Geo-physical Hazards


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What is a tectonic plate?

Earth’s outermost layer is fragmented or into several large and small solid slabs, called lithospheric plates or tectonic plates that are moving relative to one another. A tectonic plate is a rigid lithospheric slab of the fractured crustal surface of the earth floating on the semi-molten material of the asthenosphere. The theory of plate tectonics explains the global distribution of earthquakes, volcanoes, fold mountains and rift valleys. To understand why volcanoes and earthquake happen, it is necessary to understand what goes on beneath the surface of the earth. Plate tectonics is a relatively new theory and it wasn't until the 1960's that Geologists, with the help of ocean surveys, began to understand what goes on beneath our feet.


Background Knowledge


Layers of the Earth
Characteristics
Thickness and temperature
Crust
(i) Continental crust, which carries land, made up of mostly low density granite
(ii) Oceanic crust, which carries water, made up of mostly basalt
  • Outermost layer of the earth
  • Relatively thin
  • Made up of low density light weight material
  • Split into several plates floating on the semi-molten upper Mantle.
0 to70 km thick, continental crust is thicker and lighter than oceanic crust.
Mohorovicic Discontinuity or Moho
The boundary between crust and mantle
Lithosphere
  • Consist of the Earth´s crust
  • Hard, rigid, fragile surface layer of the planet.
Mantle


(i) Upper mantle or Asthenosphere


(ii) Lower mantle
Comprises 84% of the Earth's volume and 67% of its mass,
Hot material upwells, while cooler and heavier material sinks downward.

Hot and partially molten layer of the Earth which underlies the lithosphere.

Most magma that erupt at the surface as lava are derived originally by melting of the mantle

is almost exclusively solid
It has a thickness of approximately 2,900 km.


temperatures range between 500 to 900 °C
D layer and Gutenberg discontinuity
Core-mantle boundary

Core


(i) Outer core


(ii) Inner Core
Inner most layer of the Earth
consist of predominantly Iron, Nickel and Sulfur

Outer core is a layer of slow-moving liquid metal. It generates electrical currents as it flows and these create Earth’s magnetic field.

Inner core is a solid mass of hot metal reaching over 5000 °C.
Up to 5150 km


Temperatures of up to 5,500°C.
Up to 6378 km from the surface


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Forerunners of the Plate tectonic Theory

  1. Theory of continental drift by Alfred Wegener in 1912
  2. Sea floor spreading by Harry Hess in 1960s


1. Continental drift theory of Alfred Wegener (1912)

Wegener suggested that around 300 million years ago all the continents were joined together as a single supercontinent called Pangaea, surrounded by a primeval ocean called Panthalassa. The continents have since moved away or drifted apart from one another.

Why did Wegener talk about continental drifting:
  • The similarity in shape of the edge of the continents, as if they can be fitted together like the pieces of a jigsaw puzzle
  • The presence of fossils of plants such as Glossopteris, a fossil fern whose spores cannot cross wide oceans and animals who cannot swim across the wide oceans, now can be found widely-separated continents of Africa, Australia, and India
  • The presence of glacial deposits near the equator. The coal and oil reserves found in Antarctica suggest that this area was once in a different climatic zone.
  • The similarity of rock sequences and geological structures on different continents.

Why his theory was rejected at that time:
  • The mechanism for the movement which he mentioned was a combination of centrifugal force and the gravitational pull of the moon.
  • Physicists quickly proved that these forces were insufficient to move the continents and at that time little was known about the nature of the sea floor.


2. Sea-floor spreading by Princeton geologist Harry Hess in early 1960s

Basaltic magma from the mantle rises up to create new ocean floor at mid-ocean ridges and then moves apart and subsequently destroyed in subduction zones (recycled back into the mantle) to accommodate the increasing size of the oceanic crust (at that time it was not known clearly)

Supporting Arguments:
  • Age of the seafloor rocks
    • Near the mid-oceanic ridges, the basaltic rocks of the sea floor are much younger than those found towards the continental shelves at the edge of the oceans.
  • Paleomagnetism
    • Study of the Earth's magnetism and reverse polarity suggests that the rocks have been formed at the ridge and then moved away in both directions. There is a parallel pattern of magnetism in the rocks of the sea floor. As magma cools and solidifies, the iron bearing elements within it are magnetized in the direction of the earth magnetic field. There have been many reversals of the earth’s magnetic field over time and the pattern of the rock alignment on the sea floor relates to this alternating pattern of polarity.



