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when do rocks start to melt?
As you can see from the figure below, the rock will start to melt when temperatures reach about 1200oC (look for the "solidus" line in the figure). But we also know that some minerals start melting way before 1200oC (quartz, for example, start melting at temperatures as low as 600oC ).
litlification
the process by which sediment hardens into sedimentary rock
happens at temperatures even lower than 600oC. - the heat and pressure is more than normal but not enough to cause melting
diagenesis
a collective term for all the chemical, physical, and biological changes that take place after sediments are deposited and during and after lithification
metamorphism
the process in which one type of rock changes into metamorphic rock because of chemical processes or changes in temperature and pressure
metamorphism or diagenesis?
low temperature minerals (felsic)
also called LOW-TEMPERATURE MINERALS because they are the last ones to crystallize (and the first ones to melt, at temperatures just above 600oC
metamorphic
high temperature minerals (mafic)
hey are the first ones to crystallize (and the last ones to melt, at temperatures above 900oC)
diagenic
metamorphic mineral assemblage
index minerals
sillimanite, kyanite, staurolite, andalusite, and garnet
metamorphic texture
distinctive arrangements of mineral grains not found in other rock types.
Commonly, the texture results in metamorphic foliation, due to the
-parallel alignment of platy minerals (such as mica)
-presence of alternating light-colored and dark-colored layers.
prolith
The pre-existing rock, before metamorphism. Can be igneous, sedimentary, or metamorphic.
Recrystallization
original mineral crystals re-grow and become bigger (change shape and size) but they remain the same minerals (same composition and same crystal structure). In the example below - the recrystallization of sandstone
Neocrystallization
Initial minerals become unstable and change to new minerals, original minerals are digested by chemical reaction
chemical reactions digest minerals of the protolith to produce new minerals of the metamorphic rock
pressure solution
metamorphic formation process; happens when a WET rock is squeezed more strongly in one direction that in others at relatively low pressures and temperatures, in the presence of water. Mineral grains dissolve where their surfaces are pressed against other grains, producing ions that migrate through the water to precipitate elsewhere.
plastic deformation
when a rock is squeezed or sheared at elevated temperatures and pressures. Under these conditions, grains behave like soft plastic and change shape without breaking. Such deformation can take place without changing either the composition or the crystal structure of the mineral.
phase change
one mineral is transformed into another WITH THE SAME COMPOSITION but a different crystal structure. On an atomic scale, phase change involves the rearrangement of atoms under extreme pressure. For example, the transformation of quartz into a denser mineral called coesite represents a phase change
solid-state diffusion (metamorphism due to heat)
movement of atoms or ions through a solid to arrange into a new crystal structurem
Metamorphism due to pressure
when minerals are subjected to extreme pressure, denser minerals tend to form. Such transformations involve phase changes and/or neocrystallization. Rocks subjected to ultra-high pressure contain grains of coesite, a phase of SiO2 that is much denser than familiar quartz.
metamorphism due to heat and pressure
pressure and temperature change together as the depth increases
Metamorphism resulting from DIFFERENTIAL STRESS
when the stress applied to a rock is applied UNEQUALLY from different sides (i.e., a rock is being squeezed or stretched strongly in one direction). When that happens, we say that the rock is being subjected to differential stress
shear stress
moves one part of a material sideways relative to another
as shape changes ______ changes
internal structure
preferred mineral orientation
The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction.
pressure on wet rock
rock is being exposed to differential stress. In one case (bottom left) the compression is being applied to a WET rock (i.e., in the presence of hydrothermal fluids). This leads to the development of a metamorphic process called pressure solution (mineral grains dissolve under pressure and re-precipitate). After compression, the equant minerals all become inequant (elongated), and align in a specific direction (preferred mineral orientation).
pressure on dry rock
the compression is being applied to a DRY rock under high temperature conditions (solids behave like plastic). This leads to the development of a metamorphic process called plastic deformation (mineral grains deform under pressure). After compression, the equant minerals all become inequant (elongated), and align in a specific direction (preferred mineral orientation). Note that, in both cases, the preferred mineral orientation is PERPENDICULAR to the direction of the differential stress.
hydrothermal fluids
hot water with dissolved ions and volatiles
chemically reactive and can dissolve certain minerals
metasomatism
(a) represents the composition of both rocks BEFORE the movement of fluids. (b) show fluid movement from the chromitite into the melanorite, carrying dissolved chromite and transition metals derived from the partial dissolution of sulfides. Infiltration of water into melanorite promotes the breakdown of orthopyroxene to olivine to form troctolites (a mafic intrusive rock type rich in olivine).
foliated metamorphic rock
a metamorphic rock with a texture that gives the rock a layered appearance'
slate, phyllite, metaconglomerate, schist, gneiss, and migmatite
slate has salty cleavage
salty cleavage
foliation that develops at low temperature and pressure (metamorphosed shale).
develops when pressure solution removes portions of clay flakes that are not perpendicular to the compression direction, while clay flakes that are perpendicular to the compression direction grow
Phyllite
a fine grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained white mica. The name comes from the Greek word phyllon, meaning leaf, as does the word phyllo, the flaky dough in Greek pastry.
