Physical geology lecture outline
- Igneous Rocks
rock -> naturally occurring
aggregate of mineral matter.
Major divisions of rock types:
- Is this a classification based on description
or on genesis?
- This classification directly linked to the rock cycle.
Sketch of part of Yosemite National Park, deep in the heart of an igneous world, the Sierra Nevada Batholith. Image Source: http://geomaps.wr.usgs.gov/parks/yos/I_1639.html
How do we tell an igneous rock from a sedimentary one from a metamorphic one?
- by using multiple traits such as texture, context, association, and mineralogy.
- e.g. if it has fossils it is a sedimentary rock (although it could be a sedimentary rock that has been weakly metamorphosed, scuh as slate, and thus be a metamorphic rock that once was sedimentary).
- e.g. if it has large crystals of a mineral that only grows in the solid state under higher pressures and temperatures, such as garnet, in which case it is likely metamorphic.
- e.g. if it has larger well formed crystals in a fine-grained matrix of interlocking crystals, in which case it is likely volcanic and therefore igneous in origin.
Igneous rocks in general:
- Igneous rocks and the process of crystallization:
- disassociated ionic groups --> lattice
- liquid (solution) --> crystalline solid.
- salty water --> ice and salts.
- magma/lava --> plutonic rock/volcanic
- granitic melt --> granite.
what different ways can the crystallization process be studied
- Bowen's reaction series - a schema for the order of mineral crystallization from silicate melts.
This is a very useful framework for understanding igneous rocks.
= coarse grained, identifiable by eye; aphanitic = fine
controls the grain size of an igneous rock?
- Volcanic vs.
- Given the above there is the initial enigma of phenocrysts
- why are only some of the grains coarse, while the surrounding matrix is aphanitic (and thus of volcanic origin)?
This rock is from a very shallow intrusion in Death Valley. It shows the larger phenocrysts (the larger white and rectangular crystal of feldspar) in the finer grained matrix. Much smaller feldspars also exist. Why the difference in grain size?
- how does one identify a rock as volcanic?
- volcanic rock types from poorer in SiO2 to richer in SiO2: komatiite (rare) -->
basalt --> andesite/dacite --> rhyolite.
- 3 tectonic settings for volcanism:
- volcanic arcs associated with subduction - Pacific "ring of fire".
- rift zones (two types continental and oceanic).
- hot spots.
- how do you melt source rocks to create magma?
Figure of phase space for silicate melt. The space
makes intuitive space. The hotter the more likely it will be
molten. The higher the pressure the more likely it will be solid.
Note that material can move from the solid to the melt field
in at least three different manners: reducing pressure while
keeping temperature constant, increasing temperature, and by
shifting the boundary .
- types of eruptions.
- flows vs. pyroclastic.
- types of pyroclastic:
- shield volcanoes
versus strato- or composite volcanoes.
- shield volcanoes:
- composite volcanoes (also known as stratovolcanoes):
- interlayered flows and pyroclastics, usually
intermediate to sialic in composition.
- higher angle slopes (cone shape).
- larger calderas.
- Mt. Fuji and Mt. St Helens.
- USGS video of Mt. St. Helens eruption.
- Image to right is of Mt. Shasta in northern California, one of the more southern volcanoes in the Cascade chain.
- Why the difference?
- flood basalts
and volcanic plateaus and large igneous provinces (LIPs):
- textures in volcanic rocks:
What is the magmatic plumbing beneath volcanic activity?
Schematic diagram showing how volcanic rocks and features at the surface are connected to shallow and deeper plutonic rocks and features at depth. Diagram from USGS site: http://3dparks.wr.usgs.gov/lame/html/lame_history.htm
- map and/or cross
section relationships of plutonic rocks and inferred genesis.
- intrusion geometries:
- planar intrusions: dikes and sills (discordant
and concordant), see above diagram.
- irregular intrusions (proverbial blobs):
- in what tectonic setting do batholiths form?
- La Paz batholith, Baja California.
- Yosemite National Park - deep in the heart of a batholith.
- Photo to right is of the iconic Half Dome. While there is a distinctive set of fractures, note how all the rock has basically the same color. It is all massive granite. The granite extends much farther than this, and dominates the landscape for 360 degrees around the point of this view. Many granite intrusions from below combined to make a massive composite intrusion that is the center of the Sierra Nevada Mountains in eastern California. The associated cliffs are a mecca for climbers.
- Not always but commonly a distinction type of fracturing occurs in these large granite bodies, a style of fracturing known as exfoliation jointing. It is well displayed here in this image from Yosemite, and the pattern has reminded some of the peels of an onion skin. Again you are looking at a landscape dominated by granite and shaped by fracturing and erosion by glaciers.
- Above is a close up image (camera lense for scale) of the Yosemite granite with some foreign bits of the rock that the granite intruded caught up and then frozen into the granite. These pieces are known as xenoliths. Note the massive and crystalline (interlocking) texture in the granite.
- In this photo there is almost more xenolith material than the lighter colored granite. That is because at this point we are at the very edge of the granite and of the Sierra Neveda batholith in Yosemite, near its contact with the surrounding country rock that it intruded. If you look carefully at the shapes and margins of the xenoliths, there is evidence of interaction between the xenoliths and granites.
- plutonic rock types from ultramafic to mafic to sialic: peridotite, pyroxenite,
dunite (all ultramafic) -> gabbro -> diorite -> granite.
- how and why do magmas ascend through the crust?
- like a blob?
- squeezed along cracks?
Image of a sheet like (tabular) intrusion cutting the
Madison limestone in Spearfish canyon in the Black Hills. This
body is part of a suite of intrusions of Tertiary age found concentrated
in the N part of the Black Hills uplift. Devil's Tower is a part
of the suite.
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