A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano Island of Sicily which in turn, was named after Vulcan, the Roman god of fire.
VOLCANO TYPES
A volcanic vent is an opening exposed on the earth's surface where volcanic material is emitted. All volcanoes contain a central vent underlying the summit crater of the volcano. The volcano's cone-shaped structure, or edifice, is built by the more-or-less symmetrical accumulation of lava and/or pyroclastic material around this central vent system. The central vent is connected at depth to a magma chamber, which is the main storage area for the eruptive material. Because volcano flanks are inherently unstable, they often contain fractures that descend downward toward the central vent, or toward a shallow-level magma chamber. Such fractures may occasionally tap the magma source and act as conduits for flank eruptions along the sides of the volcanic edifice. These eruptions can generate cone-shaped accumulations of volcanic material, called parasitic cones. Fractures can also act as conduits for escaping volcanic gases, which are released at the surface through vent openings called fumaroles.
Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their eruptive patterns and their general forms. The form and composition of the three main volcano types are summarized here:
VOLCANO | VOLCANO | COMPOSITION | ERUPTION |
| Basalt tephra; occasionally andesitic | ||
| Basalt lava flows | ||
| Highly variable; alternating basaltic to rhyolitic lavas and tephra with an overall andesite composition |
SUBORDINATE VOLCANO TYPES -- Lava and tephra can erupt from vents other than these three main volcano types. A fissure eruption, for example, can generate huge volumes of basalt lava; however, this type of eruption is not associated with the construction of a volcanic edifice around a single central vent system. Although point-source eruptions can generate such features as spatter cones and hornitos,
these volcanic edifices are typically small, localized, and/or associated with rootless eruptions (i.e., eruptions above the surface of an active lavaflow, unconnected to an overlying magma chamber) . Vent types related to hydrovolcanic
processes generate unique volcanic structures, discussed separately under hydrovolcanic eruptions.
Scoria cones, also known as cinder cones, are the most common type of volcano. They are also the smallest type, with heights generally less than 300 meters. They can occur as discrete volcanoes on basaltic lava fields, or as parasitic cones generated by flank eruptions on shield volcanoes and stratovolcanoes.
Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of the volcano types. Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua Shield volcanoes are broad, low-profile features with basal diameters that vary from a few kilometers to over 100 kilometers (e.g., the Mauna Loa volcano, Hawaii). Their heights are typically about 1/20th of their widths. The lower slopes are often gentle (2-3 degrees), but the middle slopes become steeper (~10 degrees) and then flatten at the summit. This gives shield volcanoes a flank morphology that is convex in an upward direction.
in Guatemala.
PRODUCTS OF VOLCANIC ERUPTION
Two types of lava are named according to the surface texture: ʻAʻa (pronounced [ˈʔaʔa]) and pāhoehoe ([paːˈho.eˈho.e]), both words having Hawaiian origins. ʻAʻa is characterized by a rough, clinkery surface and is the typical texture of viscous lava flows. However, even basaltic or mafic flows can be erupted as ʻaʻa flows, particularly if the eruption rate is high and the slope is steep.
Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Usually, only mafic flows will erupt as pāhoehoe, since they often erupt at higher temperatures or have the proper chemical make-up to allow them to flow with greater fluidity.
- Tephra and Pyroclasts
The rapid eruption of expanding gases results in the obliteration and fragmentation of magma and rock. The greater the explosivity, the greater the amount of fragmentation. Individual eruptive fragments are called pyroclasts ("fire fragments"). Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.
Types of Pyroclasts
- Lahar
is an Indonesian term for a volcanic mudflow. These lethal mixtures of water and tephra have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water. These mud slurries carry debris ranging in size from ash to lapilli, to boulders more than 10 meters in diameter. Lahars can vary from hot to cold, depending on their mode of genesis. The maximum temperature of a lahar is 100 degrees Centigrade, the boiling temperature of water. - Volcanic gases
Other than free oxygen, generated by photosynthesis, all atmospheric gases were derived from inside the earth and released by volcanic eruptions. The gaseous portion of magma varies from ~1 to 5% of the total weight. Water vapor constitutes 70-90%. The remaining gases include CO2, SO2, and trace amounts of of N, H, CO, S, Ar, Cl, and F. These subordinate gases can combine with hydrogen and water to produce numerous toxic compounds, such as HCl, HF, H2SO4, H2S, which are typical products of fumarolic activity.