Phyllite forms by the metamorphism of shale at a temperature high enough to cause neocrystallization of white micas from clay minerals in the original protolith. The parallel orientation of translucent fine-grained micas gives phyllite a silky sheen known as phyllitic luster (figure below).
Schist
a medium- to coarse-grained metamorphic rock that possesses a special type of foliation called schistosity. Schistosity is defined by the preferred orientation of large mica minerals (muscovite and biotite). Like SLATE and PHYLLITE, the protolith of SCHIST (a metamorphic rock) is also SHALE (a sedimentary rock). However, schist forms at a higher temperature than slate and phyllite (we say that schist results from higher grade metamorphism).
Metaconglomerate
a coarse-grained metamorphic rock formed from the flattening of pebbles and cobbles in conglomerate. The protolith of METACONGLOMERATE (a metamorphic rock) is CONGLOMERATE (a sedimentary rock). The flattening of conglomerate clasts into pancake-like shapes occur as a result of pressure solution and plastic deformation. The alignment of inequant clasts defines the foliation.
Gneiss
(pronounced just like the word “nice”) is a compositionally layered metamorphic rock, typically composed of alternating dark-colored and light-colored mineral layers that range in thickness from millimeters to meters. This compositional layering is called gneissic banding. The contrasting colors represent contrasting compositions. Light-colored layers contain predominantly FELSIC minerals (such as quartz and feldspar), whereas the dark-colored layers contain predominantly MAFIC minerals (such as amphibole, pyroxene, and biotite). Chemical reactions segregate the minerals into different layers. Gneiss usually results from the high-grade metamorphism (higher temperatures and pressures) of granite or schist.
. Gneissic banding
alternating layers or lenses of light and dark medium to coarse grained minerals
Some evolved directly from the original bedding in a rock. For example, metamorphism of a protolith consisting of alternating beds of sandstone and shale produces a gneiss consisting of alternating beds of quartzite and mica
can also form when the protolith undergoes an extreme amount of shearing under conditions in which the rock can flow like soft plastic
migmatite
: low-temperature minerals begin to melt first! If the melt freezes again before flowing out of the source area, a mixture of igneous rock (cooling of the melt) and relict metamorphic rock (the 'unmelted' part of the rock) forms. This mixture is a metamorphic rock called migmatite
what minerals start to melt?
the minerals from gneiss that start to melt to form migmatite are the FELSIC minerals (low-temperature minerals, usually quartz and feldspar)
nonfoliated
that metamorphism occurred in the absence of compression and shear, or that most of the new crystals can only grow in an equant form. We list below some of the rock types that can occur without foliation.
hornfels
a fine-grained nonfoliated rock that contains a variety of metamorphic minerals. The specific mineral assemblage in a hornfels depends on the composition of the protolith and on the temperature and pressure of metamorphism (figure below).
Quartsite
forms by the metamorphism of pure quartz sandstone. During metamorphism, pre-existing quartz grains recrystallize, creating new, larger grains. In the process, the distinction between cement and grains disappears, open pore space disappears, and the grains become interlocking. When quartzite cracks, the fracture cuts across grain boundaries in contrast, fractures in sandstone curve around grains. Quartzite looks glassier than sandstone and does not have the grainy, sandpaper-like surface characteristic of sandstone (figure below). Depending on the impurities it contains, quartzite can vary in color from white to gray, purple, or green.
progressive metamorphism of shale
pressure temperature graph
dynamic metamorphism
it occurs as a consequence of shearing alone under metamorphic conditions, without requiring a change in temperature or pressure. The resulting rock, a mylonite, has a foliation that roughly parallels the fault (figure below). Mylonites are very fine-grained, due to processes during dynamic metamorphism that replace larger crystals with a mass of very tiny ones. Dynamic metamorphism takes place anywhere that faulting occurs at depth in the crust. Thus, mylonites can be found at all plate boundaries, in rifts, and in collision zones.
dynamothermal metamorphism (reigonal)
: (1) it heats up because of the geothermal gradient and because of igneous activity; (2) it endures greater pressure because of the weight of overburden; and (3) it undergoes compression and shearing. As a result of these changes, the protolith transforms into foliated metamorphic rock. The type of foliated rock that forms depends on the grade of metamorphism: slate forms at shallower depths, whereas schist and gneiss form at greater depths
exhumation
The process (involving uplift and erosion) that returns deeply buried rocks to the surface.
