Earthquakes and Volcanoes

How do earthquakes occur

1. Rock masses on either side of a fault are pushed by tectonic forces

2. Friction causes them to get locked and stress builds up

3. When the stress exceeds the strength of the fault (or rock), the rock snaps or suddenly moves to a new position

4. This sudden movement causes seismic waves to be released resulting in ground shaking

   • The point in the Earth’s crust where seismic waves are released is known as focus

   • The point on the Earth’s surface directly above the focus is called the epicenter

How are earthquakes measured

They are measured using seismometers
Which are sensitive instruments that detect ground vibrations and determine the magnitude of an earthquake

Richter scale (ML)

• The Richter scale calculates earthquake magnitude using the height of the largest wave recorded on seismometers

• Thus earthquake magnitude is measured based on the maximum seismic intensity reached rather than the total seismic energy released throughout the earthquake

• The scale is numbered from 1 to 10, with 10 being the greatest magnitude

• Scale is logarithmic - an earthquake of magnitude 6 releases about 32 times more than energy than a magnitude 5 earthquake

• Limitations

  • This underestimates longer earthquakes which release more overall energy by rating them as having lower magnitudes even though they are likely to do more damage

  • Hence they are no longer commonly used

Moment Magnitude Scale (Mw)

• The Mw scale rates earthquake magnitude based on the total energy released during the earthquake

• It estimates the total energy released during an earthquake instead of just the largest wave

• Hence it is generally more accurate especially in measuring earthquakes of magnitude 8 and above

• Scientists have adjusted the magnitude of past earthquakes which were initially measured using the Richter scale

• Example

  • Prince William Sound Earthquake in Alaska USA, in 1964 was initially rated 8.6 on the Richter Scale, but was later adjusted to 9.2 based on the Mw scale

• The Mw scale is logarithmic - an earthquake of magnitude released about 32 times more energy than a magnitude of 5 earthquake

How do divergent plate boundaries result in Volcanic eruptions

1. Plates move apart, the crust stretches and fractures develop

2. The decrease in overlying pressure causes part of the underlying mantle to melt, forming magma

3. Magma contains dissolved gases and is less dense than the surrounding materials

4. Therefore , magma rises through weak areas in the crust to the Earth’s surface to erupt as lava, causing a volcanic eruption

5. The lava cools , solidifies and accumulates over time , forming a volcano

How do convergent plate boundaries result in volcanic eruptions

1. Plates move towards each other and the denser plate subducts under the other

2. As the subducting plate sinks into the mantle , the high pressure forces water out of the oceanic crust. Water lowers the melting point of the overlying mantle , causing it to melt , forming magma

3. Magma contains dissolved gases and is less dense the surrounding materials

4. Therefore , magma rises through weak areas in the crust to the Earth’s surface to erupt as lava, causing a volcanic eruption

5. The lava cools , solidifies and accumulates over time forming a volcano

High Silica magma / Low Silica magma

1. More viscous magma / Less viscous magma

2. As magma rises towards the Earth’s surface, the dissolved gases in the magma cannot escape easily / can escape easily

3. More pressure builds up until gases escape explosively / less pressure builds up

4. Resulting in violent , explosive eruptions / gentle effusive eruptions

5. Generally associated with stratovolcanoes / shield volcanoes

6. However, there are volcanoes with viscous magma that do not result in explosive eruption as the magma rises in a way that allows gases to escape

   • Example : Mount Merapi is a stratovolcano , its eruption in 2006 was not explosive as the viscous magma rose in a way that allowed dissolved gases to escape easily

When do stratovolcanoes form

1. When High viscosity magma rise through weak areas in the crust to the earths surface and erupts explosively as lava , ash and rocks

2. The ash and the rocks settle on the sides of the volcano and are later covered by lava

3. Over successive eruptions , a tall volcano consisting of alternating layers of ash and lava develops

4. As the highly viscous lava travels a shorter distance before cooling and solidifying the volcano has steep sides and a narrow submit

   • Example : Mount Mayon in the Philipines

When do shield volcanoes form

1. Low viscosity magma rises through weak areas in the crust to the Earth’s surface and erupts effusively

2. Over successive eruptions, a volcano consisting of layers of lava develops

3. As the less viscous lava travels a longer distance before cooling and solidifying , the volcano has gently sloping sides with a broad summit

   • Example : Kilauea in Hawaii , USA

How are volcanic eruptions measured?

