Match the definition with the following terms.
|1. mechanical deformation
|a. metamorphic rocks in which particles are reduced to a small size
|b. developed on a regional scale in response to both deformation and mineral reaction
|c. readjustment of the material
|4. cataclastic rock
|d. formed by a significant increase in temperature in the absence of differential stress
|5. regional metamorphic rocks
|e. mechanical deformation
|6. contact metamorphic rocks
|f. preexisting mineral assemblages are destabilized by changes in the temperature, pressure or chemical milieu
3.2 INFORMATIVE READING: Metamorphic rocks Read the following text and fill in the chart after the text.
The third category is rock that has been changed. The minerals in an igneous rock, or the particles in a sedimentary rock, may look fairly permanent, but in certain circumstances (under extreme conditions of pressure or temperature) they can change and recrystallize into something new. The new rock product is called a metamorphic rock.
There are two types of metamorphic rock. The first is regional metamorphic rock in which the altering force is one of pressure rather than of temperature. These are found deep within the interior of mountain chains and are believed to constitute the lower parts of the crust. Different degrees of pressure produce different grades of metamorphic rock. Slight pressure will produce a low-grade metamorphic rock, in which the only difference will be that the minerals will have been realigned in a different direction. Often this produces flat crystals of mica that are orientated perpendicular to the direction of the applied pressure. The result is a rock that has planes of weakness running in one direction and which can split easily into flat slabs. Slate and phyllite are typical low-grade metamorphic rocks. On the other hand, intense pressure will completely change the mineralogical makeup of the rock and produce a high-grade metamorphic rock. The chemical components may recrystallize into a totally different set of minerals from the original rock and the new minerals may form in distinct bands, often
crumpled and contorted as evidence of the great pressures involved. Gneiss is the typical high-grade metamorphic rock showing distinct banding.
Fig. 41 Metamorphic facies
A typical sequence of rocks – from unconsolidated sediment, through sedimentary rock, through different grades of metamorphic rock, depending on the depth in the crust at which different conditions are found:
|SURFACE – Mud (sediment)
|Mudstone (sedimentary rock)
|3 miles (5 km) deep
|6 miles (10 km) deep
|Slate (low-grade metamorphic rock) Different kinds of micas develop
|9 miles (15 km) deep
|Schist (medium-grade metamorphic rock)
|12 miles (20 km) deep
|Gneiss (high-grade metamorphic rock)
|15.5 miles (25 km)
|Migmatite (ultrametamorphic rock)
The last rock in this series, hornfels belongs to the second type of metamorphic rock – thermal metamorphic rock, sometimes called contact metamorphic rock. Heat – is the most important influence in the formation of such rocks.
As a result, thermal metamorphic rocks are less common and much more restricted in distribution than their regional counterparts. The usual place to find them is at the edge of an intrusive igneous rock, where the heat of the cooling mass has cooked the native rocks at each side. This will produce a metamorphic aureole around the igneous rock, which may only be an inch or two wide. Unlike regional metamorphic rock, thermal metamorphic rock shows no internal structure and can often be mistaken of an igneous rock.
Different minerals crystallize at different temperatures in a metamorphic aureole, and so the mineralogy of the rock close to the intrusion will be different from that further away. The amount of heat given off as the body cools is another important variable. The chemical constituent of the original rock determines the new minerals that are formed. In sandstone that contains nothing but quartz fragments, the quartz recrystallizes in a more compact mosaic, forming the thermal metamorphic rock called quartzite. In a pure limestone the calcite will recrystallize to form marble. Displacement or dynamic metamorphism is local alteration caused by friction as one mass slides over another.
In all this complexity the important point to note is that metamorphism takes place in solid rock. The minerals recrystallize without passing through a molten phase. If the minerals melt, at any stage of this process, then the result would not be a metamorphic rock, but an igneous one. (Barret E. , Hunt A. And Milner B.” Earth and Atmosphere”, 1993, Longman)