Metamorphic reactions usually take place in the presence of hydrothermal fluids (chemically reactive solutions that can dissolve certain minerals). Which of the following options are correct regarding the origin of these hydrothermal fluids? (mark all that applies)
They were originally bonded to the minerals in the protolith, before experiencing metamorphism
They seeped up into the protolith from a nearby igneous intrusion
They seeped down into the protolith from overlying groundwater reservoirs
The type of metamorphism represented in the figure below (due to the proximity to an igneous intrusion) is called ... (mark all that applies)
Contact metamorphism
thermal metamorphism
Which of the following options correctly indicate the metamorphic process responsible for the formation of marble, a common metamorphic rock containing coarse, interlocked grains of the mineral calcite?
Recrystallization of calcite grains
Geologists use the term protolith to refer to the original rock existing before metamorphism. Marble is one of the most common metamorphic rocks on Earth. The protolith of marble is a sedimentary rock called ...
limestone
Most metamorphic rocks form in one of TWO geologic settings. One of this settings occurs where cold rock is intruded by a hot magma. This type of metamorphism is called ...
contact metamorphism
What is the name of a rare metamorphic rock that forms under very unusual conditions, very HIGH pressures (corresponding to a depth of 15 to 30 kilometers) and LOW temperatures (between 200 to 400 degrees Celsius)?
Blueschist
In some circumstances, the temperature of a deeply buried rock can become so great that the rock starts melting. When this happens, a rock having both igneous and metamorphic features results. This intermediate rock type is called ...
Migmatite
Most metamorphic rocks form in one of TWO geologic settings. In which of the following types of metamorphism BOTH temperature and pressure are required for causing metamorphic reactions?
regional metamorphism
Most metamorphic rocks form in one of TWO geologic settings. In which of the following types of metamorphism temperature is the predominant cause of metamorphic reactions?
contact metamorphism
At Earth's surface, the pressure we experience is equal all around us. Deep underground, however, pressure is not equally applied. When opposing pressures are applied on different parts of the rock, as indicated in the figure below, the rock experiences which of the following types of stress?
shear stress
The figure below shows a magnified view of elongated flakes of mica minerals in a layered metamorphic rock called mica schist. These slanted layers result from the preservation of a stress field generated within a subduction zone environment. These slanted mica layers are produced by which of the following types of stress?
shear stress
Which of the following rock features/characteristics are changed by metamorphism? (mark all that applies)
It changes the rock's shape
It changes the rock's age
It changes the rock's chemical structure
It changes the rock's appearance
It changes the rock's mineral composition
In the metamorphic region of the northeastern United States, snowball garnets preserve an important record of the building of the Appalachian Mountains. These Vermont garnets began growing about 380 million years ago, and preserve a record of which of the following geologic events?
The formation of the supercontinent Pangaea
Geologists use the term protolith to refer to the original rock existing before metamorphism. Amphibolite is a dark metamorphic rock rich in the ferromagnesian mineral amphibole. The protolith of amphibolite is a volcanic igneous rock called ...
basalt
Which of the following options are correct regarding the conditions in which metamorphism occurs? (mark all that applies)
Just BEFORE melting takes place
BEYOND depths of 2 km down the Earth
Generally at temperatures GREATER than 200 degrees centigrades
In addition to directed stress, rising temperatures also affect minerals in a metamorphic rock, resulting in the formation of new crystal lattices, causing minerals to grow larger, and developing an interlocking texture. This metamorphic process is called ...
recrystallization
Directed stress involving compression helps explain the origin of a very common metamorphic structure. As temperature and pressure increase, minerals recombine to make new, more stable minerals. The minerals grow in the direction of lowest pressure, perpendicular to the directed stress (figure below), resulting in a layering which geologists call ...
foliation
Metamorphism is a process of progressive change. As rocks are exposed to higher temperature and pressures, they are altered in a predictable manner. As the intensity of metamorphism increases, the rocks become harder, more coarsely crystalline, and develop special metamorphic textures. Geologists refer to progressive metamorphism as an increase in metamorphic grade. Which of the following options are true regarding the figure below? (mark all that applies)
GNEISS has a higher metamorphic grade than SCHIST
MIGMATITE has a higher metamorphic grade than SLATE
SCHIST has a higher metamorphic grade than SLATE
outcrop is undeformed
it contains no geologic structures other than a few joints
The Alpine cliff emphasize that during deformation, rocks can undergo one or more of the following
A change in location (displacement);
A change in orientation (rotation);
A change in shape (distortion).
distortion. is often referred to as a
strain
shear strain
If a change in shape involves the movement of one part of a rock body past another, so that angles between features in the rock change
brittle deformation
the cracking and fracturing of a material subjected to stress
ductile (plastic) deformation
The bending and flowing of a material (without cracking and breaking) subjected to stress.
