Chapter 6 - Volcanoes

Volcano characteristics

- A volcano is a vent where magma and other volcanic products erupt onto the surface. - Many geologists reserve the term volcano for hills or mountains that have been constructed by volcanic eruptions. - Most volcanoes have a crater, a roughly circular depression usually located near the top of the volcano. - Volcanoes consist of volcanic rocks, which form from lava, pumice, volcanic ash, and other products of volcanism. - Magma erupts from fairly linear cracks called fissures and from huge circular depressions called calderas.

Flood Basalts

- Basaltic lava flows covering vast areas and commonly being several kilometers thick. - Fed by a series of long fissures. - Involve huge volumes of magma and represent the largest igneous eruptions on Earth. - Form large ocean plateaus at or below sea level. - Erupt from one or more long, nearly continuous fissures or from a discontinuous string of vents. There are typically multiple eruption centers, allowing large volumes of magma to erupt onto the surface in a geologically short period of time. - Probably begin at hot spots as rising mantle plumes first encounter the lithosphere. It is uncertain whether the magma comes from melted lithosphere, melted asthenosphere, or directly from the plume.

Caldera

- Caldera eruptions are among nature's most violent phenomena. They can spread volcanic ash over huge areas, and the largest erupt more than one thousand cubic kilometers of magma. As the magma withdraws from the magma chamber, the roof of the chamber collapses to form a depression tens of kilometers across. The depression may then fill with ash, lava flows, and sediment. - A large, basin-shaped volcanic depression, which typically has a low central part surrounded by a topographic escarpment, referred to as the wall of the caldera. - The formation of a caldera and the associated eruption occur simultaneously - the caldera subsides in response to rapid removal of magma from the underlying chamber. The largest caldera eruptions produced volcanic ash layers more than 1,000 meters thick.

Volcanic Dome

- Dome-shaped features that may be hundreds of meters high. They consist of solidified lava, which can be highly fractured or mostly intact. Domes include some volcanic ash intermixed with rock fragments derived from solidified lava in the dome. They form where felsic or intermediate magma erupts and is so viscous that it piles up around a vent. Many domes are within craters of composite volcanoes or within large calderas. - When domes collapse, they sometimes release deadly pyroclastic flows that rush downhill at hundreds of kilometers an hour. - Some domes have a nearly symmetrical dome shape, but most have a more irregular shape because some parts of the dome have grown more than other parts or because one side of the dome has collapsed downhill. - Domes can be destroyed by collapse or explosion.

How We Monitor Volcanoes

- Geologists monitor volcanoes using instruments that measure changes in topography, ground shaking, heat flow, gas output, and water chemistry. - How gas output changes before eruption: Gasses dissolved in a rising magma may come out of solution, expand, and provide the driving force for an eruption. An increased flow of gases from a volcano may indicate that magma is rising and losing it's gas.

Shield Volcano

- Have broad, gently curved slopes and can be relatively small (less than a kilometer across) or can form huge mountains tens of kilometers wide and thousands of meters high. They commonly contain a crater or line of craters and have fissures along their summit. Shield volcanoes consist mostly of basaltic lava flows with smaller amounts of scoria and volcanic ash. - The largest volcanoes on earth. - Mauna Loa is the world's largest volcano. - Kilauea is probably the most active volcano in the world. - These volcanoes erupt mostly low-viscosity basaltic lava and so are dominated by relatively nonexplosive outpourings of lava from fissures and vents. Early phases of eruptions are commonly marked by spectacular lava fountains.

How Viscosity Affects Gases in Magma

- More Viscous: Felsic magmas contain a lot of silica, and so they are relatively viscous. The high viscosity prevents gas from escaping easily. Gas builds up in the magma and, when it expands, greatly increases the pressure on the surrounding rock. This can cause explosive eruptions. - Less Viscous: Less viscous magma, such as one with a basaltic composition, allows gas bubbles to escape relatively easily. This can lead to a fairly nonexplosive eruption, such as this basaltic lava flow that flows smoothly downhill from the vent.

