Diastrophism / Geologic Hazards : Volcanic Eruption and Earthquake

Diastrophism

Diastrophism is the process of deformation that changes Earth’s surface. It produces many of the basic structures you see on the surface, such as plateaus, mountains, and folds in the crust.

The movement of magma is called vulcanism or volcanism.

With the increasing pressure, four (4) different and separate responses can be observed (including rocks):

  1. showing no change
  2. showing an elastic change with recovery
  3. showing a plastic change with no recovery
  4. finally breaking from the pressure

A stress is a force that tends to compress, pull apart, or deform a rock. Rocks in Earth’s solid outer crust are subjected to forces as Earth’s plates move into, away from, or alongside one another.

Three Types of Forces that cause Rock Stress:

  1. Compressive stress is caused by two plates moving together or by one plate pushing against another plate that is not moving.
  2. Tensional stress is the opposite of compressional stress. It occurs when one part of a plate move away, for example, and another part does not move.
  3. Shear stress is produced when two plates slide past each other or one plate slides past another plate that is not working.

slide_3A rock is able to withstand stress up to a limit. Then it might undergo elastic deformation, plastic deformation, or breaking with progressively greater pressures. The adjustment to stress is called strain.

Three Types of Strain:

  1. Elastic – in elastic strain, rock units recover their original shape after the stress is released.
  2. Plastic – in plastic strain, rock units are molded or bent under stress and do not return to their original shape after the stress is released.
  3. Fracture – in fracture strain, rock units crack or break, as the name suggests.

The relationship between stress and strain, that is, exactly how the rock responds, depends on at least four (4) variables. They are:

  1. the nature of the rock
  2. the temperature of the rock
  3. how slowly or quickly the stress is applied
  4. the confining pressure on the rock

In general, rocks are better able to withstand compressional than pulling-apart stresses. Cold rocks are more likely to break than warm rocks, which tend to undergo plastic deformation.

Folding

Stress on buried layers of horizontal rocks can result in plastic strain, resulting in a wrinkling of the layers into folds. Folds are bends in layered bedrock.

A vertical, upward stress, on the other hand, can produce a large, upwardly bulging fold called a dome. A corresponding downward bulging fold is called a basin. An arch-shaped fold is called an anticline. The corresponding trough-shaped fold is called a syncline.

 

Faulting

Rock layers do not always respond to stress by folding. Rock near the surface are cooler and under less pressure, so they tend to be more brittle.

A sudden stress on these rocks may reach the rupture point, resulting in a cracking and breaking of the rock structure. If there is breaking of rock without a relative displacement on either side of the break, the crack is called a joint. When there is relative movement between the rocks on either side of a fracture, the crack is called a fault. When faulting occurs, the rocks on one side move relative to the rocks on the other side along the surface of the fault, which is called the fault plane.

faulting

Faults are generally described in terms of:

  1. the steepness of the fault plane, that is, the angle between the plane and an imaginary horizontal plane.
  2. the direction of relative movement.

 

Three ways that rocks on one side of a fault can move relative to the rocks on the other side:

  1. up and down (called dip)
  2. horizontally, or sideways (called strike)
  3. with elements of both directions of movement (called oblique)

fault

Three Basic Classes of Faults:

  1. A normal fault is one in which the hanging wall has moved downward relative to the footwall.
  2. In a reverse fault, the hanging wall block has moved upward relative to the footwall block.
  3. A thrust fault is a special type of reverse fault. In a thrust fault, the fault surface is at a very low angle or even horizontal in some places.

A transform fault is a fault along which there is a horizontal movement similar to that when you rub your hands together. This is caused by shear stress.

 

Volcanic Eruption

What is a volcano?

A volcano is a vent that connects molten rock (magma) from within the Earth’s crust to the Earth’s surface. The volcano includes the surrounding cone of erupted material.

The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano.

volcano

http://www.pgfmobile.org/mms/pages/plan/structure.html

How Volcanoes Erupt

  1. High temperatures and pressures cause solid rock to liquefy into magma.
  2. Once a large body of magma has collected, it rises through denser rock layers to the surface.
  3. The magma flows through the central vent and explodes from the crater at the surface
  4. Ash, rock, and lava propel from the opening and into the atmosphere.