MAIN GASES | TRACE GASES | TOXIC GASES |
H2O (70-90%) | N, H, S, | HCl, HF |
A variety of sulfur aerosols may be present and sulfur itself may condenses around the fumarole into a crystalline accumulation called sulfaterra (yellow ground). On some volcanoes, enough sulfur is present to be mined as an economic resource. H2S is sometimes called "sewer gas" because it has a rotten egg odor. It is an insidious poison that irritates the eyes, nose, and throat. SO2 on the other hand, is the biting, chocking gas that you smell right after you've lit a kitchen match. When these two gases occur together, they react quickly with each other (within minutes) to produce sulfaterra and water vapor.
Terms on Continental Sea Floor Spreading
asthenosphere — a portion of the mantle which underlies the lithosphere. This zone consists of easily deformed rock and in some regions reaches a depth of 700 km.
continental drift — The first hypothesis proposing large horizontal motions of continents. This idea has been replaced by the theory of plate tectonics.
convergent plate boundary — a boundary between two lithospheric plates that move towards each other. Such boundaries are marked by subduction, earthquakes, volcanoes, and mountain-building.
Curie point — the temperature (about 580 degrees C) above which a rock loses its magnetism.
deep-sea trenches — long, narrow, and very deep (up to 11 km) basins oriented parallel to continents and associated with subduction of oceanic lithosphere.
divergent plate boundary — a boundary between two plates that move away from one another; new lithosphere is created between the spreading plates.
lithosphere — the rigid, outermost layer of the Earth; includes crust and uppermost mantle and is about 100 km thick.
mid-ocean ridge — a continuous mountain chain on the floor of all major ocean basins which marks the site where new ocean floor is created as two lithospheric plates move away from one another.
normal polarity — a magnetic field that has the same direction as the Earth's present one.
paleomagnetism — the permanent magnetization recorded in rocks that allows reconstruction of the Earth's ancient magnetic field.
Pangaea or Pangea — the proposed "supercontinent" that began to break apart 200 million years ago to form the present continents.
plate tectonics — the theory that proposes that the Earth's lithosphere is broken into plates that move over a plastic layer in the mantle. Plate interactions produce earthquakes, volcanoes, and mountains.
reversed polarity — a magnetic field with direction opposite to that of the Earth's present field.
transform plate boundary — a boundary between lithosphere plates that slide past one another.
sea-floor spreading — a hypothesis, proposed in the early 1960s, that new ocean floor is created where two plates move away from one another at mid-ocean ridges.
subduction zone — a long, narrow zone where one lithospheric plate descends beneath another.
Figures in Continental Sea floor Spreading
Continental Sea-Floor Spreading
The Earth's layers — The Earth is a layered planet consisting of crust, mantle and core (Fig. 1). The outer 100 km or so is a rigid layer called the lithosphere, which is made up of the crust and uppermost mantle. The lithosphere is broken into a number of large and small plates that move over the asthenosphere, a plastic layer in the upper mantle. Earthquakes and volcanoes are concentrated at the boundaries between lithospheric plates. It is thought that plate movement is caused by convection currents in the mantle (Fig. 2), although the exact mechanism is not known. Lithosphere plates are moving at rates of a few cm per year.
Types of plate boundaries — There are three types of boundaries between lithospheric plates (Fig. 3):
1) convergent boundary — plates converge, or come together. If a plate of oceanic lithosphere collides with thicker and less dense continental lithosphere, the denser oceanic plate will dive beneath the continent in a subduction zone (Fig. 2).
2) divergent boundary — two plates diverge, or move apart and new crust or lithosphere is formed.
3) transform fault boundary — plates slide past one another with no creation or destruction of lithosphere.
The Ocean Floor — A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and deep trenches. Mid-ocean ridges are part of chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the longest mountain chain on Earth. These ridges are spreading centers or divergent plate boundaries where the upwelling of magma from the mantle creates new ocean floor.
