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Metamorphic rocks begin as either igneous, sedimentary, or pre-existing metamorphic rocks and undergo a major change, or metamorphosis. The change is caused by high levels of heat and pressure - levels found deep in the earth's crust, below where sedimentary rocks are formed but not so deep and hot that the rocks are melted into a magma.

Metamorphism changes an original or parent rock into a new type of rock. This change happens in one of three ways.

• Contact with heat: Contact metamorphism occurs when magma moves up through crustal rock and brings with it high levels of heat. The surrounding rock (the aureole) is heated enough to cause changes in the mineral structures. Contact metamorphism may also be called thermal metamorphism. Sometimes water is heated by magma in this way and then enters a rock, causing changes to the mineral structure; this process is called hydrothermal metamorphism.
• Burial under rocks and sediment: Burial metamorphism affects rocks buried at great depth (usually more than 9.656 kilometres (6 miles)). The rocks are exposed to heat and pressure that cause changes to the minerals in the parent rock.
• Direct pressure and heat from plate collisions: When two crustal plates collide, the result is mountain building or subduction. The pressure and heat of two plates crashing into each other causes dynamothermal metamorphism.

Both burial and dynamothermal metamorphism affect large areas of crustal rocks and are considered regional metamorphism. Contact metamorphism, on the other hand, is very local and affects only the rocks immediately surrounding the heated materials.

Grades of metamorphism and index materials

As parent rocks are exposed to heat and pressure, they begin to change. The degree of change depends on the levels of heat and pressure they experienced. The resulting metamorphic rocks are described by the degree of change, or metamorphic grade:

• Low-grade metamorphic rocks retain characteristics of the parent rock. If they are sedimentary rocks, they may still show signs of bedding planes or other structures. Low grade metamorphic rocks have been exposed to relatively low temperatures and pressures.
• High grade metamorphic rocks look very different from their parent rock. Rocks exposed to very high levels of heat and pressure change dramatically; their internal structure no longer resembles the original rock.

In regional metamorphism, large areas of crustal rocks are being subducted or buried and changed. The rocks deeper in the crust are subjected to higher temperatures and pressures than the rocks closer to the surface. The result is that across a region you see rocks of different metamorphic grade corresponding to the increasing degree of metamorphism.

Metamorphic rocks are identified by the minerals in the rock because certain minerals called index minerals form only under certain conditions of temperature and pressure. Figure 7.14 illustrates the different minerals formed as the sedimentary rock, shale, moves from low grade metamorphism to high grade metamorphism. At some point, high enough levels of heat will cause the minerals to melt, resulting in magma and eventually an igneous rock instead of a metamorphic rock.

Between the mineral sheets: foliation, or maybe not.

Pressure is one of the causes of metamorphism. The squeezing of rock minerals, under the conditions of high pressure, forces them to change. Two types of pressure are applied to metamorphic rocks. Indirect pressure pushes on the rocks from all sides, compacting the materials and removing any spaces between crystals or particles. Direct pressure comes from two opposite directions and elongates the minerals into parallel layers. The elongation of minerals by direct pressure creates a texture specific to metamorphic rocks called foliation. Foliation occurs when the minerals line up in the layers under the application of direct pressure. The minerals are compressed or reshaped into long, linear forms.

However, not all metamorphic rocks are foliated. Rocks that are metamorphosed by contact with heated magma or indirect pressure still experience changes in the organisation of the mineral grains, but the minerals do not create sheets or layers. New minerals created by this process are called metamorphic minerals. Other minerals respond to metamorphism by growing larger (marble is an example).

Categorising metamorphic rocks

The table below summarises the classification of metamorphic rocks, including the parent rock, conditions of metamorphism, and texture or foliation.

Transforming sedimentary rocks

When the sedimentary rock shale, composed of tiny clay particles, is metamorphosed, it first transforms into slate. Slate breaks along flat, smooth layers of foliation (which is why slate is used for chalkboards). Under increasing temperature, slate transforms to phyllite, which has foliated layers of shiny microscopic mica minerals. When the pressure and temperature are high enough to produce foliated minerals large enough to see without a microscope, the rock is called schist.

When temperatures reach about 650 degrees Celsius, the minerals stop flattening into foliated layers. Instead they try to escape the stress of all that pressure! Certain minerals handle the stress better than others, so the minerals begin to move from high stress areas to lower stress areas. The result is gneiss (pronounced nice): a rock with alternating bands of light (felsic) and dark (mafic) minerals. The separation of light and dark minerals is called metamorphic differentiation. This process can create a gneiss from any metamorphic rock, not just shale.

If the pressure and temperature exceed gneiss forming conditions, the gneiss begins to melt on its way to becoming magma. When a rock forms from these conditions, it is a migmatite. Migmatites are gneisses that have partially melted and then solidified into rock. The minerals are still differentiated into dark and light foliated layers, but they are usually swirled or curvy from all of the pressure that nearly melted them into magma.

Limestones do not proceed through the sequence of metamorphism that has been described for shale. Instead, under conditions of high temperatures and pressure, limestone (and dolostone) minerals are compressed until all the space between crystal grains is squeezed out. The result is a very hard, smooth rock called marble. The solid, smooth feature of marble - where the crystals form one continuous body - makes it a great material for sculpting.

Sandstone also creates a very hard metamorphic rock called quartzite. Similar to marble, quartzite is formed by compressing all space from between mineral grains until the crystals are smashed together in one continuous body of mineral grains.

Transforming igneous rocks

As basalts are exposed to high pressure (but still relatively low temperature), the minerals transform and become foliated. Low pressures create minerals with a green colour, so the metamorphic rock is called greenschist (which has a foliated texture as well as the green colour). Exposed to higher levels of pressure, the green coloured minerals transform into blue coloured minerals, called creating blueschist. Under increasing temperature and pressure these schists transform into gneiss, as described for sedimentary rocks previously. Intrusive igneous rocks such as granite transform into gneiss rocks as the temperature and pressure force metamorphic differentiation of the minerals into dark and light layers.

Creating hornfels

Hornfels are metamorphic rocks created through contact metamorphism. When magma first moved into a rock closer to the surface, it increases temperature enough to change the mineral composition and texture of the surrounding rocks. However, because no pressure is applied, hornfels are not foliated. Also, they have very small mineral grains because the heating by the magma occurs only for a short time; the minerals don't have time to grow very large before the rock cools again.

Logging Task

At GZ, there are three largish rocks formed by contact metamorphism. Taking the largest of these rocks as the example, describe what you see. Things to include in your answer are volume, colour, grain size, texture and any impurities that may be there.

To successfully log this Earth Cache please message me, preferably via the message centre or by email to here, your answers to the above logging tasks: (If you would like to do a basic log to keep your stats and TB distance in order that is fine.

Glossary

• Felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. The most common felsic rock is granite.
• Mafic rocks are relatively richer in magnesium and iron.