Igneous rocks 2

From GeoClasses

Contents 1 Process of melting 2 fractional crystallization 3 magmatic intrusions 4 Pulling it all together:

Process of melting

• table 4.3; see figure
• partial melting
• decompression melting: Decompression melting which occurs because of a decrease in pressure. The solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process (from Wikipedia)
• effect of water on melting: Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but near or above about 1500°C in the absence of water.[4] Water is driven out of the oceanic lithosphere in subduction zones, and it causes melting in the overlying mantle. (from Wikipedia)
• magma chambers: DISCUSSION OF READING: mid-ocean ridges

fractional crystallization

• fig. 4.5
• magmatic differentiation

crystal settling from: Tulane website

inward crystallization in a magma chamber from: Tulane website

evidence of magma mixing from glass chemistry, from: Tulane website

• example 3: porphyritic texture
  • what is the history of this rock?
  • review of crystallization processes
  • Explanation from Tulane website:

• igneous rock types:
  • website and handout to do exercise
    • Individually go through this exercise, then before going to next page, compare answers in pairs, go over each page as a class.

magmatic intrusions

• plutons: usually refers to a distinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape like those of dikes and sills. Batholiths commonly are aggregations of plutons.
• batholiths: a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite. They are composed of multiple masses, or plutons, bodies of igneous rock of irregular dimensions (typically at least several kilometers) that can be distinguished from adjacent igneous rock by some combination of criteria including age, composition, texture, or mappable structures.
• sills: a tabular pluton that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock.
• dikes: An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it.
• veins: Veins are formed by fluids carrying mineral constituents into a rock mass as a consequence of some form of hydraulic flow within the rock. Usually this is the result of hydrothermal circulation.

Half Dome, a granite monolith in the Sierra Nevada batholith, in Yosemite National Park (carved by glaciers)	sill in Edinburgh, Scotland	small dike in Alaska	large dike radiating away from volcanic neck in Shiprock, NM
vein weathering out of host rock near Cape Jervis, South Australia

Pulling it all together:

• see fig. 4.11 (plate tectonics and igneous rocks)
• ophiolites
  • source: peridotite
  • process: decompression melting
  • product: oceanic crust
• subduction zones
  • source: ocean crust +/- continental crust + mantle peridotite + water
  • process: fluid-induced melting
  • product: volcanic arcs