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v 0037 of 'Igneous Geology' by Greg Pouch at 2011-01-19 17:47:30 LastSavedBeforeThis 2011-01-19 17:38:20
C:\Users\GregAdmin\Documents\Geo101\15IgneousGeology.ppt on 'GWPOUCHDELL1720'
The mantle is mostly peridotite. So are most meteorites.
For iron-carbon (steel): iron melts at 1535C, carbon melts at 4200C, and a eutectic iron-carbon mixture melts at about 1154C and 4.3%wt Carbon.
As crystals form in a melt, the melt becomes depleted in elements that are incorporated in those minerals and enriched in elements that are not incorporated into those minerals. If the crystals are removed from contact with the melt (by settling to the bottom of a magma chamber, for example), the final melt can have a very different composition than the initial melt.
As a melt cools and changes in composition, components that were once miscible can become immiscible. (Think of grease separating from chili, or one type of grease separating from another.) This phenomenon is called exsolution and controls formation of some ores and whether volcanoes are explosive. It can also generate porphyries by changing the melting temperature of the remaining material, that becomes the groundmass.
Magmas contain volatiles (gases and liquids) in solution at high temperature. As temperature decreases, the volatiles can ex-solve producing a separate gas or liquid phase. At the surface, this can be explosive. Below the surface, this can result in a fracture network and extensive metasomatic activity.
Magmas can move upward in two main ways.
Extrusives are igneous rocks that erupt onto the earth’s surface (are extruded from the earth)
Floods (book calls these plateau basalts)are extensive layers of extrusive igneous rock that moved liquidly and are almost always basaltic.
The lava usually comes out of fissures which are fed by dikes. Areas like the Columbia River Plateau are covered by hundreds of 3-100 meters thick basalt flows, each covering hundreds to tens of thousands of square kilometers. Basalt flows like this frequently show columnar jointing. (see text).
There are not any currently active regions of flood basalts.
Falls are extensive layers of ash and other debris, usually transported by air and are often very violent. Typical of granite/rhyolite. They can include nuée ardente (glowing mix of pyroclasts and hot gases).
Plugs are the volcanic equivalent of toothpaste being squeezed out of a tube. Rhyolitic.
Pillow basalts are extruded below water. In a pillow basalt, lava breaks through a hole in the already-frozen-part-of-the-flow and flows out there, resulting in a tube of hardened rock. In cross section, they look like a stack of pillows, with the outer edge showing evidence of quenching (Good video on the CD)
Extruded rocks cool quickly, and are fine-grained (aphanitic).
Figures are from Petrology by Ehlers & Blatt p97. Y-axis is pressure in kilobars. Multiply by 3 to get depth in kilometers of rock.