Characteristics Of Sedimentary Igneous And Metamorphic Rocks – All rocks on Earth’s surface experience stress due to overburden and the weight of the rock layer, and there is a corresponding increase in stress with increasing depth. This pressure – called lithostatic pressure – is the same in all directions Lithostatic pressure can change the overall rock volume of a rock, such as by compacting clasts in a sedimentary rock and reducing the open space between clasts, but it does not change the shape of the rock. To change shape, one part of the rock must deform more than the other, and this means that the forces acting on the rock cannot be equal in all directions.
In contrast to lithostatic stress, differential stress (sometimes called directed stress) does not act uniformly in all directions, and thus can cause significant changes in the appearance of a rock. Figure 5.1 shows how a mineral can change shape due to differential pressure, with the greatest pressure applied from above and below. Two initially spherical mineral grains (Figure 5.1A) are experiencing the greatest amount of stress at the grain-to-grain contact (red arrow) in a sedimentary rock, and the bonds connecting the atoms in these grains will break. The atoms will migrate to an area of lower stress and form rebonds with other atoms in the mineral grain (Figure 5.1b). As a result, the grains will develop a planar shape where the grains are wide in the horizontal direction, and perpendicular to the top-down direction of stress (Figure 5.1C).
Characteristics Of Sedimentary Igneous And Metamorphic Rocks
Figure 5.1 Mineral grain deformation due to differential stress (gray arrows). a) Spherical grains experience the greatest stress at the contact between two grains b) Grains change shape when bonds break in areas of high stress, and atoms migrate to areas of low stress (areas of new crystal growth). c) Finally, the two grains appear flat Source: Karen Teffend (2015) CC BY-SA 3.0 visual source
Scoria Igneous Rock
Figure 5.1 shows the deformation of only two grains In fact, this is happening with all the grains in the rock, and eventually, the whole rock will have minerals that are equilibrated (meaning they are long in the same direction). The result is a rock with a metamorphic pattern called a foliation Foliation gives the rock a layered appearance and separates between layers Layers can be fairly flat if the grains are small, or wavy if the grains are large You will see some examples of the different forms that can be taken later Being able to identify foliation in metamorphic rocks is useful for the purpose of understanding how the rocks were formed, because we can infer something about what forces were acting on the rocks.
An increase in pressure can lead to metamorphism, and often an increase in pressure is accompanied by an increase in temperature. Heat is also favorable for the presence of chemically reactive fluids, which can accelerate metamorphic changes.
Metamorphic rocks are classified into low, medium and high grade metamorphic intensity mainly due to the effect of heat on mineral stability. Heat causes atoms to vibrate; The higher the temperature, the more vibrations and the weaker the bonds between atoms Thus, it is easier to break the bonds between atoms in a mineral structure at higher temperatures, leading to the type of change shown in Figure 5.1.
The process of changing the shape and size of crystals in metamorphic rocks is called recrystallization Often small crystals are aggregated into smaller, larger crystals In Figure 5.2, the protolith (A) is a sedimentary rock with small rounded grains, but recrystallization produces larger interlocking grains (B), and the openings are gone. Larger grains are more chemically stable because they have less surface area to volume ratio
Section 3: Classifying Rocks
Figure 5.2 Rearrangement of grains into larger crystals (a) Fallen rock with rounded quartz grains, and open spaces between grains b) Metamorphic rocks formed from sedimentary rocks with large, interlocking quartz crystals. Source: Karen Teffend (2015) CC BY-SA 3.0 visual source
Increased grain size is not the only metamorphic transformation Sometimes when certain temperature and pressure conditions are reached, the minerals within the rock can be transformed into new minerals. Because these minerals are indicative of specific metamorphic conditions, they are called index minerals.
Differential pressure can create a metamorphic pattern that develops in metamorphosed rocks known as foliation. There are certain types of foliation that commonly occur in metamorphic rocks, and the type is a function of the mineral composition that defines the foliation.
