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.

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:

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)

4. REVISION

4.1 Choose the correct variant