Lab 3: Igneous rocks - Magma


Have a molten origin. The name comes from ignis , the Latin word for fire.


Partial melting


Total melting can obviously generate a magma, but much magma originates when some part of solid rock, of any time, partially melts. Magma is the word for the melted part, or the "melt." You can use the word magma for any melt, whether it is underground or on the surface. If it is on the surface, you can also call it lava.


Extrusive vs. Intrusive


Extrusive igneous rocks formed from magma extruded onto the Earth's surface. Intrusive igneous rocks formed from magma intruded into the solid rocks underground, sometimes along fracture systems, sometimes in massive intrusions of immense scale. Extrusive = volcanic (from the fire god Vulcan). Intrusive = plutonic (from the god of the underworld, Pluto).




Magma is not all liquid; there are gases associated, such as water vapor, carbon dioxide, and sulfur dioxide. Bubbles of gas can escape if the magma is near the surface, or actually on the surface.




Magma cools eventually. On the surface, it can cool rather quickly, over a period of days. Underground, it cools more slowly, even on the order of years. Minerals grow in an order as cooling happens. Certain minerals will crystallize first (that just means they grow from crystal nuclei, larger and larger, over time), while others will come later, or last. Remember those silicate minerals form lab 1 (plagioclase, quartz, muscovite, olivine, etc.)? Those are the ones involved in magma crystallization. The silicon-oxygen tetrahedra form and then bond with various elements such as aluminum, sodium, iron, magnesium, calcium, etc. The business you learned about isolated, single chain, double chain, sheet, and framework is important here, as we'll see. Depending on the rate of cooling, the composition of the magma, and other factors at play during crystallization, a distinctive texture will result. Texture describes the crystal sizes and their arrangement within the rock.


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