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Hazardous environments: where and why
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Earthquakes, volcanoes and tropical cyclones are not spread evenly. Their patterns can be explained.
Distribution and measurement
Earthquakes and volcanoes occur mostly along plate boundaries, especially the Pacific Ring of Fire.
Tropical cyclones form over warm tropical oceans.
Measuring: earthquakes with the moment magnitude scale (which replaced the Richter scale) and the Mercalli scale of observed effects; volcanoes with the Volcanic Explosivity Index (VEI); cyclones with the Saffir-Simpson scale.
Tropical cyclones form over warm tropical oceans.
Measuring: earthquakes with the moment magnitude scale (which replaced the Richter scale) and the Mercalli scale of observed effects; volcanoes with the Volcanic Explosivity Index (VEI); cyclones with the Saffir-Simpson scale.
Plate boundaries and hotspots
Destructive (convergent): an oceanic plate subducts beneath a continental plate, causing explosive volcanoes and strong earthquakes, as in the Andes.
Constructive (divergent): plates move apart and magma rises, giving gentler eruptions, as at the Mid-Atlantic Ridge and Iceland.
Conservative (transform): plates slide past each other, causing earthquakes but no volcanoes, as on the San Andreas Fault.
Collision: two continental plates push together forming fold mountains, such as the Himalayas, with earthquakes.
Hotspots: plumes of magma rise in the middle of a plate, as in Hawaii.
Constructive (divergent): plates move apart and magma rises, giving gentler eruptions, as at the Mid-Atlantic Ridge and Iceland.
Conservative (transform): plates slide past each other, causing earthquakes but no volcanoes, as on the San Andreas Fault.
Collision: two continental plates push together forming fold mountains, such as the Himalayas, with earthquakes.
Hotspots: plumes of magma rise in the middle of a plate, as in Hawaii.
What tropical cyclones need
Ocean temperature of about 26.5 °C or more.
Low atmospheric pressure: warm, moist air rises rapidly.
Low wind shear: winds at different heights must be similar, or the storm is torn apart.
Coriolis force from the Earth's spin makes the storm rotate. It is too weak near the equator, so cyclones rarely form within about 5° of it.
Low atmospheric pressure: warm, moist air rises rapidly.
Low wind shear: winds at different heights must be similar, or the storm is torn apart.
Coriolis force from the Earth's spin makes the storm rotate. It is too weak near the equator, so cyclones rarely form within about 5° of it.
Explain why there are volcanoes along the west coast of South America.
- The oceanic Nazca plate is moving towards the continental South American plate.
- The denser oceanic plate subducts and melts.
- Magma rises through the continental crust and erupts, forming the volcanoes of the Andes.
Answer: Subduction at a destructive plate boundary.
Earthquakes and volcanoes cluster on plate boundaries (destructive, constructive, conservative, collision) and at hotspots. Measure with moment magnitude, Mercalli, VEI and Saffir-Simpson. Cyclones need warm oceans (26.5 °C+), low pressure, low wind shear and the Coriolis force.
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