Download metamorphics

Metamorphism:
The permanent altering of a rock
through heat and/or pressure
In order to undergo metamorphism, a rock must
become ductile: capable of bending when stress is
applied.
This is the opposite of brittle: a state where rocks
under stress just break.
The conditions needed to form a metamorphic rock are
found deep beneath the earths surface, at the contact zone
of igneous intrusions, and along fault zones.
Parent Rock: What the rock was before it was
metamorphosed.
What happens to a rock when it metamorphoses?
It can:
1. recrystallize and change texture
2. react chemically to form new minerals from the
original elements present
What factors affect the final characteristics
of a metamorphic rock?
1. composition of the parent rock
2. how much heat and pressure the rock underwent
3. the effects of tectonic forces
4. the effect of fluids present during metamorphism
1. Composition of the parent rock
Outside of a few rare cases, there are no new minerals
introduced during metamorphism. So the final rock can
only be made up of elements that were already present in
the minerals of the parent rock.
For example, limestone is pure calcite, so a metamorphic rock derived from
limestone has to be pure calcite, too!
Limestone (CaCO3)
Marble (CaCO3)
2a. how much heating the rock underwent
Every mineral has a temperature range at which it is
STABLE: that means it will not react with another
substance or convert to a new mineral over time.
For Example: Quartz is stable at surface pressure up to 800 degrees C.
At higher pressures it can remain stable at even hotter temperatures.
The melting point of an ultramafic igneous rock (full of
ferromagnesian silicate minerals) has to reach 1,200 degrees C before it
starts to melt, but a shale under a lot of pressure can melt at 650
degrees C.
2b. how much pressure the rock was under
Confining Pressure: Pressure applied equally over the
entire surface of an object. In geology we call this
Lithostatic Pressure.
The pressure gradient for burial beneath the earth’s
surface is 1 kilobar (1,000 times the surface pressure at
sea level) for every 3.3 kilometers of depth.
3. the effects of tectonic forces
Compressive Stress: Forces
acting on the body from
opposite directions, causing the
object to flatten.
Differential Stress: Forces
acting on a body are stronger or
weaker in one direction, so the
stress is not uniform on the
entire rock. This is usually
caused by shearing…
Sheering: Forcing one part of a body to move or slide
relative to the rest of the body
4. the effect of fluids present during metamorphism
Water vapor is though to aid in the chemical changes of
metamorphic rock by speeding up the dissolution of
minerals.
The water can come from the chemical makeup of some
minerals (like clays and micas), or it can become trapped
in sedimentary rock.
Types of Metamorphism:
Contact Metamorphism: Rock is altered by the heat of
an intruding igneous body. Usually happens less than
10 km beneath the surface.
Regional Metamorphism: Rocks over a very large area
are altered by the heat and pressure from plate
boundaries, mountain building, or just great depth.
Prograde Metamorphism: Rock is altered, then buried
deeper and altered again, and again, and so on. This is
best exhibited by shale’s progression to gneiss.
Contact Metamorphism near and igneous intrusion
Regional Metamorphism
at a converging plate boundary:
www.gly.fsu.edu/.../7_Rocks/7
Prograde
Metamorphism:
The transition
from
Shale to Gneiss
www.earth.edu.waseda.ac.jp
Foliation: the flattening and/or alignment of grains within
the rock to form a platy texture.
The 3 types of foliation are:
1. Slaty – the rock splits easily along flat, parallel planes
2. Schistose – the visible platy or needle-shaped minerals
have grown in roughly parallel planes.
3. Gneissic – the light and dark minerals have separated out
into distinct bands.
The role of water:
Water can have a big impact on the process of
metamorphism. There are two metamorphic processes that
involve large amounts of water:
Metasomatism
And
Hydrothermal precipitation
Metasomatism: Water brings in ions from outside rocks
that are exchanged with ions inside the metamorphosing
rock. New minerals then replace old ones as the water both
dissolves the rock and brings in new ions.
Hydrothermal deposits: Water dissolves minerals
from the rock it flows through. The hotter the water, the
more it is able to dissolve. Those minerals are later
deposited inside cracks and voids in other rocks.
Hydrothermal Rocks + minerals
Many commercially important minerals are hydrothermal in origin,
including zinc, lead, gold, silver, tin, mercury, and copper.
Quartz, pyrite and sphalerite
http://www.geodil.com/i
Various metallic minerals from
inside a black smoker vent.
Bingham Canyon Copper mine in Utah is the
world’s largest man-made hole: ½ mile deep
and 2 ½ miles across! It is also one of the
most productive copper mines in the world.
Identifying Metamorphic Rocks
First, you have to decide if the texture is foliated or non-foliated:
Foliation: the organization of platy minerals (like mica and chlorite)
and tabular minerals (like feldspar) into planar structure, making the
rock appear to have layers.
*If the rock is non-foliated, then it is identified by its
mineral content.
If the rock is made up of large, interlocking grains of calcite, it is a
marble.
If the rock is mostly quartz grains locked together, it is a quartzite.
If the rock is a dense, dark mass of grains too small to identify, it is
probably a hornfels.
If the rock is foliated, you have to determine what kind of foliation it
has and what minerals there are.
Slaty:
Develops into flat sheets that can usually be split off. This looks like
(Slate - ee) shale that has been glued together. These rock are called Slate.
Intermediate: Rocks that have the same sheet-like structure of slate, but show
wavy lines on the edges instead of flat like slate, and usually has
a silky luster. These rocks are called Phyllite.
Schistose: Composted of visibly platy or elongated minerals that show planar
(Sh – is - tos) alignment. These rocks are called Schist.
Gneissic: Light dark minerals have separated out into bands. These rocks are
(Nice - ick) called Gneiss.