HKDSE Geography/M1/Formation of Tectonic Hazards

Earthquake Formation

At Plate Boundaries

1. At destructive/constructive/conservative plate boundaries, plates move towards/move away from/slide past each other because of compressional/tensional/lateral force.
2. The plates are held back by friction.
3. Stress builds up in the plates.
4. As the stress accumulates, it will eventually exceed the friction created.
5. The land shakes and seismic waves are erupted from the focus of the earthquake.

Note that:

• The point on the earth's surface directly above the focus is called the epicentre, and receives the most damage
• When plates collide with or slide past each other, more friction is created, so destructive and conservative plate boundaries experience more frequent earthquakes.

Intraplate Earthquakes

1. New magma convection currents may appear in the asthenosphere.
2. This causes the plate to break up and create a new boundary.
3. During this process, earthquakes may be triggered by fault rupture or displacement in fault zones.

The East African Rift Valley is sinking below the Nubian and Somalian portions of the African Plate, which are splitting along fault lines. Friction accumulates at the fault plane and causes earthquakes.

Other Causes

Other causes of earthquakes include (CE08Q16):

• Volcanic eruptions release enough energy to shake the ground.
• Underground nuclear tests release enough energy to shake the ground (e.g. North Korea).
• The construction of large-scale reservoirs will change the amount of pressure on the ground, leading to earthquakes.

Formation of Volcanic Eruptions

Along Destructive Plate Boundaries

1. Converging magma currents in the magma build up pressure at destructive plate boundaries by compressional force.
2. At oceanic-continental or oceanic-oceanic boundaries, the melting of the subducted plate below the subduction zone intensifies the pressure.
3. Cracks develop, possibly because of earthquakes, and extend downward to the magma chamber.
4. The magma pressure is released.
5. The magma rises through cracks to the earth's surface.

Along Constructive Plate Boundaries

1. Diverging magma currents in the magma cause two plates to drift apart from one another, leaving a gap in between.
2. Lava wells out of the gap to fill it, causing vulcanicity.

Note： Water pressure above the sea floor suppresses volcanic activity, so volcanic activity here is not as violent.

At Hot Spots

Note how the movement of the Pacific Plate is related to the shape of the Hawaiian Islands.

Mauna Loa is the currently active Hawaiian volcano. The other four volcanoes are now extinct as they have moved away from the hot spot.

Certain points in the mantle experience concentrated amounts of pressure. These are called hot spots (mantle plumes) and were tested in the HKDSE Practice Paper.

1. When lines of weakness appear in the earth crust and extend to the hot spot below, magma pressure is released.
2. Magma rises through the cracks to the earth's surface.
3. Extrusive vulcanicity occurs.

Then volcanic islands are formed (materials in round brackets are related only to the Hawaiian Islands):

1. Lava continuously extrudes to the earth's surface above the hot spot.
2. As lava cools and solidifies, volcanoes are created and grown above sea level.
3. Hot spots are fixed in location.
4. When the plate moves (e.g. the Pacific Plate moves northwest), the volcano is carried away from the hot spot.
5. The volcano will become extinct.
6. A new volcano is formed at the hot spot (e.g. at the southeast of the original volcano). A volcanic island chain (Hawaii) is formed.

Formation of Tsunamis

Submarine Earthquakes

1. The plates are held back by friction.
2. Stress builds up in the plates.
3. As the stress accumulates, it will eventually exceed the friction created.
4. The land shakes and seismic waves are erupted from the focus of the earthquake.
5. If the earthquake originated close to a seabed, rocks are displaced vertically and sea floor deformation occurs.
6. Water is displaced to create big waves.

Not all submarine earthquakes trigger tsunamis. Only the major ones do.

Submarine Volcanic Eruptions

Very severe submarine volcanic eruptions cause the displacement of water, causing tsunamis.

Volcanic tsunamis are NOT common CE06Q11 confirmed that tsunamis caused by submarine volcanic eruptions are NOT common.

Submarine Landslides

Very severe submarine landslide cause the displacement of water, causing tsunamis.