Flashcards in MIDTERM CH. 5 Deck (21)
A large wave caused by a seismic event.
2. Where is a tsunami not a hazard?
In the open ocean, where this low, long fast wave is generated, it is not a hazard.
3. Where is a tsunami most hazardous?
These waves are a significant hazard when they approach a coast.
At a coastline, as water depth increases, the energy of this wave and the water it is displacing is forced upward.
Wave length decreases, but wave height increases. (You end up with a very tall wave (several meters tall))
This much larger wave then breaks against the shore.
- very small amplitudes (1-3 meters)
- very long wave lengths (hundreds of kilometres)
- very fast speeds (hundreds of kilometres per hour)
4. How are tsunamis generated?
Tsunamis are generated when energy is transferred from the crust to the water - a seismic event.
This transfer of energy may be the result of:
3. Mass wasting
5. How does an earthquake generate a tsunami?
During an earthquake, the crust beneath the ocean must move vertically.
As the surface of the ocean returns to normal - the hole fills in - a series of waves are generated that travel outward from this point of origin.
6. How does a volcano generate a tsunami?
1. An explosive eruption next to or beneath the ocean can generate a tsunami.
2. The collapse of an underwater volcano as the magma chamber empties during an eruption.
3. Underwater landslides from a volcano between eruptions. (Movement of a large mass of rock down the flank of a volcano can create a wave)
7. What particular features of a shield volcano contributes to the creation of a tsunami? (part 1) (see diagrams 1-3)
As a shield volcano forms, it does so on the sea floor.
Because lava cools very quickly on contact with water, the rock that forms has little to no crystalline structure.
Also, pillow structures may form.
As a result, the rock that forms the underwater part of the volcano is structurally weak.
8. How does a landslide or rockfall generate a tsunami
The movement of a large mass of rock beneath or into the water can cause a tsunami.
9. What two factors control the formation of a tsunami by a rockfall?
The size of a wave generated by mass wasting will depend on two things:
1. The volume of rock that moves
2. The heigh that it falls from.
Pillow structures (see diagram)
These are spherical or bulbous shapes in the rock.
7. What particular features of a shield volcano contributes to the creation of a tsunami? (part 2) (see diagram)
Once the volcano reaches seal level and above, the rate of cooling is slower and the crystalline structure of the rock is better and the rock is stronger.
As a result, there is a solid cap of rock on top of weaker rock beneath it. In places like Hawaii, this exposed (above sea level) rock is prone to failure.
If enough rock suddenly falls into the ocean, a tsunami can be generated.
10. What is run-up height? (see diagram)
Elevation on land that the water reaches
11. What is run-out distance? (see diagram)
Distance a wave reaches horizontally on land.
The maximum height and distance reached on land by the wave as recorded by the damaged done to vegetation, usually trees, by the water.
13. Describe the events that took place in Lituya Bay, Alaska, July 9, 1958.
In this fjord, an earthquake generated a large landslide from a mountain next to the ocean.
Upon impact with the water, a 150 meter wave was generated.
There were 6 people in the bay and 4 survived.
The run up height was 524m.
14. Burin Peninsula, Newfoundland, Nov 18 1929.
On this date, a large earthquake happened in the Gulf of St. Lawrence.
As a result, a large underwater land took place off the coast of Newfoundland near the Burin Peninsula.
This landslide create a tsunami that caused significant damage and the loss of 29 lives.
15. What are tsunami laid sediments? (part 1)
Waves break against a shoreline because they run out of room. And as the wave breaks, water travels onto the land.
Water slows down until it reaches zero velocity at the maximum run-up height, run-out distance.
At this point, some water may stay on the land - pooled on the surface - or infiltrate into the ground.
As a result, less water flows back into the ocean.
This return flow may also be slower because it is driven only by the force of gravity.
15. What are tsunami laid sediments? (part 2)
Sediment originally on the coast (e.g. beach) may be eroded and transported on land.
It may remain there because return flow is too weak to move it back to the ocean.
The end result is a layer of sediment - a tsunami deposit - that is thicker and coarser (larger grains) near the coast and thinner and thinner (smaller grains) further inland.
While a lot of coarser sediment can be moved where wave energy is high near the coast, further inland where wave energy is less only smaller amounts of finer sediment is moved.
The structure of a tsunami deposit will reflect the different processes as water moves on land and returns to the ocean.
Seiche (see diagram)
A wave generated in an enclosed body of water that behaves like a tsunami.
However, a seiche is wind generated, not seismic.
At A, water is pushed up against the shoreline by the wind.
Eventually, this extra water flows back into the lake.
As the moving water bounces back and forth across the lake basin it creates waves that impact the shoreline.