Volcanoes by benbenzhou


									          THE STRUCTURE OF THE EARTH
•   The earth is made up of 4 distinct layers:

•   The inner core is in the centre of the earth and is the hottest part of the earth.
    The inner core is solid. It is made up of iron and nickel with temperatures of
    up to 5500°C. With its immense heat energy, the inner core is like the engine
    room of the Earth.
    The outer core is the layer surrounding the inner core. It is a liquid layer, also
    made up of iron and nickel. It is still extremely hot here, with temperatures
    similar to the inner core.
    The mantle is the widest section of the earth. It has a diameter of
    approximately 2900km. The mantle is made up of semi-molten rock called
    magma. In the upper parts of the mantle the rock is hard, but lower down,
    nearer the inner core, the rock is soft and beginning to melt.
    The crust is the outer layer of the earth. It is a thin layer between 0-60km
    thick. The crust is the solid rock layer upon which we live.
    There are two different types of crust: continental crust, which carries land,
    and oceanic crust, which carries water.
    The diagram below shows the structure of the earth. In geography, taking a
    slice through a structure to see inside is called a cross section.
• Plates and plate boundaries
• The earth's crust is broken up into pieces. These
  pieces are called plates. Heat rising and falling
  inside the mantle creates convection currents.
  The convection currents move the plates. The
  movement of the plates, and the activity inside the
  earth, is called plate tectonics.
• Plate tectonics cause earthquakes and volcanoes.
  The point where two plates meet is called a plate
  boundary. Earthquakes and volcanoes are most
  likely to occur either on or near plate boundaries.
Subduction zone
• The San Andreas fault in Western
Convection currents
    Conservative plate margins

• Conservative margins occur where two plates slide
  horizontally past each other. There is no subduction zone
  with one plate being destroyed beneath another nor is there
  a constructive zone as at the Mid-Atlantic ridge. As the
  two plates slide past each other, the high levels of friction
  create large strains along the slippage zone faults.
  Eventually the strain energy is released in an earthquake.
  When this happens there is likely to be large scale
  movement along the transform fault with substantial
  damage to nearby buildings.
• The best known example of a conservative plate margin
  can be found along the west coast of the USA where the
  Nazca plate is moving in a North Westerly direction and
  sliding past the North American Plate. The North
  American plate is also moving in a North-Westerly
  direction but not so fast as the Nazca plate so the two
  appear to be moving in opposite directions. The fracture
  zone between the two plates is the San Andreas Fault - a
  huge fault running for hundreds of km along the
  Californian coastal region from San Francisco to Los
  Angeles. The region has a number of other substantial
  faults running parallel to the major San Andreas fault - the
  Hayward Fault runs almost parallel and to the east of the
  San Andreas fault with the modern city built across the two
  active fault planes. Earth tremors and earthquakes
  regularly occur along these fault planes - in 1906 the city
  of San Francisco was destroyed in a magnitude 8.2 quake
  on the San Andreas fault; in 1989 substantial damage and
  some 69 fatalities resulted from the magnitude 7.1 Loma
  Prieta earthquake.
The 1989 Loma Prieta

The 1906 San
• Processes which lead to the eruption of lava,
  gases, and pyroclastic materials onto the surface
  and into the atmosphere
• Active volcanoes An active volcano has erupted recently and is likely to erupt again.
• Dormant volcanoes A dormant volcano has not erupted in 2000 years.
• Extinct volcanoes An extinct volcano will never erupt again.
            What are Volcanoes?
• Conical mountains formed
  around a vent where lava,
  gases, and pyroclastic
  materials are erupted
  – Variations in the type of lava
    and other factors distinguish
    three types
  – Most have a central crater,
    while calderas and fissures
    are also common
Cone volcanoes:
•These are usually found at destructive boundaries.
•Cone volcanoes are tall and steep-sided.
•Cone volcanoes are formed by eruptions of thick, viscous (sticky) lava.
•The thick lava moves relatively slowly and hardens quickly to form new
rock - this explains the formation of a cone shape.
•Eruptions tend to be violent.
Shield volcanoes:
•These are usually found at constructive boundaries.
•They are low, with gently sloping sides.
•Shield volcanoes are formed by eruptions of thin, runny lava.
•Eruptions here tend to be frequent but relatively gentle.
Composite volcanoes:
•These volcanoes are composed (made up) of alternating layers of lava and
ash (other volcanoes just consist of lava).
•The eruptions from these volcanoes may be a pyroclastic flow rather than a
lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
•A pyroclastic flow can roll down the sides of a volcano at very high speeds
and with temperatures of over 400° C.
What is this guy doing?
Why is he dressed like this?
              Crater Lake

Cinder Cone
Lava Types
                             Debris Avalanche, Mt. Adams

Pyroclastic Flow, Mt. St. Helens
Mt St Helens Eruption
Volcanic eruptions can have a devastating
effect on people and the environment.
However, unlike earthquakes, volcanoes can
also have a positive impact on an area. These
positive impacts can help to explain why people
choose to live near volcanoes.
• Montserrat is a small island in the Caribbean. There is a volcanic area
  located in the south of the island, called Soufriere Hills.
• The volcanic peak in this area is called Chances Peak, which had been
  dormant for over 300 years. Then in 1995, the volcano began to give
  off warning signs of an eruption (small earthquakes and eruptions of
  dust and ash). Once Chances Peak had woken up it then remained
  active for a period of 5 years. The most intense eruptions occurred in
• During this time, Montserrat was devastated by pyroclastic flows. The
  small population of the island (11,000 people) was evacuated in 1995
  to neighbouring islands. The evacuees became refugees.
• Despite the evacuations, 19 people were killed by the eruptions. This is
  because a small group of people chose to stay behind on the island and
  watch over their crops.
• Volcanic eruptions and lahars have destroyed large areas of
  Montserrat. The capital, Plymouth, has been covered in layers of ash
  and mud. Homes and buildings have been destroyed.

    (Lahars are similar to pyroclastic flows but contain more water )
The graphic shows the progress of the eruption and its impact on
the island.

  Volcanic activity has calmed down in recent years and people
  have begun to return to the island.

                                                            The island of
                                                            Montserrat in the

How deep is the ash in this photo?
What effect might it have had on the people living here?
Few people died in this eruption but it is still a disaster. Why?
   Why do people live close to volcanoes?
People live close to volcanoes because Geothermal energy can be harnessed by using the steam from
underground which has been heated by the Earth's magma. This steam is used to drive turbines in
geothermal power stations to produce electricity for domestic and industrial use. Countries such as Iceland
and New Zealand use this method of generating electricity.

Volcanoes attract millions of visitors around the world every year. Apart from the volcano itself, hot
springs and geysers can also bring in the tourists. This creates many jobs for people in the tourism
industry. This includes work in hotels, restaurants and gift shops. Often locals are also employed as tour

Lava from deep within the earth contains minerals which can be mined once the lava has cooled. These
include gold, silver, diamonds, copper and zinc, depending on their mineral composition. Often, mining
develop around volcanoes.

Volcanic areas often contain some of the most mineral rich soils in the world. This is ideal for farming.
Lava and material from pyroclastic flows are weathered to form nutrient rich soil which can be cultivated
to produce healthy crops and rich harvests.

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