Deep-sea trenches are long, narrow basins which extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle (Fig. 2).
Continental drift — The idea that continents move is an old one; Alfred Wegener, a German meteorologist, proposed the hypothesis of continental drift. in the early 1900's. Wegener used several lines of evidence to support his idea that the continents were once joined together in a supercontintent called Pangaea and have since moved away from one another: (1) the similarity in shape of the continents, as if they once fit together like the pieces of a jigsaw puzzle; (2) the presence of fossils such as Glossopteris, a fossil fern whose spores could not cross wide oceans, on the now widely-separated continents of Africa, Australia, and India; (3) the presence of glacial deposits on continents now found near the equator; and (4) the similarity of rock sequences on different continents.
Wegener's hypothesis of continental drift was not widely accepted because he had no mechanism to explain how the continents move. The idea was not revived until new technology made exploration of the ocean floor possible.
Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle (Fig. 2). A test of the hypothesis of sea-floor spreading was provided by studies of the Earth's magnetism.
The Earth's Magnetic Field — The Earth's magnetic field is thought to arise from the movement of liquid iron in the outer core as the planet rotates. The field behaves as if a permanent magnet were located near the center of the Earth, inclined about 11 degrees from the geographic axis of rotation (Fig. 4). Note that magnetic north (as measured by a compass) differs from geographic north, which corresponds to the planet's axis of rotation.
Placing a bar magnet beneath a piece of paper with iron filings on it will create a pattern as the filings align themselves with the magnetic field generated by the magnet. The Earth's magnetic field is similar to that generated by a simple bar magnet. At present, the lines of force of the Earth's magnetic field are arranged as shown in Figure 4; the present orientation of the Earth's magnetic field is referred to as normal polarity. In the early 1960s, geophysicists discovered that the Earth's magnetic field periodically reverses; i.e. the north magnetic pole becomes the south pole and vice versa. Hence, the Earth has experienced periods of reversed polarity alternating with times (like now) of normal polarity. Although the magnetic field reverses at these times, the physical Earth does not move or change its direction of rotation.
Basaltic lavas contain iron-bearing minerals such as magnetite which act like compasses. That is, as these iron-rich minerals cool below their Curie point, they become magnetized in the direction of the surrounding magnetic field. Studies of ancient magnetism (paleomagnetism) recorded in rocks of different ages provide a record of when the Earth's magnetic field reversed its polarity.
During World War II, sensitive instruments called magnetometers were developed to help detect steel-hulled submarines. When research scientists used magnetometers to study the ocean floor, they discovered a surprising pattern. Measurements of magnetic variations showed that, in many areas, alternating bands of rocks recording normal and reversed polarity were arranged symmetrically about mid-ocean ridges (Fig. 5).
In 1963, F. Vine and D.H. Matthews reasoned that, as basaltic magma rises to form new ocean floor at a mid-ocean spreading center, it records the polarity of the magnetic field existing at the time magma crystallized. As spreading pulls the new oceanic crust apart, stripes of approximately the same size should be carried away from the ridge on each side (Fig. 5). Basaltic magma forming at mid-ocean ridges serves as a kind of "tape recorder", recording the Earth's magnetic field as it reverses through time. If this idea is correct, alternating stripes of normal and reversed polarity should be arranged symmetrically about mid-ocean spreading centers. The discovery of such magnetic stripes provided powerful evidence that sea-floor spreading occurs.
The age of the sea-floor also supports sea-floor spreading. If sea-floor spreading operates, the youngest oceanic crust should be found at the ridges and progressively older crust should be found in moving away from the ridges towards the continents. This is the case. The oldest known ocean floor is dated at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea trenches.
It took exploration of the ocean floor to discover sea-floor spreading, the mechanism for the movement of continents that Alfred Wegener lacked. The hypothesis of continental drift gained renewed interest and, when combined with sea-floor spreading, led to the theory of plate tectonics. The history of thought about the movement of continents provides a wonderful example of how hypotheses such as continental drift and sea-floor spreading are thoroughly tested before a new theory emerges.
