Metamorphic Rock Characteristics Help (page 2)

Index Minerals

A Scottish geologist, George Barrow, noticed that rocks having the same overall mineral make up (like shale) could be seen to go through a series of transformations throughout specific zones in a metamorphic region. He found that minerals in individual zones had specific mineral configurations. As he studied minerals across a zone, he found that when new metamorphic mineral configurations were created, it was predictable.

The first appearance of index minerals marks the boundary of low-to high-grade metamorphic rock changes in a specific regional zone.

Barrow found that mineral (shale) configuration changes happened with regard to index minerals . These index minerals acted like milestones in the low- to high-grade metamorphic rock transformation process. Barrow found that the domino effect of metamorphism happened in the following series:

chlorite biotite garnet staruolite kyanite sillimanite Low grade → High grade

When Barrow and his team studied the geological maps of the Scottish Highlands, they were able to plot where certain minerals started and stopped. They marked the locations of certain minerals and called these connected places isograds .

An isograd is a marker line on a map connecting different areas of certain minerals found in metamorphic rock.

Metamorphic Rock Textures

Metamorphic rocks are divided into two categories, foliated and nonfoliated . Foliate comes from the Latin work folium (meaning leaf) and describes thin mineral sheets, like pages in a book. Metamorphic minerals that align and form bands, like granite gneiss and biotite schist, are strongly banded or foliated . Figure 8-4 gives you an idea of how mineral grains line up (foliate) after metamorphism has taken place.

Fig. 8-4. Foliated metamorphic rocks have parallel mineral grains.

When metamorphic mineral grains align parallel in the same plane and give rock a striped appearance, it is called foliation or foliated rock.

Initially, the weight of sedimentary rock strata keeps the sheet-like formation of minerals parallel to the bedding planes. As the mineral layers are buried deeper or compressed by tectonic stresses, however, folding and deformation take place. The sedimentary strata are shoved sideways and are no longer parallel to the original bedding. In fact, metamorphism changes the texture enough that when broken, the metamorphic rock breaks in the direction of the foliation not the original mineral’s composition. Table 8-1 lists some of the main foliated and nonfoliated metamorphic rocks and their mineral compositions.

Table 8-1 Metamorphic rocks are foliated and nonfoliated in composition.

Foliates are made up of large concentrations of mica and chlorite. These minerals have very clear-cut cleavage. Foliated metamorphic rocks split along cleavage lines that are parallel to the alignment of the rock’s minerals. For example, mica can be separated into thin, flat nearly transparent sheets. Mica is said to have good schistosity , from the Latin word schistos meaning easily cleaved.

Schistosity is the parallel arrangement of coarse grains of sheet-structure minerals formed during metamorphism and increasing pressure.

For fine-grained rocks with microscopic mineral grains, the breakage property is known as rock cleavage or slaty cleavage .

Slaty cleavage is found in an environment of low temperature and pressure. In these less-intense conditions, grain sizes increase and single grains are easily seen. Foliation is present with slaty cleavage, but not in a flat plane. Intermediate and high-grade metamorphic rock commonly breaks along rolling, or somewhat distorted surfaces similar to the orientation of the grain of quartz, feldspar, and other minerals.

Rock cleavage or slaty cleavage describes the way rock breaks into plate-like pieces along flat planes.

Large crystal textures can also be formed in a fine-grained, support rock during metamorphism. When this happens, crystals found in both contact and regional metamorphic rock are called porphyroblasts . They grow as the elements are rearranged by heat and temperature.

We learned that structural deformation goes on during metamorphism. When two rock surfaces deep in the Earth’s crust grind against each other, crushing and stretching into bands, myolites are formed. These rocks are deformed under very high pressure. This deformation can take place before, during, or after metamorphic changes have happened and is part of the ongoing recycling of rock.

For example, shale may be changed into schist during deep burial without any deformation. Then, much later, when tectonic action hauls the schist layer upward in mountain building, higher-grade metamorphism may cause foliation and deformation. Then, if the rock is living a really interesting life, it may be heated during contact metamorphism and change yet again.