Igneous Geology. Igneous geology focuses on the process and structure (arrangement of parts) of igneous intrusions and extrusions.
v 0067 of 'Igneous Geology' by Greg Pouch at 2012-09-08 10:45:15
LastSavedBeforeThis 2011-08-31 14:00:25
5 Melting > Generation of Magma
6 Melting > Partial Melting
7 Melting > Partial Melting > Continuous
8 Melting > Partial melting > Eutectic
9 Melting > Partial Melting > Results
10 Melting > How to Melt Rock
11 Melting>Melting Temperature Varies with Pressure
12 Melt > Melting Temperature with Pressure
13 Processes > Ex-solution
14 Processes > Movement
15 Processes > Heating of Country Rock
17 Products > Extrusive Features > Volcanoes
18 Products > Extrusive Features > Others
19 Flood Basalts
20 Products > Extrusive Materials
21 Products > Intrusions > Some Vocabulary
22 Products > Intrusions > Tabular
23 Products > Intrusions > Equant, Irregular
24 Products > Intrusions > Features > Other
25 Plate Tectonics and the Origin of Igneous Rocks
26 Plate Tectonics > Divergent Basalt
27 Plate Tectonics > Subduction Andesite
28 Plate Tectonics > Collision Granite
29 Plate Tectonics > Hotspots Basalt, maybe Granite/Rhyolite
39 Plate Tectonics > Transform Boundaries NONE
Mantle is mostly peridotite (ol+pyr+Ca-plag). So are most meteorites.
For iron-carbon (steel):
Pure iron melts at 1535C,
Pure carbon melts at 4200C, and a
Eutectic iron-carbon mixture (a cast iron) melts at about 1154C and 4.3%wt Carbon, which is more than 400C colder than iron.
At 1 atm pressure: Olivine, the magnesian endmember Mg2SiO4 Forsterite (Fo) melts around 1890°C whereas the Fe endmember Fe2SiO4 Fayallite (Fa) melts at 1205°C.
Plagioclase, the sodic endmember Albite (Ab) NaSiAlSi3O8=NaAlSi3O8 melts at 1118°C, whereas the calic endmember Anorthite CaAlAlSi3O8=CaAl2Si2O8 (An) melts at 1500°C.
As minerals crystallize from a melt, the melt can become depleted in elements that are incorporated in those minerals and enriched in elements that are not incorporated into those minerals (incompatible elements). Magmas contain volatiles (gases and liquids) in solution at high temperature, but the combination might be unstable at low temperature. Amongst others, water concentrates into the melt, as do lots of rare elements like Ag and Pb.
As a melt cools and changes in composition, components that were once miscible can become immiscible, like grease separating from broth, separating into two fluids. This phenomenon is called ex-solution.
The combined volume of the two fluids is often greater than the volume of the single fluid. At the surface, this can be explosive. Below the surface, this can result in a fracture network and extensive metasomatic activity.
Intermediate and felsic magmas often ex-solve into a water-rich phase and a silica-rich phase. Ex-solution can generate porphyries by changing the melting temperature of the siliceous liquid which is suddenly NOT as water-rich, thus suddenly below its freezing temperature, and so freezes, becoming the groundmass.
The water-rich phase contains lots of elements like Au and Cu and Cl that didn't go into early-formed minerals like plagioclase and can lead to cool ore deposits and pegmatites.
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).
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)
Plugs and Domes are the volcanic equivalent of toothpaste being squeezed out of a tube. Rhyolitic.
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.