Ch 1.4 How do plate movements create tectonic hazards?

Question 1

What is the definition of earthquake?

Answer 1

Earthquake is the sudden shaking of the land.

Question 2

What is the occurrence of earthquakes?

Answer 2

When stress is applied to rocks, energy build up in the rock layer and a fracture is formed. When the energy exceeds the strength of rock, the rock breaks and displaces along the fracture. The sudden release of energy stored in the rocks beneath the earths surface caused earthquakes.

Question 3

Describe the distribution of earthquakes around the world.

Answer 3

1. It is found at all types of plate boundaries, but strong earthquakes occur more frequently at convergent and transform plate boundary.
2. Most common in the Circum-Pacific Belt (75%) and the Alpine-Himalayan Belt.
3. Some are found in the interior of a plate, especially in the location where fault planes are found, such as African Plate in Central Asia.
4. Some are found in hot spots such as the Hawaiian Islands.

Question 4

What is the depth of earthquake foci at different types of plate boundaries?

Answer 4

Convergent: Shallow to deep
Divergent: Shallow
Transform: Shallow

Question 5

Explain the depth of focus.

Answer 5

Subduction of plates causes friction at a great depth, therefore generating intermediate to deep focus earthquakes. Non-subduction plate movements cause shallow focus earthquakes. In general, deep earthquakes are less damaging than shallow ones. This is because seismic waves from deep earthquakes lose energy as they travel a long distance from focus to the surface.

Question 6

How can the severity of earthquakes be expressed?

Answer 6

1. Depth of earthquakes.
2. Intensity.
3. Magnitude.

Question 7

How can intensity be measured?

Answer 7

Modified Mercalli Intensity scale. The scale ranges from 1 (not felt at all) to 9 (total destruction). It is based on the damage caused by the earthquake.

Question 8

How can magnitude be measured?

Answer 8

It is measured in Richter Scale, which is recorded by a seismograph. Magnitude is related to the amount of energy released at the focus.

Question 9

Except for the plate boundaries causing earthquake, what can cause earthquakes?

Answer 9

Human activities may also trigger some minor earthquake. For example, water in large reservoirs put pressure on rocks. Besides, nuclear tests and mining may also cause earthquakes.

Question 10

What are the types of volcanic materials?

Answer 10

1. Volcanic Gas: Water vapour, carbon dioxide, sulphur dioxide.
2. Lava: basaltic lava, acidic lava.
3. Pyroclastic Materials: Volcanic ash, blocks and bombs.

Question 11

What are lava structures?

Answer 11

Columnar Joints. When lava flows cool and solidify, they shrink. Long vertical cracks or joints within the brittle rock allow for the shrinkage. As temperature dropped, the fractures extended downwards forming the vertical rock column.

Question 12

Describe the distribution of active volcanoes around the world.

Answer 12

Active volcanoes are found at convergent and divergent plate boundaries. They form a rough circle around the Pacific Ocean and is known as the Pacific Ring of Fire. Some volcanic eruptions have occurred in scattered places away from plate boundaries / within the plate like Hawaiian Volcano, Yellowstone volcano

Question 13

How does volcano occur in convergent plate boundaries?

Answer 13

Water released part of the asthenosphere into magma. When cracks develop and extend downwards to the asthenosphere due to folding and subduction, magma will rise through these cracks and result in volcanic eruption.

Question 14

How does volcano occur in divergent boundaries?

Answer 14

Tensional force along the plate boundaries form fissures. Pressure in the asthenosphere reduces. Rocks melt to form magma and magma rises through the fissures resulting in volcanic eruptions.

Question 15

How are volcanoes, submarine volcanoes and volcanic islands formed?

Answer 15

A mantle plume rises from between the core and the mantle. It is called a hot spot. When the plume head reaches the lithosphere, the heat melts the rock into magma. Magma pushes through cracks and erupts at the earth’s surface, forming volcanoes, submarine volcanoes and volcanic islands.

Question 16

How does the Hawaiian Islands move?

Answer 16

Alignment of the Hawaiian Islands is controlled by the direction of plate movement. As the Pacific Plate move north-westwards, old volcanic islands or submarine volcanoes are carried north-westwards. The supply of magma is cut off. Vulcanicity ceases and the volcanoes become extinct. These islands and volcanoes are subject to erosion and become lower in height. Further erosion may cause volcanic islands to sink below sea level. They are called sea mounts. Above the hot spot, new volcanoes are then created. The process is repeated and this results in a line of volcanic islands.

Question 17

What are tsunamis?

Answer 17

Tsunamis are a series of big sea waves that can travel 700km per hour or more.

Question 18

When do tsunami generate in ?

Answer 18

Once generated, they travel in all directions. Tsunami is generated when there is a displacement of a large body of water caused by submarine earthquakes, submarine volcanic eruptions and submarine landslides.

Question 19

What are the causes of tsunami?

Answer 19

1. Submarine earthquakes cause vertical displacement of rocks on the seabed. The seabed is lifted up.
2. Very violent eruptions of submarine volcanoes displace water.
3. Large scale submarine landslides cause large scale collapse of solid coastal materials under water that displace water.
   This displaces a large body of water. Water forms a bulge on the sea surface. The bulge collapses due to gravity. This creates a succession of waves or tsunamis.