CHAPTER 8

What are the different types of pressure or stress that a rock might experience?

(1) HYDROSTATIC/ LITHOSTATIC STRESS -Stresses are EQUAL on an object and it experiences a VOLUME CHANGE (2) DIFFERENTIAL STRESS -Stresses are NOT EQUAL on an object and a SHAPE CHANGE will occur

What is the geothermal gradient?

-Describes how temperature changes with depth -Above 150 km depth change is large -Below 150 km, the change is small

What are the three basic metamorphic paths that a rock might follow in the Earth?

-Low-P and high T -P and T increase at similar rates -High P and low T

What is eclogite - in what tectonic setting does it form?

-Occurs at the base of the crust -Ultra high pressure required -Garnet and Pyroxene materials -Contient-Continent collision will move fertile rocks to high pressures and temperatures

What is prograde and retrograde metamorphism?

-PROGRADE -Rocks are moved towards HIGHER TEMPERATURES and HIGH PRESSURES -RETROGRADE -Rocks are moved toward LOWER PRESSURES and LOWER TEMPERATURES -hydrothermal fluids must enter the rock to add back water

What is shock metamorphism? What causes this type of metamorphism?

-Results from the impact of meteorites with Earth's surface -Extreme conditions-temperatures (700 to 1400 degrees C) and pressures (<500 bar) -Unusual minerals that only form in extreme conditions (rocks—TEKTITES and minerals—STISHOVITE)

What is an isotherm?

A line on a map or chart of the Earth's surface that show equal temperature.

GARNET GNEISS

GARNET GNEISS -alternating dark-colored and light-colored layer that range in thickness from millimeters to meters, this GNEISS BANDING gives it a striped appearance -Needs more temperature than SCHIST

Greenstone

GREENSTONE -PROTOLITH is a low grade mafic volcanic rock

What are geobarometers and geothermometers used to calculate?

Geobarometers are used to calculate pressure conditions at which the rock forms Geothermometers are used to calculate temperature conditions at which the rock forms

What causes metamorphism?

HEATING -Heat causes atoms to vibrate rapidly and lose the chemical bonds that they are connected to. So they form bonds with new atoms. This causes the makeup of the grains to be different (RECRYSTALLIZATION and/or NEOCRYSTALLIZATION) takes place (2) PRESSURE -When minerals are subject to extreme pressure, the atoms pack more closely together (resulting in less space between the atoms) and denser materials tend to form. This cause PHASE CHANGES or NEOCRYSTALLIZATION. -Tectonic forces -Burial of sedimentary rocks -Kinetic energy from meteorite impacts (3) FLUIDS -Water will interact with minerals and cause chemical reactions -Water may also carry elements into and out of rock

hornfels

HORNFELS -requires a high temperature -no deformation -fine-grained non foliated rock

What is metamorphic grade? How is grade defined?

Intensity of metamorphism, meaning the amount or degree of metamorphic change The classification of metamorphic grade depends on the temperature in which rocks change since temperature plays a dominant role in determining the extend of recrystallization and neocrstallization Metamorphic rocks that form at low temperatures are LOW GRADE rocks. Metamorphic rocks that form at relatively high temperatures are HIGH GRADE rocks. Metamorphic rocks that form between these two extremes are INTERMEDIATE GRADE rocks.

How are the minerals kyanite, andalusite, and sillimanite related to possible metamorphic paths as defined by P and T?

KYANITE: low P high T ANDULUSITE: intermediate P and T SILLIMANITE: high P and T

marble

MARBLE -Forms by the metamorphism of LIMESTONE or DOLOSTONE (PROTOLITH) -relatively soft -Reacts with HCl

Relate metamorphic conditions (e.g. regional metamorphism) to tectonic environment.

MID-OCEAN RIDGES (DIVERGENT BOUNDARIES) -Sea water carries dissolved metals from igneous rocks to fractures (veins) CONVERGENT BOUNDARIES -Water from descending oceanic lithosphere migrates into the mantle and the crust -HYDROTHERMAL METAMORPHISM PLUTON -Groundwater carries dissolved metal from igneous rocks to fracture (veins) -This is happening in YELLOWSTONE -HYDROTHERMAL METAMORPHISM

MUSCOVITE+BIOTITE SCHIST

MUSCOVITE+BIOTITE SCHIST -medium to coarse grained -Forms at higher temperature than PHYLITTE -Has larger grain size that PHYLITTE

quartzite

QUARTZITE -forms by the metamorphism of pure quartz sandstone (PROTOLITH)

SLATE

SLATE -Forms by metamorphism of shale or mudstone—rocks composed of clay—under low pressures and temperatures -it had SLATY CLEAVAGE -Finest grained foliated metamorphic rock

Relate metamorphic rocks to metamorphic grade, including grain size, foliation type (i.e. slaty cleavage vs. banding).

SLATE—LOW GRADE fine grained slaty cleavage PHYLITTE—INTERMEDIATE GRADE fine grained MUSCOVITE+BIOTITE SCHIST—INTERMEDIATE GRADE -medium to coarse grained GARNET GNEISS—HIGH GRADE -Foliated -Banding

What is metamorphism?

When a new rock forms when a pre-existing rock, or protolith, undergoes a solid-state change in response to the modification of its environment at the depth in the Earth

Amphibolite

AMPHIBOLITE -PROTOLITH is an intermediate grade mafic volcanic

Explain foliation and how it forms.

Minerals will re-grow (recrystallize) perpendicular to the maximum stress orientation. Other rock fragments may change shape in response to stress. The more temperature that is applied, the more it becomes foliated. The more it becomes foliated, the grain size gets coarse and the minerals begin to separate into light and dark bands. ie. Slate goes to Phyllite with low temperature and large differential stress and then Phyllite goes to Muscovite+ Biotite Schist with medium temperature and large differential stress. Finally Muscovite goes to Garnet Gneiss with high temperature and large differential stress. DIFFERENTIAL PRESSURE is applied to form a foliation. Rocks that do not have a large fraction of platy minerals won't develop planar structure or foliation.

Describe metasomatism/hydrothermal metamorphism.

Occurs when hot, chemically active, mineral laden waters interact with a surrounding pre existing rock. Usually takes place at low pressures and relatively low temperature.

PHYLITTE

PHYLITTE -Fine grained -Caused by the preferred orientation of very fine grained white mica -Forms when slate is subjected to a temperature high enough to produce a new assemblage of metamorphic minerals

Define the term 'parent rock' and relate it to different types of metamorphic rocks.

Parent rock is what a metamorphic rock originally was. For example, Garnet Gneiss's protolith or parent rock is shale. Marble's parent rock is limestone or dolostone.

How are metamorphic grade and metamorphic facies different?

While metamorphic grade chiefly depends on intensity, metamorphic facies are indicated by minerals and rock texture in a similar realm. The type of minerals and rock texture indicate what range of pressure and temperature the metamorphic rocks will form at.