In previous exercises and in your reading you learned that magmas are produced by partial melting of the mantle in the asthenosphere, or by partial melting of the base of crust above a down-going (subducting) lithospheric plate, adjacent to convergent plate boundaries. Partial melting in the asthenosphere gives rise to basaltic magma, which is extruded onto the sea floor at divergent plate boundaries or is extruded from mantle plumes (hot spots), quite unrelated to, and often very distant from, a plate boundary. On the other hand, the product of melting of the base of the crust above a subduction zone is mostly andesitic and rhyolitic magma. The two magma types, basaltic versus andesitic/rhyolitic, have fundamentally different chemical compositions and fundamentally different physical characteristics.
Basaltic magma is low in SiO2 (~50 wt. %), low in dissolved gasses such as CO2, H2O and SO2, but high in FeO and MgO. The magma composition reflects the composition of the upper mantle (peridotite) from which it is derived. The temperature of the magma can be as high as 1400 C. The high temperature and low SiO2 make for a very fluid lava (low viscosity) that can flow easily for great distances, sometimes quite rapidly down slopes in thin sheets or as rivers of lava. As the lava moves down slope and cools, the flows become thicker, more viscous, and finally break up into a mass of rubbly chunks of almost solid lava that barely moves at a snail's pace. The low amount of dissolved gasses mean that the eruptions are predictable and not explosive, well at least not catastrophically explosive.
In contrast, andesitic and rhyolitic magmas are high in SiO2 (60-70 wt. %), high in dissolved gasses such as CO2, H2O, and SO2, but low in FeO and MgO. The composition is more like the composition of continental crust. The magma is a lot cooler (~800 C) than basaltic magma (~1400 C). The low temperature and high SiO2 make for a thick and pasty lava (high viscosity) that can barely flow out of its own way. This doesn't sound too dangerous, and wouldn't be, if it weren't for the dissolved gasses. The high amount of dissolved gasses leads to catastrophically explosive eruptions when the gasses form bubbles that expand almost instantly as the magma gets near the surface. The magma literally explodes into hot fragments that are hurled as high as 30 km up into the atmosphere. This hot, airborne material is referred to as pyroclastics. Pyroclastic fragments come in all sizes from the finest dust (ash) up to huge chucks as big as a house (bombs). Eruptions involving pyroclastic material, in one or more of it's forms, represent the most unpredictable and hazardous of volcanic phenomena. Mixed with air, pyroclastic material can form extremely hot, dense, cloud-like masses, called pyroclastic flows or nuee ardente. Such ground-hugging pyroclastic flows rush down the flanks of a volcano, reaching speeds up 700 km/hour (420 mph), incinerating anything along the way. To make matters even worse, loose, pyroclastic material that has already been deposited on the flanks of a volcano and that might otherwise seem stable can be remobilized as land slides or, when mixed with water, as mud flows, known as lahars. Following a catastrophic eruption, degassed andesitic or rhyolitic magma commonly emerges at the surface, and just piles up on top of itself, forming a biscuit shaped hill, called a lava dome, effectively plugging the vent of the volcano, and closing out the eruptive event. Volcanic materials in the lava dome include obsidian and pumice, among other things.
You might ask your instructor to show you some examples of various kinds of volcanic rocks, especially if you have forgotten what they look like.
Just as the two main magma types (basaltic versus andesitic/rhyolitic) have fundamentally different physical and eruptive characteristics, so too do they produce characteristic types of volcanoes. In this exercise you will learn to recognize the two main types of volcanoes, the shield type volcano and the strato-type volcano. Shield volcanoes are formed by basaltic magma, typically above a mantle plume, whereas the stratovolcanoes (sometimes referred to as composite volcanoes) are formed by andesitic/rhyolitic magma. You will also come to understand the relationship between volcanic phenomena and plate tectonics.
The exercise is organized into three parts. Answer sheets will be provided, or you may print copies from this site.
PART 1: Shield Volcanoes, the Hawaiian Islands.PART 2: Stratovolcanoes, Mount St. Helens eruption of May 18, 1980.