Lab_4_Igneous Rock.docx

Laboratory 4: Volcanoes, Igneous Settings and Igneous Rock Identification (Image by J.D. Griggs, courtesy of USGS)

Volcanoes and Igneous Settings Part 1: Where the Rocks Melt The lava we see on the earth’s surface, gushing out of Mt. Kilauea and spewing out (in the form of ash) from Iceland’s unpronounceable volcanoes, all comes from the earth’s mantle. The mantle itself is solid, but it is capable of melting through several means, and this melted rock is then able to rise to the surface because it is less dense than the non-melted rock surrounding it. Before we get into how rock is melted, we should explain what we call this liquid rock. Melted rock has three names: lava, magma, or simply ‘melt’. ‘ Melt ’ is the generic term for melted rock, ‘ lava ’ is the name for melt that is on the earth’s surface, and ‘ magma ’ is the name for melt that is below the earth’s surface (underground). Figure 3.1: Cross-section of the Earth. The Crust (the outermost layer) is cool and brittle. The Upper and Lower Mantle are solid but much hotter and more malleable than the Crust, and capable of flowing. The Outer Core is liquid, while the Inner Core is solid. (Image by D.T.) There are three ways in which rocks in the mantle can be melted: by heating (increasing temperature), by decompression (decreasing pressure), and by adding water to the rocks (called “flux melting”). Decompression and flux melting are common occurrences in the mantle, where they are each responsible for the creation of melt in different settings (these will be explained shortly). Rocks are sometimes melted by heating , but this typically happens in relatively minor quantities the crust as magma melts some of the rocks it moves past (called “country rock”) on its way to the surface.

Figure 3.2: Pressure-Temperature diagram showing the three possible ways for rocks to melt. Decompression lowers pressure until the solidus line is crossed, at which point a rock begins to melt. Heating increases temperature until the solidus line is crossed, at which point a rock can also begin to melt. Adding water moves the solidus line to where the ‘wet’ solidus is shown, thereby increasing the range of temperatures and pressures at which rocks will begin to melt. This last melting type is what happens in flux melting. (Image by D.T.) Though the earth’s mantle is solid, it is capable of flowing, the same way a large clump of bread dough might flow if you place it on a steeply-inclined slope. The deeper parts of the mantle get heated by the hot outer core, making the rock there less dense than the rock above it and causing it to rise. As this hot rock rises it experiences less and less pressure. Eventually the pressure of the rock reaches a critical point where it is no longer stable as a solid, and it begins to melt from decompression . This kind of melting can happen in any part of the mantle where convection is bringing material up to the surface. The melt found in hot spots (like the Hawaiian Islands) and mid-ocean ridges (like the Mid-Atlantic Ridge) is typically the result of this sort of melting. Flux melting typically happens at subduction zones , where a hydrated oceanic plate (this is, oceanic crust that’s full of water) gets pushed beneath a continental plate and into the upper mantle. As the plate gets pushed deeper into the mantle it experiences greater amounts of pressure, forcing the water out of the plate and into the surrounding mantle rocks. This water causes the already hot mantle rocks to melt. Volcanism in the Japanese Islands and the Andes Mountains is caused by melt from this sort of process.

Figure 3.3: Map of Tectonic Plates (Modified from USGS). Name: Fisher Charles Problem 1: Volcanic Settings Using the two world maps below, identify the probable source of magma (heating, decompression, or flux melting) for each of the locations marked by a pink star in the second figure. The locations used as examples above are marked by red stars, so that you can see where on the map they lie (though you, hopefully, already knew that!). Give a short (1 sentence) explanation as to how you made each of your decisions.

(Image by D.T) The Cascades: Heating Yellowstone: Heating & Decompression Iceland: Decompression Krakatoa: Heating

Part 2: Magmatic Flavors While they are all derived from a similar source (the mantle), by the time they reach us on the surface not all melts are the same. Melts are composed largely of silica , a tetrahedral- shaped compound formed by a single silicon atom surrounded by 4 oxygen atoms, but also contain other elements such as Mg, Na, Fe, K, Al, and Ca. Melts are classified based on how much silica they have: if they have a large amount of silica they are called felsic , if they have less than they are called intermediate , less still and they are called mafic , and melts with the least amount of silica are called ultramafic . The mantle has an ultramafic composition (very low silica), and so all melts derived from the mantle start out as being ultramafic. As the melt cools, different minerals will begin to crystallize at different temperatures. This temperature gradient for the common igneous minerals is known as Bowen’s Reaction Series . Figure 3.4: Bowen’s Reaction Series. Different igneous minerals crystallize at different temperatures. As melts cool they start out crystallizing the minerals listed near the top of the reaction series, and they progressively begin to crystallize minerals farther down as they get cooler and cooler. While the Bowen’s Reaction Series progresses downward as melts cool, melts in the real world actually tend to get cooler as they rise closer to the earth’s surface. (Image by D.T.)

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