Plate Tectonic Theory

by Tuzo Wilson, W. J. Morgan, McKenzie and Parker in 1965

Mechanism of plate movement:
  • The Earth's surface is made up of a series of large rigid crustal slabs called plates, floating on the semi-molten material of the asthenosphere. These plates are in constant motion in relation to each other, moving typically at rates of a few centimeters per year.
  • Basaltic magma from the mantle rises up to create new ocean floor at mid-ocean ridges and then moves apart and subsequently destroyed in subduction zones (recycled back into the mantle) to accommodate the increasing size of the oceanic crust
  • The most likely cause of plate movement is the Mantle Convection currents that generate in the asthenosphere push the plates in different directions. The source of heat creating the convection current cells is a combination of radioactive decay in the core and the residual primary heat of the Earth.
  • The convection currents cause the magma to circulate that pulls the lithosphere apart.
  • Plates are hottest near the mid oceanic ridges and cool down as the move away. As the plate materials cool down, density increases so it starts sinking into the molten rock beneath and is easily dragged downwards into the subduction zone. This is known as slab pull means the subducting slab pull on the rest of the plate.


Types of Plate Boundaries
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Divergent and Constructive Boundaries


Where plates are moving away from each other. The movement is due to divergence of two cenvection cells, which brings magma from the asthenosphere towards the surface
Types of divergence possible:
  • divergence of two oceanic crust as is happening along the mid-Atlantic ridges.
  • divergence of two plates of continental crust as is happening in East African rift valley.

Example:
The divergence of Eurasian plate and the North American plate in the Mid-Atlantic Ridges is an example of a constructive margin.


external image Iceland_Mid-Atlantic_Ridge_Fig16.gif

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East African Rift System: divergent continental margin


The East African Rift System is a 50-60 km wide active volcanic and faulting zone that extends north-south in eastern Africa for more than 3000 km ( app. 1800 miles) from Ethiopia in the north to Zambezi in the south. It is a rare example of an active continental rift zone, where a continental plate is attempting to split into two plates, which are moving away from one another. A new oceanic curst is in the process of formation as molten magma from the mantle rises to fill any possible gap between the two plates. The African Plate is in the process of dividing into two new tectonic plates called the Somali Plate and the Nubian Plate.
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Convergent boundaries or Destructive margin


Destructive plate margins occur where two plates converge due to the existence of the descending limbs of convection cell currents in the asthenosphere beneath the lithosphere. This type of margins are common where oceanic crust meets continental crust. For example, convergence of Nazca plate beneath the South American plate. The more denser crust is subducted down into the asthenosphere beneath the less dense crust.

Types of convergence and it’s effects
1. Oceanic crust meets Continental crust
    • Oceanic crust is denser than continental crust. The continental crust, being more buoyant, is not subducted but is uplifted, buckled and folded and forms a range of fold mountains such as the Andes.
    • The subducted plate is heated and eventually melts under pressure at about 100 km below the surface. This melted material rises up through any lines of weakness and may create volcanic mountains. It may also cool and solidify beneath the surface forming intrusive igneous features called batholiths and other granite structures. High magnitude earthquakes are common.

Example: Nazca Plate moving under the South American Plate
    • The Oceanic Nazca plate is moving towards east at a rate of app.12 cm per year and converges with and then subducts beneath the continental South American plate, which is moving west at an average rate of 1 cm per year. The Peru-Chile trench reaches the depth of 8000 meters at the zone of subduction. Earthquakes are often of high magnitude.
    • The Andes, a chain of fold mountains rising nearly 7000 m above the sea level, has been formed as the continental crust has been buckled and lifted up. Volcanoes such as Cotopaxi, occur along the mountain chain.
2. Oceanic crust to another oceanic crust
    • Where two plates of oceanic crust come together
    • Subduction still occurs, as one plate is likely to be older and denser than the other.
    • Volcanic activity leads to a formation of a chain of volcanic islands, known as island arc such as Mariana and Guam island chains at the convergence of Pacific and Philippines plate.
Example: Pacific plate and Philippines plate





Collision Margin: Continental to Continental crust
  • The convergence of two plates of continental crust is known as collision margins.
  • No subduction occurs, as both plates are buoyant and of low density.
  • Intervening oceanic sediments trapped between the two converging plates are heaved upwards, resulting in the formation of fold mountain ranges.
  • No volcanic activity is found in this type of margin as no crust is being destroyed by subduction.
  • Deep focus earthquakes often occur with limited surface impact.