Gneiss (pronounced “beautiful”; Fig. 5.3) is a metamorphic rock with foliation that presents as alternating bands of dark and light minerals throughout the rock. This type of foliation is called genetic banding In Figure 5.3A, the specimen sits with color bands running almost horizontally, indicating that the greatest stress was applied to the rock in the up-down direction (ie, at right angles) in the photo. banding). If differential pressure is applied to the gneiss after the bands have formed, they may not be flat as in Figure 5.3A, but may be folded or consolidated as in Figure 5.3B.
It All About Types Of Rocks And Their Characteristics.
Figure 5.3 Two examples of metamorphic rocks, gneisses Each stone exhibits alternating dark and light mineral bands in alternating directions Credits: Karla Panchuk (2020), Karan Teffend (2015) CC BY-SA 3.0 Adapted from view source.
Biotite and amphibole are common minerals in the darker bands, and the lighter bands contain feldspar and quartz. The protolith for a genus can be any rock that contains more than one mineral, such as earthy quartz and feldspar with its clay minerals, or both dark-colored ferromagnesian minerals and light-colored non-ferromagnesian minerals. For genetic foliation to develop, temperature and pressure must be sufficiently high; Because of this, gneiss rocks represent a high grade of metamorphism
Other types of foliated rocks break in more or less parallel layers This property is called rock cleavage If you look closely at rocks with well-developed rock fragments, you can identify areas where foliation gives the appearance of book pages. As with some page edges in Figure 5.4, foliation can create flat with wavy surfaces.
Figure 5.4 The edge-view of book pages is similar to what you can see on some stones with foliation Source: Tom Murphy VII (2005) CC BY-SA 3.0 See source
Rocks Rock!!! (igneous, Metamorphic, & Sedimentary)
Metamorphic rocks with a foliation pattern defined by layers of platy minerals (such as muscovite or biotite micas) are called schists. The rock name is usually modified to indicate which mica it contains For example, Figure 5.5 is muscovite schist with garnet, so the correct name for this rock is a garnet muscovite schist. By convention, when naming a metamorphic rock, the least abundant mineral is mentioned first.
Muscovite micas define a very wavy foliation in the rock, called schistose foliation (Fig. 5.5B). Shell is usually the protolith for schists; During metamorphism, very small clay minerals in the shale are recrystallized into mica that is large enough to be seen on a large scale. The temperatures and pressures required for schistose foliation are not high for gneiss, so schists represent an intermediate grade of metamorphism.
Figure 5.5 Photo of a garnet muscovite schist A) Muscovite micas are large enough to appear as very shiny minerals in the photograph above. b) Side view of schist showing distinctive wavy foliation composed of platy micas. This metamorphic rock also contains the index mineral, garnet Source: Karen Teffend (2015) CC BY-SA 3.0 visual source
The rock-forming crystals in schist and gneiss are visible without magnification, but this is not the case for all foliated metamorphic rocks. Phyllite (Figure 5.6) is a metamorphic rock composed of fine crystals that give the rock its satin shine. The rock exhibits phyllitic foliation, with fine, sometimes wrinkled or folded, clasts. Phyllites are usually formed from a shell protolith, in which clay minerals are aligned and recrystallized as small mica minerals. This foliation develops at temperatures and pressures that are lower than those of schist and represents a low to moderate grade of metamorphism.
The Rock Cycle Objective: Student Will Identify And Classify The Characteristics Of The Rock Cycle By The End Of The Lesson.
Figure 5.6 Including side view showing slightly wavy surfaces resulting from phyllite, folitic foliation. Source: Karla Panchuk (2020) CC BY-NC-SA
Slate (Fig. 5.7) is formed by low temperature and pressure changes in a shale protolith. The clay-sized minerals in the shale are recrystallized into very fine mica, which is larger than the clay minerals, but too small to be visible. The arrangement of these mics breaks into flat sheets parallel to the mica alignment This is called slaty cleavage The specimen in Figure 5.7 is interesting because there is evidence of the original sedimentary layers (highlighted in the lower right of the specimen), but due to metamorphism the floor slab has now cut into those layers. Slate is an example of a low grade
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