What determines weather the rock deforms brittlely or plastically
Temperature: Warmer rocks tend to deform plastically, whereas colder rocks tend to deform brittlely. Heat makes materials softer.
Pressure: Under great pressures deep in the Earth, rock behaves more plastically than it does under low pressures near the surface. Pressure effectively prevents rock from separating into fragments.
Deformation rate: A sudden change in shape causes brittle deformation, whereas a slow change in shape causes ductile deformation. For example, if you hit a marble bench with a hammer, it shatters, but if you leave the bench alone for a century, it gradually sags without breaking.
Composition: Some rock types are softer than others; for example, halite (rock salt) deforms plastically under conditions in which granite deforms brittlely
Stress vs. Strain
Compression takes place when a rock is squeezed;
Tension occurs when a rock is pulled apart;
Shear stress develops when one part of a rock body moves sideways past another;
Pressure refers to a special stress condition that happens when the same push acts on all sides of an object.
stress vs strain
stress refers to the amount of force applied per unit area of a rock, whereas strain refers to a change in shape of a rock. Thus, stress causes strain
joints
develop in response to tensile stress in brittle rock: a rock splits open because it has been pulled slightly apart.
faults
planar structures, so we can represent their orientation by strike and dip.
categories of faults
fault scarp
Displacement on a dip-slip or oblique-slip fault will make a step on the ground surface
fault breccia
Faulting under brittle conditions may crush or break rock. If this shattered rock consists of visible angular fragments
hinge
a line along which the curvature is greatest
limbs
the sides of the fold that display less curvature
axial surface
imaginary plane that contains the hinges of successive layers and effectively divides the fold into two halves. With these terms in hand, we distinguish among the following types of folds
anticlines
Folds that have an arch-like shape in which the limbs dip away from the hinge
syncline
folds with a trough-like shape in which the limbs dip toward the hinge
monocline
has the shape of a carpet draped over a stair step
open vs tight
If the angle between the limbs is large, the fold is an open fold. If the angle between the limbs is small, the fold is a tight fold.
non plunging fold
If the hinge is horizontal
plunging fold
if the hinge is tilted
dome
A fold with the shape of an overturned bowl
basin
upright bowl
flexural-slip folds
a stack of layers bends, and slip occurs between the layers
passive flow folds
form when the rock, overall, is so soft that it behaves like weak plastic and slowly flows; these folds develop simply because different parts of the rock body flow at different rates.
"All physical, chemical, and biological processes that transform sediment into sedimentary rock". This is the definition of which of the following processes?
lithification
The increasing heating and pressure, without melt, as a result of burial and/or stress of pre-existing rock, will result in the formation of ...
metamorphic rock
The formation of metamorphic minerals and textures takes place very slowly and involves several processes, which sometimes occur alone and sometimes together. Which of these metamorphic processes is represented in the figure below?
Neocrystallization
The formation of metamorphic minerals and textures takes place very slowly and involves several processes, which sometimes occur alone and sometimes together. Which of these metamorphic processes is represented in the figure below?
Pressure solution
Differential stress produces a distinctive texture that is typical of metamorphic rocks (an arrangement of mineral grains not found in other rock types). The figure below shows a rock, containing six equant (rounded) minerals, that is being exposed to differential stress. In this particular case, the compression is being applied to a WET rock (i.e., in the presence of hydrothermal fluids), leading to the development of which of the following metamorphic processes?
Pressure solution
Metamorphic reactions usually take place in the presence of hydrothermal fluids (chemically reactive solutions that can dissolve certain minerals). Which of the following options are correct regarding the origin of these hydrothermal fluids? (mark all that applies)
Correct Answer They were originally bonded to the minerals in the protolith, before experiencing metamorphism
Correct Answer They seeped down into the protolith from overlying groundwater reservoirs
Correct Answer They seeped up into the protolith from a nearby igneous intrusion
The figure below is a metamorphic rock called SCHIST, which is formed from ...
NEOcrystallization of micas and garnets in a shale protolith
The foliated metamorphic rock in the figure below results from the flattening of pebbles and cobbles from a coarse-grained sedimentary rock. This metamorphic rock is called ...
metaconglomerate
"The type of metamorphism that occurs as a consequence of shearing alone under metamorphic conditions, without requiring a change in temperature or pressure". This is the definition of ...
Dynamic metamorphism
Dynamic metamorphism
Correct Answer Thermal metamorphism
Correct Answer Contact metamorphism