Example:
Indo-Australian plate collides with the Eurasian plate.
African plate and Eurasian plate (still a tiny bit of ocean is left as the Mediterranean sea)

Case of Himalayas:
  • The Indo-Australian plate is moving northwards at a rate of 5 cm per year and collides with the Eurasian plate, which is moving southeast-wards at a slightly slower rate.
  • Prior to their collision, the two landmasses were separated by the remnants of the Tethys sea. Tethys sea originated 300 million years ago at the time of break up of Pangaea.
  • Himalayas formed as the two plates collided, the continental crust including the shallow sea sediments have been buckled and lifted up.



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Examples of Collision and Convergent boundaries

  • The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.
  • The collision between the Australian Plate and the Pacific Plate that formed the Southern Alps in New Zealand.
  • Subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands.
  • Subduction of the Nazca Plate beneath the South American Plate to form the Andes.
  • Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate, forming the island complex from New Zealand to New Guinea.
  • Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey.
  • Mariana Trench parallel to the Mariana Islands marks the convergent boundary between Pacific plate against the slow moving Philippine Plate.
  • Subduction of the Juan de Fuca Plate beneath the North American Plate forming the cascade volcanic arc which is a part of Pacific ring of fire.

Transform or Conservative Boundaries

  • Conservative margin occurs when two plates laterally glide past each other.
  • Like the collision zone boundary, no volcanic activity is found here. Volcanic activity is normally not present because the typical magma sources of upwelling convection current or a melting subducting plate are not present here.
  • Shallow focus high frequency-low magnitude earthquakes are common along this type of margin. The earthquakes are usually shallow because they occur within and between plates that are not involved in subduction.
  • Major earthquake events may take place after a significant built-up of pressure, typically when high levels of friction restrict movement of the crust along fault lines.
  • Most of the lithospheric plates have a constructive margin at one edge and a destructive margin at other. Conservative margins make up the other two sides.



Activities


Using the smart board label the following
1. a constructive (divergent boundary)
2. a destructive (convergent) boundary
3. a collision boundary
4. a conservative boundary

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Transform Fault


A transform fault is a special type of strike-slip fault commonly found along the conservative plate boundary. A strike slip fault is one in which the adjacent blocks are displaced laterally or horizontally relative to each other or parallel to the line of the fault as a consequences of shear stresses.These faults accommodate the relative horizontal slip between two tectonic plates. Transform faults are also common along the edges of the plates in mid-oceanic ridge regions. The lateral displacement along transform faults often ends or changes abruptly. Transform faults are locations of recurring earthquake activity and faulting.
Example: Best known conservative boundary is between the Pacific plate and the North American plate along the coast of California.
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Hotspots


Hot spots are the places where plumes of magma rise up from the mantle if the crust is thin or weak, even though they are not necessarily near a plate margin. Lava may build up over time above the present day sea level, giving rise to a volcanic island.
  • The Hawaiian Islands are a chain of volcanic islands lying over a stable hot spot. The pacific plate has been moving over the hot spot for about 70 million years and a succession of volcanic islands and underwater volcanoes has formed over time.
  • As the plate moved so the volcanoes have been carried away from the hot spot in a northwesterly direction, forming a chain of extinct under-water volcanoes called seamount. Kauai is the oldest of the main Hawaiian Islands now, having formed some 5 million years ago, with its volcano considered to be extinct and in the process of erosion.
  • These extinct volcanoes are often eroded into flat-topped remnants called guyots, extending all the way to Aleutian Islands.
  • At present the seafloor of nearly 20 miles to the southeast of Hawaii is an active volcanic area with periodic eruptions. This area is called Loihi and will be the site of the next Hawaiian Island in 10000 years.
  • Most hotspots, also known as "mantle plumes," occur beneath oceanic plates. Yellowstone in U.S., however, is a good example of a hotspot beneath a continental part of a plate. Yellowstone caldera is well known for its bubbling fumaroles (vents from which volcanic gas escapes into the atmosphere) and hot water geysers like "Old Faithful.