Explanation

  • Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface. At depths >20 km the temperature = 800 – 1,600 degrees Celsius.
  • High temperature of the Earth’s interior.
  • When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it  as to how it erupts.
  • Radioactive decay, leading to convection in the mantle.

Two Styles of  Volcanic Eruption:

  • Explosive:
    • Large amounts of gas and a high viscosity (sticky ) magma will form an explosive eruption.
    • Where rapidly escaping gas bubbles rip apart the magma, fragmenting it.
    • Erupts 10’s – 1000’s cubic kilometer of magma
    • Send ash clouds >25 km into the stratosphere.
    • If you release the pressure of a magma chamber (by cracking the surrounding rock or breaking through to the surface, the gas dissolved in the magma will start to exsolve (separate from the melt forming bubbles). These bubbles, called vesicles, rapidly expand and rise through the magma. The rapid escape of gas (volatiles) causes magma to fragment and erupt explosively.
    • An explosive volcanic eruption will propel large volumes of volcanic rock, ash, and gas into the atmosphere. The larger (most dense) particles will fall out of the air quickly and close to the volcanic vent. The smaller particles (ash) can be suspended in the atmosphere for days to weeks before they fall back to Earth. Whilst in the atmosphere the wind can transport the ash particles large distances.
    • Three Products from an Explosive Eruption:
      • Ash fall: the fallout of rock, debris, and ash from an explosive eruption column.
      • Pyroclastic flow: are hot, turbulent, fast-moving, high particle concentration clouds of rock, ash and gas. Pyroclastic flows can reach > 100 km from a volcano. They can  travel 100s km/h and are commonly >400C.
      • Pyroclastic surge: are low particle concentration (low density) flows of volcanic material. The reason they are low density flows is because they don’t have a high concentration of particles and contain a lot of gas. Pyroclastic surges are very turbulent and fast (up to 300 km per hour). Pyroclastic surges usually do not travel as far as pyroclastic flows, but pyroclastic surges can travel up to at least 10 kilometers from the source.
  • Effusive:
    • Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption.
    • where the magma leaks out onto the surface passively as lava flows.
    • Some may turn into explosive eruptions. If the magma is too viscous (sticky) it can block up the volcanic vent, trapping as inside the volcano. If this gas builds up enough to break through the blockage an extremely dangerous explosive eruption may form.
    • This happens either because there is not enough gas (volatiles) in the magma to break it apart upon escaping, or the magma is too viscous (sticky) to allow the volatiles to escape quickly.
    • Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge rate (how fast it comes out of the vent), slope of the ground over which the lava travels, and duration of eruption.

 

Volcanic Hazards

  • Pyroclastic Flow
    • is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano reaching speeds of up to 700 km/h. The gases can reach temperatures of about 1,000 Celsius.
    • Mount Mayon Volcanic Eruptionmayon
    • Mount Vesuvius Eruption 79 ADvesuvius
  • Lahars / Mud Flows
    • Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.
    • Heavy rain after an eruption or hot volcanic activity melting snow and ice will provide a large volume of water that will flow down the sides of the volcano. This water picks up the newly erupted material forming fast flowing torrents of water, mud, ash, rock and debris.
    • Lahars can flow great distances and be very destructive.
  • Pyroclastic Fall
    • An explosive eruption will produce an eruption column of hot gas, ash and debris, ejected kilometers into the air. As this debris falls back down to the ground it can cause a lot of damage.
    • Like too much snow on a roof, too much ash raining down from an eruption column can cause the roof to collapse.
    • Ash loading on power lines will cause them to fall. As little as 1 centimetre of ash accumulated on the leaves of a plant will stop it from being able to photosynthesize and therefore the plant will die.
    • Lots of fine ash falling in lakes, rivers and water reservoirs will cause contamination making it unfit to drink, or to live in if you are a fish, etc. Very fine particles, if inhaled by humans, can cause extensive damage to the lungs causing a respiratory disease called silicosis.
  • Lava Flow
    • Lava flows although generally slower and moving and less catastrophic than pyroclastic flows still remain dangerous.
    • Lava flows have temperatures in excess of 200 degrees Celsius. Therefore will burn any flammable material it contacts with.
  • Noxious Gas
    • When magma rises towards the surface the decrease in pressure causes it to lose some of its gas content. As gas is released from the magma it often vents at the surface, leaking out of small cracks in the ground or from the large volcanic vent.
    • A dormant volcano will commonly vent gas even when there is no eruption going on. This is because the magma is deep down in the crust, still releasing gas but not in the position to erupt at the surface.
    • A change in the amount of gas or the chemistry of the gas being released is another precursor to an eruption. An increase in the amount of magma in the chamber will produce an increase in the amount of gas. Also, the new magma may have slightly different abundances of gas types (CO2, SO2, H2O). For example, an increase in the ratio of carbon to sulfur can be used to indicate the arrival of a new batch of magma at the summit reservoir.
  • Earthquakes
    • Earthquake activity commonly precedes an eruption.
    • Result of magma pushing up towards the surface.
    • Increase volume of material in the volcano shatters the rock.
    • Many erupts are preceded by increased levels of seismic activity. The earthquakes are caused by fracturing and brittle failure of the subsurface rocks as new magma pushes it’s way up towards the surface.

