Chapter 4 Volcanism and Extrusive Rocks IndexIndex  

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Definitions  Magma – Molten rock, usually mostly silica.  Lava – Magma on the earth’s surface.  Pyroclasts – Fragment of rock formed by volcanic explosion.  Extrusive Rock – Any igneous rock that forms at the earth’s surface, whether it solidifies directly from a lava flow or is pyroclastic.  Volcanism – Volcanic activity, including the eruption of lava, rock fragments, and gas explosions.  Volcano – A hill or mountain constructed by the extrusion of lava or rock fragments from a vent. 

Effects on Humans The Growth of Islands Geothermal Energy In some other areas of geologically recent volcanic activity, underground heat generated by igneous activity is harnessed for human needs. Effect on Climate Occasionally, a volcano will spew large amounts of fine, volcanic dust and gas into the high atmosphere. Winds can keep fine particles suspended over the earth for years. Volcanic Catastrophes Usually hot ashes, tsunamis, lava flow, flying debris, and other mass from the volcano makes a mess for us humans. (Fig. 4.2)Fig. 4.2 

Eruptive Violence and Physical Characteristics of Lava  What determines the degree of violence associated with volcanic activity? Whether eruptions are very explosive or relatively “quiet” is largely determined by two factors: (1) the amount of gas in the lava or magma and (2) the ease or difficulty with which the gas can escape to the atmosphere.  The viscosity, or resistance to flow, of a lava determines how easily the gas escapes.  The two most important factors that influence viscosity are (1) the temperature of the lava relative to the cooler temperature at which it solidifies and (2) the silica (SiO 2 ) content of the lava. (A third factor is gas dissolved in magma – the greater the dissolved gas content, the more fluid.) 

Gas  From active volcanoes we have learned that most of the gas released during eruptions is water vapor, which condenses as steam.  A pyroclastic flow is a mixture of gas and pyroclastic debris that is so dense that it hugs the ground as it flows rapidly into low areas. (Fig. 4.6)Fig. 4.6 

Extrusive Rocks  Most extrusive rocks are named and identified on the basis of their composition and texture. However, some are named exclusively on texture (e.g., pumice).  Composition Composition  Textures Textures 

Composition  The amount of silica in a lava largely controls not only the viscosity of lava and the violence of eruptions but also which particular rock is formed.  Rhyolite, a silicic rock, is usually cream-colored, tan, or pink; it is made up mostly of feldspar but always includes some quartz.  A basalt has a relatively low amount (~50% by weight) of SiO 2. (Fig. 4.10)Fig  Andesite, which crystallizes from an intermediate lava, can be recognized by its moderately gray or green color. (Fig. 4.9)Fig. 4.9 Back

Textures  Texture refers to a rock’s appearance with respect to the size, shape, and arrangement of its grains or other constituents.  Two critical factors determine grain size during the solidification of igneous rocks: rate of cooling and viscosity. If lava cools rapidly, the atoms have time to move only a short distance; they bond with nearby atoms, forming only small crystals. With extremely rapid or almost instantaneous cooling, individual atoms in the lava are ‘frozen’ in place, forming glass rather than crystals. (Fig. 4.8)Fig. 4.8 Porphyritic Textures Textures Due to Trapped Gas Fragmental Textures Back

Porphyritic Textures  Porphyritic rock – An igneous rock in which large crystals are enclosed in a matrix (or ground mass) of much finer-grained minerals or obsidian.  Phenocrysts – Any of the large crystals in porphyritic igneous rock. Back

Textures Due to Trapped Gas  A magma deep underground is under high pressure, generally high enough to keep all its gases in a dissolved state. On eruption, the pressure is suddenly released and the gases come out of solution.  When a lava solidifies while gas is bubbling through it, holes are trapped in the rock, creating a distinctive vesicular texture. Vesicles are cavities in extrusive rock resulting from gas bubbles that were in lava.  In more viscous lavas, where the gas cannot escape as easily, the lava is churned into a froth. When cooled quickly, it forms pumice (Fig. 4.11), a frothy glass with so much void space that it floats in water.Fig Back

Fragmental Textures  Pyroclasts, the fragments formed by volcanic explosion, can be almost any size. Dust and ash are the finest particles; cinders are about the size of sand grains; bombs and blocks are large pyroclasts. When solid rock has been blasted apart by a volcanic explosion, the pyroclastic fragments are angular, with no rounded edges or corners and are called blocks. If lava is ejected into the air, a molten blob becomes streamlined during flight, solidifies, and falls to the ground as a bomb, a spindle or lens-shaped pyroclast. (Fig. 4.12)Fig  A tuff is a rock composed of fine-grained pyroclastic particles.  Fig Fig Back

Volcano  Vent – The opening in the earth’s surface through which a volcanic eruption takes place.  Crater – A basin-like depression over a vent at the summit of a volcanic cone.  Flank Eruption – An eruption in which lava erupts out of a vent on the side of a volcano.  Caldera – A volcanic depression much larger than the original crater.  Fig Fig 

Types of Volcanoes Shield Volcanoes Cinder Cones Composite Volcanoes Volcanic Domes 

Shield Volcanoes  Shield volcanoes are broad, gently sloping cones constructed of solidified lava flows. During eruptions, the lava spreads widely and thinly due to its low viscosity. Because the lava flows from a central vent, without building up much near the vent, the slopes are usually between 2 o and 10 o from the horizontal, producing a volcano in the shape of a flattened dome or ‘shield’. (Fig. 4.15)Fig  A minor feature called a spatter cone, (Fig. 4.18) a small steep-sided cone built from lava sputtering out of a vent will occasionally develop a solidifying lava flow.Fig  Back Back

Cinder Cones  A cinder cone is a volcano constructed of pyroclasts ejected from a central vent (Fig. 4.19). In contrast to the gentle slopes of shield volcanoes, cinder cones commonly have slopes of about 30 o.Fig  Cinder cones are a product of concentration of gas in magma rather than of a particular chemical composition of magma. Most cinder cones are associated with mafic or inter-mediate lava. Silicic cinder cones, which are made of fragments of pumice, are also known as pumice cones. Back

Composite Volcanoes  A composite volcano is one constructed of alternating layers of pyroclastics and rock solidified from lava flows. (Fig. 4.20)Fig  Composite volcanoes are built over long spans of time. Eruption is intermittent, with hundreds or thousands of years of inactivity separating a few years of intense activity.  Nearly all the better known volcanoes of the world are composite volcanoes. Back

Volcanic Domes  Volcanic domes are steep-sided, dome- or spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcano vent. (Fig. 4.25)Fig  Most of the viscous lavas that form volcanic domes are high in silica. Back

Lava Floods  Not all extrusive rocks are associated with volcanoes. Lava that is very non-viscous and flows almost as easily as water, does not build a cone around its vents. Such lava is, of course, mafic (low in silica).  Plateau basalts – Layers of basalt flows that have built up to great thicknesses.  Back to the Beginning Back to the Beginning

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