external image 1280px-Hawaii_hotspot_cross-sectional_diagram.jpg


Mantle convection and hotspots volcanism


Heat constantly flows out of the Earth’s core at the base of the mantle, causing the semi-molten mantle to circulate slowly and carrying the heat outwards by convection. Convection plays a major role in driving the movement of the tectonic plates at the top of the mantle. The pattern of convection is not fully understood. Geophysicists have recently argued that the mantle convection occurs in two layers involving a deep layer circulation, which is largely separate from the rest of the upper-mantle convection. According to this model, streams of hot deep layer material from the core-mantle boundary flows upward as mantle plumes, creating hotspots. On the other hand, shallow upper mantle convection is responsible for mid-oceanic volcanism.


Planet Earth 100 Million Years In The Future - What will happen to our world?





Earthquakes


Definition:
  • Earthquakes are sudden, violent shaking or vibration of the earth crust produced by the shock waves resulting mainly from a sudden displacement along a fault.




Why does it happen?
Earthquakes are associated with all types of plate boundaries. Earthquakes occur when tension is released from inside the Earth. Plates do not always move smoothly alongside each other and may get stuck. When this happens pressure starts to build up. When the accumulated pressure (energy) is eventually or abruptly released, earthquake tends to occur. While the fault rupture can be visible at the surface, the actual displacement may occur at a considerable depth as deep as 500 km beneath the surface in case of a subduction zone. Earthquake may also develop from the movement of magma or due to sudden ground subsidence.
Quasi-natural earthquakes: Man made causes like nuclear testing, building of large dam or reservoir, oil drilling, coal mining may also cause earthquakes.



Related Terminologies
Focus or hypocenter: The place beneath the ground where the earthquake originates is called the focus. It is also the center of the fault motion where energy is released; originating different kinds of seismic waves.

Epicenter: The point on the Earth's surface directly above the focus is called the epicenter. The strongest shocks and crustal vibration are often felt on at the epicenter.

Aftershock: These are the series of small earthquake that follow a major earthquake near the original earth movement. If the initial earthquake is strong, then the aftershocks can also be very strong. Aftershocks represent the redistribution of stress on the fault zone.

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Earthquake Watch: How many earthquakes happened today?....see here



Seismic waves: The energy released in an earthquake generates different kinds of seismic waves. These waves travels outward in widening circles, like the ripples produced when a stone is thrown into a pond, marked by diminishing amplitude with increasing distance from the focus.



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Earthquake magnitude: magnitude depicts the strength of an earthquake. It measures the relative amount of energy released during an earthquake. Magnitudes are calculated on a logarithmic scale means there is a 10-fold increase every time the scale increases by 1. A magnitude 5 earthquake releases 1000 times more energy than a magnitude 3 earthquake. The most commonly used magnitude scale is Richter scale, devised in 1935 by Charles Richter. The scale is easy to use but not appropriate for comparing very large earthquakes with magnitude 7 and higher. Moment magnitude scale is now widely used to describe the size of large earthquakes. It combines several parameters like movement on the fault, rock strength, size of the rapture etc. Because of different magnitude scale in use, slightly different magnitude numbers are often reported for the same earthquake. Many Earth tremors are of such low magnitude that they are often not noticed by us.

Earthquake Intensity: means the degree of surface shaking. Every earthquake generates a wide range of local ground shaking intensities causing earthquake damages. The shaking intensities of an earthquake is qualitatively measured by Mercalli scale, originally devised by Italian geologist Giuseppe Mercalli in 1902 and was later modified in 1956. The scale varies from 1 to 12 categories based on observed effects and damages.

