Fig. 41 Metamorphic facies

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)

Definition	Types	Formation conditions	Location	Examples

4. REVISION

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

A. stable

B. ductile

C. rigid

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

B. water

C. fluids

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

B. gneissosity

C. schistosity

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