The most common causes of tsunamis are underwater earthquakes. To understand underwater earthquakes, you must first understand plate tectonics. The theory of plate tectonics suggests that the lithosphere, or top layer of the Earth, is made up of a series of huge plates. These plates make up the continents and sea floor. They rest on an underlying viscous layer called the asthenosphere.
Think of a pie cut into eight slices. The pie crust would be the lithosphere and the hot, sticky pie filling underneath would be on the asthenosphere. On the Earth, these plates are constantly in motion, moving along each other at the speed of 1 to 2 inches (2.5 – 5 cm) per year. The movement occurs most dramatically along fault lines (where the pie is cut). These motions are capable of producing earthquakes and volcanism.
When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one. This is called subduction. Underwater subduction often leaves enormous “handprints” in the form of deep ocean trenches along the seafloor. In some cases of subduction, part of the seafloor connected to the lighter plate may “snap up” suddenly due to pressure from the sinking plate. This results in an earthquake. The focus of the earthquake is the point within the Earth where the rupture first occurs, rocks break and the first seismic waves are generated. The epicenter is the point on the seafloor directly above the focus.
When the piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes the water upward above normal sea level. This is the birth of a tsunami. The earthquakes that generated December 26, 2004 tsunami in the Indian Ocean was a 9.0 on the Richter scale – one of the biggest in recorded history.
1. What is the topic of the text?
A. causes of tsunami
B. underwater earthquake
C. above normal sea waves
D. tectonic subduction
E. the birth of tsunami
2. What does the text explain?
A. what causes the layers of the earth to move
B. why sea level rises above normal
C. how tectonic subduction produces earthquake
D. how underwater earthquake causes tsunami
E. why a heavier plate slips under a lighter one
3. According to paragraph 1, continents and sea floor are part of … .
A. lithosphere
B. asthenosphere
C. plate tectonics
D. sinking plates
E. huge plates
4. What is the main idea of paragraph 2?
A. The lithosphere is like the pie crust
B. The lithosphere is on top of the asthenosphere
C. The asthenosphere is a hot, sticky layer
D. All the plates keep moving along each other
E. The plate movements are dramatic at fault lines
5. What is the main idea of paragraph 3?
A. Seismic waves are caused by the rupture
B. A heavier plate slips under a lighter one
C. Subduction produces great force
D. A lighter plate snaps up suddenly
E. Subduction could result in earthquake
6. It is implied in the text that earthquake occurs when … .
A. subduction happens
B. seismic waves are generated
C. there is no fault line
D. heavier plates and lighter ones break up
E. deep ocean trenches are along the seafloor
7. Which information is irrelevant to tsunami?
A. It is connected to the underlying viscous layer
B. It is closely linked to underwater earthquakes
C. It is related to strong movements of plate tectonics
D. It is predictable following any incidence of earthquake
E. It is related to the rise of sea level
8. The place within the Earth where the rupture first occurs is called … .
A. the focus
B. the epicenter
C. the vocal
D. the center
E. the diameter