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Earthquake magnitude calculator here



Haiti Earthquake: January 12, 2010

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Nepal Earthquake


Pictures:

Earthquake prone regions:

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Interactive map of the Major Earthquakes on the World here






Haiti Earthquake 2010
Mega-quake documentary
Nepal Earthquak






Spatial Distribution of Earthquakes


Earthquakes can be classified according to the depth of their focus. Three broad categories are recognized:
  • Deep focus: 300– 700 km deep
  • Intermediate focus: 70– 300 km deep
  • Shallow focus: 0–70 km deep

Shallow focus earthquake causes the greatest degree of damage and account for approximately 75% of all earthquake types.
Deep focus earthquakes are generally associated with plate margins (Benioff zone) where the oceanic plate is forced under the continental plates in the process of subduction. 20 percent of the subduction zones dominate the circum-Pacific ocean basin. Shallow focus earthquakes are generally located along constructive and conservative boundaries.


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Tsunami: How does it generate?
How do Tsunamis relate to earthquake?






Volcano


A volcano is an opening or fissure on the earth crust through which hot molten magma (lava), gases, molten rock and ashes are ejected onto the surface. It also depicts the typical landform that develops around the points of weakness in the earth‘s crust through which solid, liquid and gaseous materials are forced into the earth's crust.

Why does eruption happen?
When immense pressure builds up beneath the crust, primarily associated with plate movements.
Hot Spot volcanism though may not be near to the plate boundary. These are associated to the streams of hot deep layer material from the core-mantle boundary that flows upward as mantle plumes, creating hotspots.

Click here to see National Geographic photo collection on Volcanoes

Spatial distribution of volcanoes
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Pacific Ring of Fire

What comes out of a volcano?
Gases
Steam of water, carbon dioxide and sulfur dioxides, carbon monoxide, hydrogen sulphide etc
Liquid
Molten lava - may be either fluid or viscous (sticky). Acid lavas (on the continents at the convergent or destructive margin, called Rhyolite and Andesite lava) are viscous even at high temperatures; trapped gasses inside the bubble often explode violently. Basic lava or basaltic lava (on the divergent margins of the ocean floor) are fluid. Volcanic muds are called lahars.
Solid
Solid materials of
varying grain size (volcanic bomb to ash) ejected into the atmosphere are called Tephra or pyroclastic materials.
Pyroclastic material are mostly solid volcanic material consist of super hot ashes more than 700oC, lapilli or cinder (walnut-sized pieces of volcanic rock), Lava bombs (volcanic rocks larger than 64 mm in size) and pumice which may still have evidence of the bubbles of gas trapped as the rock solidified.
Nuées ardentes or pyroclastic flows are fast moving (500 km per hour) hot ashes and other material thrown out from an erupting volcano. Often incandescent (light emitting)





Classification of volcanoes is done mainly on the basis of the shape of the volcano and its vent and the nature of eruption.
Major types of volcanoes
Features
Fissure eruptions (basaltic)
Volcanic eruption can take place through long cracks called fissure. When two plates move apart basaltic lava may flow considerable distance over gentle slopes. e.g. Laki eruption in Iceland.
Composite or stratovolcano (andesitic)
Steeper slopes volcanoes with high viscosity lava and are more explosive than shield volcano. They are marked by well developed central vent and alternating layers of lava and pyroclastic material e.g. Mt. Etna, Mt. Fuji in Japan, Mt. Vesuvius in Italy.
Shield volcano (basaltic)
Mainly formed in the ocean often at the constructive plate margins. Thin lava flow built up over a central vent creating gentle upper slopes and usually have a circular or oval shaped map view e.g. Mauna Loa in Hawaii
Dome volcano (acid lava/rhyolitic)
Explosive with highly viscous acid lava e.g. Mt. Pelee
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Lava flows from Hawaii Kilauea volcano
Under water volcanoes

Crater vs Caldera
Crater
Caldera
Circular depression usually less than 1 km in diameter. Composite and shield volcanoes have crater at the summit.
Is a huge crater caused mainly due to collapse of a part of the volcanic cone into it’s party empty magma chamber after a huge eruption. Subsidence and widening of the crater enlarge the opening to several kilometers. E.g. Yellowstone caldera in the Yellowstone National Park in the USA is considered an active volcano. It has erupted with tremendous force several times in the last two million years. The caldera lie over a hot spot which produced the latest three super-eruptions from the Yellowstone hotspot. According to the analysis of earthquake data in 2013, the magma chamber is 80 km long and 20 km wide and may create another super-eruption in future.
Supervolcano
A super volcano is capable of producing exceptionally large volcanic eruptions of ejected mass greater than 1 quadrillion kilogram (means 10 to the power 15 = 1,000,000,000,000,000).

