Presentation on theme: "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."— Presentation transcript:
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:
Prograde Metamorphism: The transition from Shale to Gneiss
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 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. Various metallic minerals from inside a black smoker vent.
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 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 alignment. These rocks are called Schist. Gneissic: Light dark minerals have separated out into bands. These rocks are called Gneiss. (Nice - ick) (Sh – is - tos) (Slate - ee)