(not corrected yet)

VOLCANOES I (TYPES AND DIFFERENCES)

Shield Volcano is a gently-sloping volcano that emits mostly basaltic lava (very fluid lava) that flows in long-lasting, relatively gentle eruptions – explosions are minimal. Shield volcanoes can be very big.

Composite or Strato Volcano is a steep-coned volcano that explosively emits gases, ash, pumice, and a small amount of stiff, silica lava (called rhyolite). This type of volcano can have eruptions accompanied by lahars — deadly mudflows. Most volcanoes on Earth are of this type. Stratovolcanoes kill more people than any other type of volcanoes – this is because of their abundance on Earth and their powerful mudflows.

Lava domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in Mount Saint Helens), but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent.

Volcanic cones or cinder cones are the result from eruptions that erupt mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.

A Supervolcano is a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States. Supervolcanoes are hard to identify centuries later, given the enormous areas they cover. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted, but are non-explosive.

Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water because of volcanic gases. Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals.

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse leaving a flat-topped mountain. Then, the pillow lavas also collapse, giving an angle of 37.5 degrees.. Very good examples of this type of volcano can be seen in Iceland. 2 million years old and hasn’t erupted violently for approximately 640,000 years, although there has been some minor activity relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be an active volcano.

VOLCANOES II (PARTS AND MATERIALS)

Magma is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets.

Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava, and explosive ejection as tephra to form pyroclastic rock.

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

Magma, as liquid, preferentially forms in high temperature, low pressure environments within several kilometers of the Earth’s surface.

Magma compositions may evolve after formation by fractional crystallization, contamination, and magma mixing. By definition, all igneous rock is formed from magma.

Types of Magma:

-Ultramafic (picritic)

SiO2 < 45%

Fe-Mg >8% up to 32%MgO

Temperature: up to 1500°C

Viscosity: Very Low.

Eruptive behavior: gentle or very explosive (kimberilites).

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; komatiite and other ultramafic lavas are mostly Archean and were formed from a higher geothermal gradient and are unknown in the present.

-Mafic (basaltic)

SiO2 < 50%

FeO and MgO typically < 10 wt%

Temperature: up to ~1300°C

Viscosity: Low.

Eruptive behavior: gentle.

Distribution: divergent plate boundaries, hot spots, convergent plate boundaries.

-Intermediate (andesitic)

SiO2 ~ 60%

Fe-Mg: ~ 3%

Temperature: ~1000°C

Viscosity: Intermediate

Eruptive behavior: explosive or effusive

Distribution: convergent plate boundaries, island arcs.

-Felsic (rhyolitic)

SiO2 >70%

Fe-Mg: ~ 2%

Temp: < 900°C

Viscosity: High.

Eruptive behavior: explosive or effusive.

Distribution: hot spots in continental crust (Yellowstone National Park), continental rifts

Lava is molten rock expelled by a volcano during eruption. This molten rock is formed in the interior of the Earth. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700ºC to 1,200ºC. Although lava is quite viscous, with about 100,000 times the viscosity of water, it can flow great distances before cooling and solidifying, because of both its thixotropic and shear thinning properties

A Volcanic crater is a circular depression in the ground caused by volcanic activity. It is typically a basin, circular in form within which occurs a vent (or vents) from which magma erupts as gases and lava. A crater can be of large dimensions, and sometimes of great depth. During certain types of climactic eruptions, the volcano’s magma chamber may empty enough for an area above it to subside, forming what may appear to be a crater but is actually known as a caldera.

A Magmatic chamber is a large underground pool of molten rock found beneath the surface of the Earth’s crust. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma. If it finds a way to the surface, then the result will be a volcanic eruption; consequently many volcanoes are situated over magma chambers.

Magma chambers are hard to detect, and most of the known ones are therefore close to the surface of the Earth, commonly between 1km and 10km under the surface. In geological terms this is extremely close to the surface, although in human terms it is considerably deep underground.

Magma rises through fractures from beneath the crust because it is less dense than the surrounding rock. When the magma cannot find a path upwards it pools into a magma chamber. As more magma rises up below it, the pressure in the chamber grows. If magma resides in a chamber for a long period, then it can become stratified with lower density components rising to the top and denser materials sinking. It can also start to cool, with the higher melting point components such as olivine crystallising out of the solution, particularly near to the cooler walls of the chamber, and forming a denser conglomerate of minerals which sinks. If the magma is not vented to the surface in a volcanic eruption it will slowly cool and crystallize at depth to form an intrusive igneous body composed of granite. Often, a volcano may have a deep magma chamber many kilometres down, which supplies a shallower chamber near the summit.As a volcano erupts, emptying the magma chamber, the surrounding rock will collapse into it. If a large amount of magma is erupted, causing the chamber to reduce considerably in volume, then this can result in a depression at the surface called a caldera.

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than 2millimetres in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions and ejection of entrained particles during steam eruptions causing phreatic eruptions. The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size. Volcanic ash can lead to breathing problems, malfunctions in machinery, and from more severe eruptions, years of global cooling.

Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a lava supply. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since “supervolcano” calderas can have eruptive life-spans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.

EARTHQUAKES (MAIN CONCEPTS AND SCALES)

An Earthquake is the result of a sudden release of energy in the Earth’s crust that creates seismic waves. Earthquakes are recorded with a seismograph. The moment magnitude (or the related Richter magnitude) of an earthquake is conventionally reported, 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.

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 the hypocenter.

Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.

The Richter magnitude scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a Wood–Anderson torsion seismometer output. The effective limit of measurement for local magnitude is about 6.8 .The energy release of an earthquake, which closely correlates to its destructive power.

The Mercalli intensity scale is a scale used for measuring the intensity of an earthquake. The scale quantifies the effects of an earthquake on the Earth’s surface, humans, objects of nature, and man-made structures on a scale of I through XII, with I denoting not felt, and XII one that causes almost complete destruction. The values will differ based on the distance to the earthquake, with the highest intensities being around the epicentral area. Data is gathered from individuals who have experienced the quake, and an intensity value will be given to their location.

The Epicenter or epicentre is the point on the Earth’s surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates.

In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone.

A Tsunami is a wave in the ocean caused by earthquakes or volcanic eruptions. A tsunami is a very long wave. It can be hundreds of kilometers long. It is a chain of fast moving waves caused by fast changes in the ocean. Usually, a tsunami starts suddenly. It will begin as normal waves and change to a very big wave very quickly. The waves travel at a great speed across an ocean with little energy loss.

The water will draw back from the coast half of the period of the wave before it gets to the coast (picture 1). If the slope of the coast is shallow, the water may pull back for many hundreds of metres (picture 2). People who do not know of the danger will often remain at the shore. Tsunamis can not be prevented, but there are ways to help stop people from dying from a tsunami. Some regions with a high risk of tsunamis may use warning systems to warn the general population before the big waves reaches the land. Because an earthquake that caused the tsunami can be felt before the wave gets to the shore, people can be warned to go somewhere safe.

Picture 1

Picture 2

FOLDS (PARTS AND TYPES)

The term Fold is ,in geology, when one or a stack of originally flat and planar surfaces, are bent or curved as a result of plastic (i.e. permanent) deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones.

An Anticline is a fold that is convex up and has its oldest beds at its core. On a geologic map, anticlines are usually recognized by a sequence of rock layers that are progressively older toward the centre of the fold because the uplifted core of the fold is preferentially eroded to a deeper strato-graphic level relative to the topographically lower flanks. The strata dip away from the centre, or crest, of the fold.

If an anticline plunges (i.e., is inclined to the Earth’s surface), the surface strata will form Vs that point in the direction of plunge. Anticlines are typically flanked by synclines although faulting can complicate and obscure the relationship between the two

A Syncline is a downward-curving fold, with layers that dip toward the center of the structure. A synclinorium is a large syncline with superimposed smaller folds.

On a geologic map, synclines are recognized by a sequence of rock layers that grow progressively younger, followed by the youngest layer at the fold’s centre or hinge, and by a reverse sequence of the same rock layers on the opposite side of the hinge. If the fold pattern is circular or elongate circular the structure is a basin. Folds typically form during crustal deformation as the result of compression that accompanies orogenic mountain building.

5-FAULTS (PARTS AND TYPES)

A Fault or fault line is a planar fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Large faults within the Earth’s crust are the result of differential or shear motion and active fault zones are the causal locations of most earthquakes. Earthquakes are caused by energy release during rapid slippage along a fault. A fault that runs along the boundary between two tectonic plates is called a transform fault.

Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane. The two sides of a non-vertical fault are called the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining.

Reverse faults are exactly the opposite of normal faults. If the hanging wall rises relative to the footwall, you have a reverse fault. Reverse faults occur in areas undergoing compression (squishing). If you imagine undoing the motion of a reverse fault, you will undo the compression and thus lengthen the horizontal distance between two points on either side of the fault.

A Horst is the raised fault block bounded by normal faults or graben. A horst is formed from extension of the Earth’s crust. The raised block is a portion of the crust that generally remains stationary or is uplifted while the land has dropped on either side.

A Graben is a depressed block of land bordered by parallel faults. A graben is the result of a block of land being downthrown producing a valley with a distinct scarp on each side. Grabens often occurs side-by-side with horsts. Horst and graben structures are indicative of tensional forces and crustal stretching.

Grabens are produced from parallel normal faults, where the hanging wall is downthrown and the footwall is up thrown. The faults typically dip toward the center of the graben from both sides. Horsts are parallel blocks that remain between grabens, the bounding faults of a horst typically dip away from the centre line of the horst.

A single graben or multiple grabens can produce a rift valley.

A Thrust fault is a type of fault, or break in the Earth’s crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger. Thrust faults are the result of compressional forces

An Oblique-slip faults is a fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.

-Perseo Magallón

Advertisements