The planet we live on: The beginnings of the Earth Sciences

Figure 1.43: Basalt erupting from a fissure, with its low viscosity, running like water intoa hollow.gas are often trapped in basaltic lavas as they cool, and become preserved as sphericalvesicles; there is an illustration of this in Figure 1.41. In very active volcanic areas, manysheets of basalt can be laid down in this way, as tabular basalts over wide areas. Someof these shrink as they cool, breaking into the vertical polygonal columns of columnarbasalt (shown in Figure 1.44). If basalts are erupted under water, they are often extrudedas thin tongues of lava that cool quickly into pillow-like shapes. Each pillow has a veryfine-grained (chilled) edge, is slightly coarser inside, and also often contains vesicles. Asthe eruptions continue, the pillows build up, so a sequence of pillow basalts (shown inFigure 1.45) shows that the eruption must have happened underwater.Intermediate magmas are more viscous than mafic ones. While they can flow out of centralvolcanic vents as thick andesite lavas, they often solidify in the vent. Pressure builds upon this volcanic plug and eventually the volcano erupts violently, often ejecting volcanicblocks and huge quantities of volcanic ash high into the air. <strong>The</strong>se solid eruptionproducts are called pyroclastics (pyro = fire-formed; clastic = broken material). <strong>The</strong>blocks and ash rain down on the surrounding area, often producing the typical cone-shapeof many central vent volcanoes, as shown in the photograph in Figure 1.46. Andesiticvolcanoes are much more dangerous than basaltic ones and some eruptions have killedthousands of people. To simulate magma viscosity, try the ‘See how they run: investigatewhy some lavas flow further and more quickly than others’ activity on the http://www.earthlearningidea.com <strong>we</strong>bsite.Silicic magmas are even more viscous than intermediate ones, which is why silicic lavasare rarely found. Instead, silicic volcanoes tend to have very violent eruptions, eruptingenormous volumes of ash from central vents catastrophically (Figure 1.47).Igneous magmas that don’t reach the surface often intrude through the rocks aboveas large upside-down raindrop shapes. <strong>The</strong>se intrusive large drop-shapes, that can be29