• The Volcanic Explosivity Index (VEI) measures the magnitudes of different volcanic eruptions based on explosivity using the following criteria

  • Volume of ejected material

  • The heigh of the eruption cloud

  • Duration of the eruption

• Measured on a scale of 0 to 8

  • Effusive eruptions are generally given a value of 0 or 1 as they are non-explosive with less than 1,000km³ of material ejected

    • Example : the ongoing effusive eruption of Kilauea , Hawaii , which is a non-explosive

  • A value of 8 represent a mega-colossal explosive eruption cloud column height of over 25km

    • Example : Tova Volcano in Northern Sumatra , Indonesia , 74000 years ago

      • The volcano erupted approximately 2800 km³ of material, which covered India and parts of Southeast Asian in over 15cm of ash

• VEI scale is logarithmic - an increase of “1” on the VEI scale indicates an eruption 10 times more powerful than the number before it

How are earthquakes distributed?

• Earthquakes occur along all types of plate boundaries

• The largest concentration of earthquakes is at the Pacific Ring of Fire

• At plate boundaries, plates are pushed back by tectonic forces, stress builds up and energy is eventually released

• Examples

  • Broad belt of earthquakes along convergent plate boundaries in the the Pacific Ring of Fire

  • Narrow belt of earthquakes along the divergent plate boundary at the Mid-Atlantic Ridge

  • Narrow belt of earthquakes along the transform plate boundary of the San Andreas Fault

• Earthquakes occur more commonly along convergent plate boundaries at subduction zones as more stress is built up during subduction

• Exception : some earthquakes may occur away from plate boundaries such as within the Eurasian plate

What is the pacific Ring of fire

• Broad belt around the pacific ocean where most earthquakes and active volcanoes occur

• Comprise all three types of plate boundaries

• Examples

  • Convergent : Pacific plate moving towards the Philippine plate

  • Divergent : Pacific plate moving away from the Cocos plate

  • Transform : Pacific plate sliding past the North American plate at the San Andreas Fault

How are volcanoes distributed

• Volcanoes are generally located near convergent and divergent plate boundaries

• At these plate boundaries, magma rises up to the surface , forming volcanoes

• Belts of volcanoes may be observed along:

  • Convergent plate boundaries: A belt of volcanoes along subduction zones in the Pacific Ring of Fire

  • Divergent plate boundaries : A belt of volcanoes along divergent plate boundaries between the North American and Eurasian plates

• Hot spot volcanoes are exceptions that can be found away from plate boundaries

• Examples :

  • The piton de la Fournaise is a hot spot volcano located on the island of Reunion of the Indian Ocean, away from plate boundaries

How does ground shaking affect the natural and human systems

• Ruptures oil and chemical factories polluting land and water

• Fractures and uproot trees, causing widespread tree injury and death and damaging wildlife habitats

• Weakens building bridges roads and railways causing them to collapse make it difficult to rescue people or supply emergency did

• Snaps water and gas pipes resulting in water and disruptions to gas supply

• Electricity and communication cables break affecting important communication services such as tsunami warmings and television broadcasts

• People may get trapped under collapsed and infrastructure , resulting in loss of lives and injuries

• Example : 2010 Haiti earthquake (Mw 7.0)

  • Surrounding areas around factories were polluted

  • Debris polluted rivers

  • More than 250000 houses collapsed due to violent virations

  • About 220000 people were killed

  • water pipes ruptured causing water shortages

• 2010 Christchuch New Zealand earthquakes:

  • Many trees were damaged and more than 300 had to be removed reducing availability of habitat for terrestrial species