Volcanoes

Vol canism

Volcanism is directly related to the internal structure of Earth and interaction among tectonic plates. Volcanism occurs in divergent and convergent boundaries. Divergent boundaries occur within ocean basins and continents marked by midocean ridges and rift valleys, such as the Icelandic volcanism and Great Rift Valley of East Africa, respectively. Convergent boundaries are complex according to the kind of plate's interaction: oceanic plate/oceanic plate, oceanic plate/continental plate, or continental plate/continental plate. Intraplate volcanism occurs less frequently, either undersea as in the Hawaiian Islands or in continents such as the Yellowstone Volcanic Field. Geodynamic settings determine the physical and chemical characteristics of the magmas.

Magma is molten rock of silicate composition. Magma is generated by the partial melting at the asthenosphere or within the lithosphere. There rocks are under high pressure and temperature, and just slight changes due to increase in temperature, decompression, or addition of fluid phases can cause rocks to melt to generate magma. The lower density of the magma compared with the surrounding rock favors the ascent. During the ascent, processes such as fractional crystallization, magma mixing, assimilation, and contamination modify the original chemical magma composition and determine their temperature, viscosity, and gas contents and consequently the type of eruption.

The classification of igneous rocks is based on texture and chemical and mineralogical composition. Texture reflects the conditions of cooling. Coarse-grained textures with large crystals result when magma cools slowly beneath the surface. Fine-grained textures occur when lava cools rapidly at the surface. Basalt, andesite, dacite, and rhyolite are the common extrusive igneous rocks. Gabbro, diorite, granodiorite, and granite are their intrusive equivalents. Petrologic studies elucidate the origin and evolution of magma that generates igneous rocks.

Types of Eruptions

Eruptions occur when the magma reaches the surface through a vent or crater. However, magma [Page 3030]also erupts through fissures on the flanks of a volcano or in flat areas. An initial classification of eruptions differentiates between effusive and explosive eruptions.

Effusive eruptions are characterized by emissions of lava. Eruptions of low-viscosity and high-temperature lavas produce pahoehoe morphological features. In contrast, aa lavas present a fragmented and rough morphology due to a relatively higher viscosity. With the increase in viscosity of lavas and decrease of temperature, domes may form. The accumulation of lavas over geological time generated shields, composite, and stratovolcanoes. The eruptions of Hawaiian volcanoes are typically effusive.

Explosive eruptions are produced by the sudden separation of volatiles from magma, fragmenting it and the surrounding rocks inside volcanic edifices. Volcanic explosions eject fragmented material known as tephra into the atmosphere by eruptive columns or produce pyroclastic flows sweeping the volcanoes’ flanks. Tephra includes various sizes of particles: block (>64 mm [millimeters]), lapilli (2–64 mm), and ash (<2 mm). Due to temperature contrasts with the atmosphere, it rises until it loses buoyancy and then travels under the influence of the wind and falls, mantling the landscape with a thickness from millimeters to meters. These deposits are called fall deposits. Based on the analysis of distribution, thickness, and chronostratigraphic studies of fall deposits, volcanologists infer the magnitude and development of an eruption. When an eruptive column collapses due to its high density, dense clouds of tephra flow down over the volcanoes slopes, forming pyroclastic flows. A pyroclastic flow consists of a hot mixture of fine and coarse fragments of rock, pumice, and gas, and its flow is controlled by the topography, moving fast along canyons or ravines until it stops and the material is deposited. According to the components, ash flows, pumice or scoria flows, and block and ash flows can be distinguished.

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