1.2.3 Volcanoes and Earthquakes

Volcanoes and Earthquakes

Where do Volcanoes Occur?

• Sixty percent of all active volcanoes occur at the boundaries between tectonic plates.
• Most volcanoes are found along a belt, called the "Ring of Fire" that encircles the Pacific Ocean.
• Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called "hot spots."

Where do Earthquakes Occur?

• Earthquakes are caused by the tension that builds up at all 3 plate margins:
  • Destructive Plate Margin: Earthquakes occur when the subducted oceanic plate scrapes against the continental as it slides underneath.
  • Constructive Plate Margins: Tension builds along cracks within the plates.
  • Conservative: Tension builds when plates sliding past each other get stuck.

World's Most Earthquake-Prone Areas

Earthquakes Occur at all 4 Plate Margins

The Moment Magnitude Scale

• Measures the amount of energy released by an earthquake (called magnitude) [we used to call this the Richter Scale].
• The Moment Magnitude Scale is logarithmic - so a magnitude 7 earthquake (like Nepal) is 10 times more powerful than a 6 (like New Zealand).
• The plates eventually jerk past each other causing shockwaves.
• The shockwaves spread out from the focus (the point in the earth where the earthquake starts).
• If the focus is shallow then the waves are stronger and cause more damage.
• The epicentre is the point on the earth's surface directly above the focus.

Earthquakes and Volcanoes have Many Serious Effects

Primary Effects:

• Immediate effects of a disaster (i.e., impacts of the ground shaking in an earthquake). Secondary Effects:

• Happen later on, often as a result of the primary effect.