Pyroclastic Flow (Ash Flow)

- Some ash does not jet straight up but collapses down the side of the volcano as a dense, hot cloud of ash particles and gas. - Can be devastating because of it's high temperatures and high speeds.

Pyroclastic Eruptions

- Some explosive eruptions send molten lava into the air. A lava fountain, can accompany basaltic volcanism and results fro ma high initial gas content in a less viscous lava. The gas propels the lava and separates it into discrete pieces. - Other explosive eruptions eject a mixture of volcanic ash, pumice, and rock fragments into the air. Such airborne material is called tephra, and tephra particles that are sand-sized or smaller are volcanic ash. Ash mostly forms when bubbles blow apart bits of magma. Tephra is derived from pumice, fragmented volcanic glass, an shattered preexisting rocks.

Composite Volcano (stratovolcanoes)

- Typically fairly symmetrical mountains thousands of meters high, with moderately steep slopes and a crater at the top. They may be large, but are, on average, much smaller than shield volcanoes. Their name derives from the interlaying of lava flows, pyroclastic deposits, and volcanic mudflows. They consist mostly of intermediate-composition rocks, such as andesite, but can also contain felsic and mafic rocks. - Common above subduction zones and are especially numerous along the Pacific Ring of Fire. - Contain dikes, sills, and other intrusions. - Mount Fuji, Mount Kilimanjaro, and Mount Etna are the most famous composite volcanoes.

Lava Flows and Domes

- When magma erupts onto the surface and flows away from a vent, it crates a lava flow. Erupted lava can be fairly fluid, flowing downhill like a fast river of molten rock. Some lava flows are not so fluid and travel only a short distance before solidifying. - A lava dome forms from the eruption of highly viscous lava. The high viscosity of the lava is generally due to a high silica content and causes the lava to pile up around the vent, instead of flowing away. Domes are often accompanied by several types of explosive eruptions.

Non-vesicular Basalt

Basaltic magma may not contain enough gas to form bubbles, so the lava solidifies into non-vesicular basalt. A magma that forms non-vesicular basalt may have a low content of gas because it started out with a low content of dissolved gas or because it lost gas somewhere along the way. Most basalt has fractures.

Scoria Cone

Cone-shaped hills several hundred meters high, or higher, usually with a small crater at their summit. They're also called CINDER CONES because they contain loose black or red, pebble-sized volcanic cinders, along with larger volcanic bombs. The scoria is basaltic or, less commonly, andesitic in composition.

Vesicular Basalt

Contains abundant gas pockets (vesicles). The vesicles were gas bubbles that expanded in the magma and were trapped when the lava solidified. Vesicles occur in lava flows and in ejected material, such as highly vesicular scoria, which represents frothy, gas-rich magma.

Areas with the Highest Potential for Volcanic Hazards

In North America, volcanoes are relatively common along the west coast and virtually absent east of the Rocky Mountains. Tectonic setting, especially proximity to certain types of plate boundaries or to a hot spot, is the major factor making some places more prone to volcanic hazards than others.

Scoria

Scoria is highly vesicular. The fragments that explosively eject from the volcano during a lava fountain, may have been liquid or solid when ejected. Small blobs cool and solidify in the air to form scoria (cinders). Large blobs of magma and solid angular blocks are ejected as volcanic bombs.

Eruption Column

Tephra, which forms when magma is blown apart by volcanic gases, can erupt high in to the atmosphere, forming an eruption column. The tephra falls back to Earth as solidified and cooled pieces of rock.

Pompeii

Vesuvius is an active composite volcano near the city of Napes in southwestern Italy. In A.D. 79, a series of pyroclastic flows moved down the flank of the volcano, destroyed the coastal towns of Pompeii and Herculaneum, and killed the cities inhabitants.