Metamorphic Rocks Flashcards
Metamorphism
Metamorphic changes occur when a rock is subjected to new temperatures and pressures. Over millions of years, rocks will change mineralogically and texturally until in equilibrium again.
Main types of metamorphism
Contact, burial, and regional
Contact Metamorphism
When hot magma rises to the surface fo the crust and heats adjacent cooler rocks transforming them.
Burial Metamorphism
When sediment is buried ad accumulates under moderate temperatures and pressures transforming them.
Regional Metamorphism
When a rock is buried deep into the crust, the elevated pressures and temperatures cause metamorphism. This takes place during mountain building and occurs at much larger scales than a simple intrusion.
When at convergent plate boundaries, it occurs at moderate to deep levels under moderate to ultra-high pressure and temperatures.
Types of metamorphism
Regional, high-pressure, contact, seafloor, burial, shock, and regional
High-pressure metamorphism
Along linear belts of volcanic arcs, rocks are produced by continent-continent collision and occur at high pressure.
Seafloor Metamorphism
Occurs at mid-ocean ridges, where intruding magma drives seawater circulation through extruded basalts.
Shock metamorphism
Results from the heat and shock waves of a meteorite impact, transforming rocks at the site.
Theory of Techtonics provides a unified view for what types of metamorphism?
contact, regional, and burial
Geothermal gradient
The increase in temperature with increasing depth is called the geothermal gradient. On average, it is 20 degrees per km.
Plate tectonics affect metamorphic reactions as the processes move ____ rocks from the earth’s surface to the interior where temperatures are ___.
sedimentary
higher
As a parent rock is metamorphosed, it progresses from ____ to ____ metamorphic rock.
from low-grade to high-grade
Because fo different minerals crystallizing at different temperatures and pressures, a geologist can use a rock’s composition as a _________ to gauge the conditions at which the rock recrystallized.
Geothermometer/geobarometer
Metasomatism
Hydrothermal fluids produced during metamorphism can carry dissolved ions and metals. As this water percolates upwards, they react with overlaying rocks they penetrate, changing their chemical and mineral composition.
Foliated rocks
Metamorphic rocks show an alignment of minerals, which arises when directed stresses cause minerals to align.
How foliated rocks form
- direct pressure causes sedimentary rocks to form cleavage planes
- original bedding in a sample is marked by thin sandstone layers
- regionally metamorphic rock shows foliation by compressive forces
- Rocks develop foliation when they contain platy of minerals aligning in a preferred orientation (caused by compression).
As the intensity of metamorphism increases, so does ____ size and coarseness of _____.
crystalization
foliation
Why a number of rocks do not form foliation
They are not initially composed of minerals that naturally align, or the rocks were not subjected to regional metamorphism
An example of non-foliated rock formation
contact metamorphism
Granoblastic rocks
non-foliated rocks
Examples of non-foliated rocks
Hornfel, quartzite, marble, argillite,greenstone, amphibolite, and granulite
Hornfel
a high temperature rock derived from shales of uniform grain size that has crystals randomly oriented
Quartzine
very hard, dense rocks derived from sandstones
Marble
heat and pressure acting on limestone
greenstone
low-grade metamorphosed mafic volcanic rocks that tend to form when lavas and ash deposits react with percolating seawater
amphibolite
a medium- to high-grade metamorphic product of mafic volcanic rocks
granulite
a high grade metamorphic rock formed by the metamorphism of sandstone and many kinds of igneous rocks
Porphyroblasts
Some metamorphic minerals may grow into large crystals, surrounded by a much finer-grained matrix. The large crystals called porphyroblasts are found in both contact and regionally metamorphosed rocks. The minerals that grow as the chemical components of the matrix are dissolved or re-organized.
Examples of minerals that form porphyroblasts
staurolite and garnet
Characteristic of foliated rock
slaty cleavage, schistosity, or gneissic foliation; mineral grains show preferred orientation
Characteristics of granoblasts (non-foliated)
granular, characterized by coarse or fine interlocking grains; little or no preferred orientation
Metamorphic Facies
a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. Additionally, the kind of metamorphic rocks that results from a given grade of metamorphism depends partly on the mineral composition of the parent rock.
Examples of metamorphic facies (from graph)
Zeolite (lowest pressure and temperature), Eclogite and Granulite (highest temperature and pressures)
Two essential points of metamorphic facies
- different kinds of metamorphic rocks form from parent rocks of different compositional the same grade of metamorphism
- Different kinds of metamorphic rocks form at different grades of metamorphism from parent rocks of the same composition.
Isograds
Isograds based on index minerals can be used to plot metamorphic grades over a regional metamorphic belt. This can help us identify where a mountain used to be based on the grades of rocks.
