Domes

Felsic lavas are so sluggish they barely flow. This type of lava usually makes up a volcanic dome globbed out from a central or side vent. Domes are rounded and steep-sided masses of rock that can plug vents and trap gases. When this happens, the pressure builds from below until the dome is blasted to pieces in a violent eruption. This is what happened when Mount St. Helens erupted in 1980.

Practice problems of this concept can be found at: Volcano Practice Test