Yellowstone National Park and Old faithful geyser in the U.S. state of Wyoming


Mid-ocean volcanoes



Comparative features of the major types of volcanoes
Types of Volcano
Shield Volcano with basaltic magma
Composite or stratovolcano with andesitic magma
Lava dome with rhyolitic magma
Location
Constructive margin at the ocean ridges
Hot spots
Destructive margin with an oceanic plate
On the continental crust
Structure
Broad, gently sloping
Giant steep sided, symmetrical cone
Steep sided cone, irregular shape
Type of eruption
Regular and mostly non explosive, very little pyroclastic material erupted
Explosive with dormant phases,
Mainly rhyolitic
Lava type
Usually Basaltic, quiet eruption of fluid lava
Mostly Andesitic (acidic), explosive eruption with viscous lava and pyroclastic flow
High silica content, very viscous lava, often explosive, quickly solidifies on exposure to air
Example
Mauna Loa in
Hawaiian Islands
Mt. Fuji in Japan, Mt. Vesuvius in Italy, Mt. St Helen in USA
Pacific ring of fire also called Andesite line (75% of world volcano)
Lassen peak, California
Mt.Pele in French Caribbean Island of Martinique.
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Hazards associated with tectonic events
Earthquake and related hazards- ground displacement, landslides, liquefaction, tsunami, destruction and loss of lives and property.
Volcano and related hazards- explosive blast, lava flows, ash flows and ash falls, mud flows, lahar, hot avalanches or Nuées ardentes, ground displacement, landslides, climate change, destruction, loss of lives and property etc.

Why do people live near volcanoes?

More than 500 million people (approximately 7% of the world population ) live close to volcanoes and in areas at risk from volcanic hazards. Major cities like Mexico City in Mexico is only 50 miles away from the active volcano of Popocatapetl.
Fertile soil
Volcanic rocks are rich in minerals, but when the rocks are fresh the minerals are not available to plants. The rocks need thousands of years to become weathered and broken down before they form rich soils. The slopes of Vesuvius in Italy have very productive soils. The area is intensively cultivated and produces grapes, vegetables, herbs, flowers and has become a major tomato growing region.
Mineral Mining
Most of the metallic minerals mined around the world, particularly copper, gold, silver, lead and zinc are associated with rocks found deep below extinct volcanoes. Most of sulfur mine is located around active volcanoes.
Tourism
Volcanoes and related features like hot springs, bubbling mud pools and steam vents and geysers are always popular tourist attractions, such as Old Faithful in the Yellowstone National Park, US attract millions of visitors every year. Tourism creates jobs and supports local economy.
Geothermal energy
It is derived from the Earth's internal heat. Tapping of naturally occurring hydrothermal convection to generate electricity is very common in Iceland and in New Zealand. It is commonly used for heating and cooking.
Others
Overcrowding and shortage of land, Poverty, inertia to relocate and boding to the soil etc.

What can be done to minimize the risk from volcanoes?
Lava flow diversion
Canal or lava channel is dug to divert lava flow.
Cooling of lava to stop flowing
Salty water is sprayed to cool down and solidify lava
Mudflow barrier
Walls built across valleys to trap mudflows to protect settlements and property.
Volcano monitoring
Research aircraft, satellites and remote sensing, observation borehole measures composition of escaping gases, changes in temperatures and swelling of ground. Magnetometer measures changes in local magnetic fields.
Hazard mapping and planning
Past pattern of eruption is projected to predict the future prediction. High risk areas are identified to increase community preparedness and mitigation responses. Mitigation is the effort to reduce loss of life and property by lessening the impact of disaster.
Dealing with volcanic eruption in Hawaii


Case Study: The Eruption of Eyjafjallajoekull in Iceland in 2010




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Geology of the New York City....New York Plate tectonics