4.1 Choose the correct variant
1.A rock is metamorphosed when
A. heat and pressure melt the rock
B. heat and pressure change the rock composition
C. heat and pressure alter the mineral texture
2. Rocks exposed to lots of heat are
3. Pressure in metamorphism involves
A. buried rocks influenced by stress
B. buried rocks pressurized by their own weight
C. buried rocks subjected to weight of overlying rocks
4. Recrystallization is the result of
A. temperature and pressure
B. pressure and stress
C. only stress
5. Another factor influencing metamorphism is
A. carbon dioxide
6. The two main types of metamorphism are
A. contact and regional
B. contact and foliation
C. burial and lineation
7. Aureole is
A. metamorphosed intruding magma
B. metamorphosed continental crust
C. metamorphosed country rock
8. You can ____________ and _____________ foliation.
A. smell and feel
B. feel and see
C. see and taste
9. Slate has
A. slaty cleavage
10. Another term for schistosity is
A. slaty schist
B. schistose lineation
C. schistose foliation
11. Gneiss is formed when granite withstands
A. pressure and stress
B. high temperature and pressure
C. heat and stress
12. Hornfels are formed from
A. contact metamorphism
B. regional metamorphism
C. burial metamorphism
4.2 Match the definitions with the following terms
|a. metamorphic rocks that don’t have foliation, lineation and directional texture
|2. contact metamorphism
|b. scaly foliation composed of large-grained minerals that you can see without magnifying them
|c. locking minerals that don’t align themselves in any particular direction or fashion.
|4. regional metamorphism
|d. when granite withstands the high heat and pressure of metamorphism
|e. hot magma comes into contact with the country rock and the heat from the intruding magma causes the country rock to metamorphose
|6. slaty cleavage
|f. minerals in an igneous rock, or the particles in a sedimentary rock in certain circumstances (under extreme conditions of pressure or temperature) change and recrystallize
|g. the part of the country rock that metamorphoses as a result of contact with the magma
|h. the heat and pressure is great enough to cause rocks to change their mineral composition and crystalline textures
|i. a texture caused by the alignment of minerals into parallel bands
|10. metamorphic rocks
|j. the coarsest foliation characterized by alternating bands of light and dark minerals.
|k. high pressures and temperatures affect huge areas of the Earth’s crust, metamorphosing huge regions of rocks
|12. hornfelsic (granular) metamorphic rocks
|l. it can easily be split along cleavage planes and the planes often have bumpy parallel ridges
5.1 You will hear part of a lecture. For statements 1- 10, complete the
notes, which summarize what the speaker says. You will need to write a
word or short phrase in each box.
1. The formation of metamorphic rocks depends on parent rock, passing fluids, and _____________ .
2. There are two main agents of change: ____________ and ________ .
3. Thermal metamorphism forms __________________ .
4. Higher pressure and heat produce _____________________ .
5. Foliation means that minerals are flattened and ____________ .
6. Minerals that indicate the rock’s metamorphic grade are called _____ .
7. In comparison to sedimentary rocks, metamorphic rocks are
______________ . Clay-rich rocks change more significantly than
8. Metamorphic rocks are subdivided into types, according to _______ .
9. There are the following types of metamorphic rocks: __________ ,
_____________ , ____________ , etc.
5.2 You will hear a conversation between a student and a teacher in
Geology at Aberdeen University. For statements 1-10, choose the best
answer A, B, or C.
1. What are metamorphic rocks?
A. altered igneous and sedimentary rocks
B. changed sedimentary rocks
C. pressed igneous and sedimentary rocks
2. What are the main factors in metamorphism?
A. pressure and climate
B. temperature and pressure
C. heat and agents
3. According to the talk, how many forms of metamorphism exist in nature?
A.4 B.6 C.5
4. Which of the following is the most significant type of metamorphism?
A. regional and impact
B. dynamic and contact
C. contact and regional
5.Where does thermal metamorphism take place?
A. mass of magma intrudes and bakes country rock
B. mass of magma erodes and changes country rock
C. mass of magma extends and bakes country rock
6. What term is used to define «surrounding older rock»?
A. country rock
B. altered rock
C. contact rock
7.What is the definition for the following term «bake»?
A. try under certain conditions
B. harden under certain conditions
C. change under certain conditions
8.Where does regional metamorphism occur?
A. in subducting continental plates
B. in colliding continental plates
C. in eroding mountain ranges
9.Which of the following types of metamorphism covers the largest area?
10.What do you think would be the topic of the next lecture?
A. classification of contact and regional metamorphism
B. conditions of the formation of metamorphic rocks
C. major types of metamorphic rocks
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