Volcano Monitoring

Volcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behavior of the volcano in question.

  1. Seismicity – earthquake activity is measured by Seismographs. Seismographs are stationed on the flanks of the volcano. These record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.
  2. Deformation – any deformation on a volcano can be measured by GPS surveys or tiltmeters. Tiltmeters can measure tiny changes in slope angle. A slope change of one part per million is equivalent to raising the end of a board one kilometeres long only one millimeter. Tiltmeteres can tell the scientists when new magma has entered a magma chamber in the volcano.
  3. Gas Output – gas samples are collected from fumaroles and active vents. Gas levels may also be monitored by remote sensing techniques, such as spectrometer or COSPEC.

Precursors to an Eruption:

  • Increased earthquakes in the area (increased seismicity)
  • Swelling and cracking of the ground (deformation)
  • Change in the amount of or chemistry of the gas coming out of the volcano.
  • Change in the groundwater levels or chemistry.

Earthquakes

The shaking of the earth’s crust caused by a release of energy. Earthquakes can be caused by:

  1. Eruption of a volcano
  2. Collapse of a cavern
  3. Impact of a meteorite
  4. Strain built up along boundaries between plates

A fault is a break in the lithosphere along which movement has occurred. Most earthquakes occur in this way.

Elastic Rebound Theory

Friction between plates prevents them from moving, so strain builds up. The rock deforms. Eventually, the strain becomes great enough that the rock moves, and returns to normal shape. This causes an earthquake.

Focus : the point at which the rock first breaks and moves in an earthquake. This is below the surface.

Epicenter : the point on the earth’s surface directly above the focus.

Seismic Waves

The energy released in an earthquake travels in waves. There are three types:

  1. Primary waves (P waves)
    • Compression waves-squeeze and stretch rock (Push and Pull)
    • Can travel through any material – solid, liquid, and gases.
    • this wave travel the fastest.
  2. Secondary waves (S waves)
    • Side to side movement.
    • can travel only through solid material, not liquids or gases.
    • travel a little more than half the speed of P waves.
  3. Surface waves (Rayleigh and Love)
    • Seismic waves that travel along Earth’s surface.
    • when P and S waves reach the surface, they make surface waves.
    • There are two types, Love waves . and Rayleigh waves.

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Measuring and Locating Earthquakes

  • Seismograph : instrument used to measure an Earthquake.
  • Seismogram is the paper record of the Earthquake data (shaking) is called a seismogram.
  • The S-P Time interval is the time between the start of the P wave and the S wave.
  • Because P waves and S waves travel at different speeds, the difference in their arrival times can be used to determine the distance away an earthquake occurred.
  • If you know the distance an earthquake occurred from at least three different seismic stations, you can determine the location of the epicenter.
  • Triangulation : Using the S-P time interval data from 3 stations to determine the epicenter.
  • Earthquake Magnitude : strength measured by the amount of released energy.
  • Richeter Scale by Charles Richter – each increase in number represents 10 times an increase in power.
  • Mercalli Scale : The effect of an earthquake on people and buildings can be used to determine the relative intensity of the earthquake. The scale expresses such intensities with Roman numberals that range from I to XII.
  • Earthquake Hazards
    • Fire : causes the most damage in an earthquake, some utility lines and roads get damaged.
    • Liquefaction : when the ground turns to quicksand due to the shaking.
    • Tsunamis : are caused by underwater earthquakes that make a big wave.

 

 

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