Metamorphic Rocks

Pictures of Foliated and Non-Foliated Rock Types

Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are provided below.

Metamorphic Rock Types Menu
Amphibolite	Gneiss	Hornfels	Marble
Phyllite	Quartzite	Schist	Slate

Sedimentary Rocks

Picture Gallery of the Most Common Rock Types

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1) clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3) organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris. Pictures and brief descriptions of some common sedimentary rock types are shown below.

Sedimentary Rock Types Menu
Breccia	Chert	Coal	Conglomerate	Iron Ore
Limestone	Rock Salt	Sandstone	Shale	Siltstone

Igneous Rocks

Pictures of Intrusive and Extrusive Rock Types

Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface. Pictures and brief descriptions of some common igneous rock types are shown below.

Igneous Rock Types Menu
Andesite	Basalt	Diorite	Gabbro	Granite	Obsidian
Pegmatite	Peridotite	Pumice	Rhyolite	Scoria	Tuff

Saturday, November 22, 2008

The topographic map illustrated in Figure 10l-1 suggests that the Earth's surface has been deformed. This deformation is the result of forces that are strong enough to move ocean sediments to an eleveation many thousands meters above sea level. In previous lectures, we have discovered that this displacement of rock can be caused by tectonic plate movement and subduction, volcanic activity, and intrusive igneous activity.

Figure 10l-1: Topographic relief of the Earth's terrestrial surface and ocean basins. Ocean trenches and the ocean floor have the lowest elevations on the image and are colored dark blue. Elevation is indicated by color. The legend below shows the relationship between color and elevation. (Source: National Geophysical Data Center, National Oceanic and Atmospheric Administration).

Deformation of rock involves changes in the shape and/or volume of these substances. Changes in shape and volume occur when stress and strain causes rock to buckle and fracture or crumple into folds. A fold can be defined as a bend in rock that is the response to compressional forces. Folds are most visible in rocks that contain layering. For plastic deformation of rock to occur a number of conditions must be met, including:

• The rock material must have the ability to deform under pressure and heat.
• The higher the temperature of the rock the more plastic it becomes.
• Pressure must not exceed the internal strength of the rock. If it does, fracturing occurs.
• Deformation must be applied slowly.

A number of different folds have been recognized and classified by geologists. The simplest type of fold is called a monocline (Figure 10i-2). This fold involves a slight bend in otherwise parallel layers of rock.

Figure 10l-2: Monocline fold.

An anticline is a convex up fold in rock that resembles an arch like structure with the rock beds (or limbs) dipping way from the center of the structure (Figure 10l-3).

Figure 10l-3: Anticline fold. Note how the rock layers dip away from the center of the fold are roughly symmetrical.

A syncline is a fold where the rock layers are warped downward (Figure 10l-4 and 10l-5). Both anticlines and synclines are the result of compressional stress.

Figure 10l-4: Syncline fold. Note how the rock layers dip toward the center of the fold and are roughly symmetrical.

Figure 10l-5: Synclinal folds in bedrock, near Saint-Godard-de-Lejeune, Canada. (Source: Natural Resources Canada - Terrain Sciences Division - Canadian Landscapes).

More complex fold types can develop in situations where lateral pressures become greater. The greater pressure results in anticlines and synclines that are inclined and asymmetrical (Figure 10l-6).

Figure 10l-6: The following illustration shows two anticline folds which are inclined. Also note how the beds on either side of the fold center are asymmetrical.

A recumbent fold develops if the center of the fold moves from being once vertical to a horizontal position (Figure 10l-7). Recumbent folds are commonly found in the core of mountain ranges and indicate that compression and/or shear forces were stronger in one direction. Extreme stress and pressure can sometimes cause the rocks to shear along a plane of weakness creating a fault. We call the combination of a fault and a fold in a rock an overthrust fault.

Figure 10l-7: Recumbent fold.

Faults form in rocks when the stresses overcome the internal strength of the rock resulting in a fracture. A fault can be defined as the displacement of once connected blocks of rock along a fault plane. This can occur in any direction with the blocks moving away from each other. Faults occur from both tensional and compressional forces. Figure 10l-8 shows the location of some of the major faults located on the Earth.

Figure 10l-8: Location of some of the major faults on the Earth. Note that many of these faults are in mountainous regions (see section 10k).

There are several different kinds of faults. These faults are named according to the type of stress that acts on the rock and by the nature of the movement of the rock blocks either side of the fault plane. Normal faults occur when tensional forces act in opposite directions and cause one slab of the rock to be displaced up and the other slab down (Figure 10l-9).

Figure 10l-9: Animation of a normal fault.

Reverse faults develop when compressional forces exist (Figure 10l-10). Compression causes one block to be pushed up and over the other block.

Figure 10l-10: Animation of a reverse fault.

A graben fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys (Figure 10l-11).

Figure 10l-11: Animation of a graben fault.

A horst fault is the development of two reverse faults causing a block of rock to be pushed up (Figure 10l-12).

Figure 10l-12: Animation of a horst fault.

The final major type of fault is the strike-slip or transform fault. These faults are vertical in nature and are produced where the stresses are exerted parallel to each other (Figure 10l-13). A well-known example of this type of fault is the San Andreas fault in California.

Figure 10l-13: Transcurrent fault zones on and off the West coast of North America. (Source: U.S. Geological Survey).

Latitude, usually denoted symbolically by the Greek letter phi (Φ) gives the location of a place on Earth (or other planetary body) north or south of the equator. Lines of Latitude are the horizontal lines shown running east-to-west on maps. Technically, latitude is an angular measurement in degrees (marked with °) ranging from 0° at the equator (low latitude) to 90° at the poles (90° N for the North Pole or 90° S for the South Pole; high latitude). The complementary angle of a latitude is called the colatitude.

Magma (Plurals include: magmas and magmata) is molten rock that sometimes forms beneath the surface of the earth (or any other terrestrial planet) that often collects in a magma chamber inside a volcano. Magma may contain suspended crystals and gas bubbles. By definition, all igneous rock is formed from magma.

Hawaiian lava flow (lava is the extrusive equivalent of magma *Pahoehoe)

Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700 °C to 1300 °C (or 1292 °F to 2372 °F), but very rare carbonatite melts may be as cool as 600 °C, and komatiite melts may have been as hot at 1600 °C. Most are silicate solutions.

Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava, and explosive ejection as tephra to form pyroclastic rock.

Environments of magma formation and compositions are commonly correlated. Environments include subduction zones, continental rift zones, mid-oceanic ridges, and hotspots, some of which are interpreted as mantle plumes. Environments are discussed in the entry on igneous rock. Magma compositions may evolve after formation by fractional crystallization, contamination, and magma mixing.

Contrary to some impressions,^[clarify] the bulk of the Earth's crust and mantle is not molten. Rather, the bulk of the Earth takes the form of a rheid, a form of solid that can move or deform under pressure. Magma, as liquid, preferentally forms in high temperature, low pressure environments within several kilometers of the Earth's surface.

This article is about the natural seismic phenomenon. For other uses, see Earthquake (disambiguation).

An earthquake or seism is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.

In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above this.

Posted by regelle tayo at 1:59 AM