The hard, rigid plates that form the outermost portion of the Earth are about 100 kilometers thick. These plates include both the Earth's crust and the upper mantle. The rocks of the crust are composed mostly of minerals with light elements, like aluminum and sodium, while the mantle contains some heavier elements, like iron and (5) magnesium. Together, the crust and upper mantle that form the surface plates are called the lithosphere. This rigid layer floats on the denser material of the lower mantle the way a wooden raft floats on a pond. The plates are supported by a weak, plastic layer of the lower mantle called the asthenosphere.
Also like a raft on a pond, the lithospheric plates are carried along by slow currents in this more fluid layer beneath (10)them. With an understanding of plate tectonics, geologists have put together a new history for the Earth's surface. About 200 million years ago, the plates at the Earth's surface formed a “supercontinent” called Pangaea. When this supercontinent started to tear apart because of plate movement, Pangaea first broke into two large continental masses (15)with a newly formed sea that grew between the land areas as the depression filled with water. The southern one ― which included the modern continents of South America, Africa, Australia, and Antarctica ― is called Gondwanaland. The northern one ― with North America, Europe, and Asia ― is called Laurasia. North America tore away from Europe about 180 million years ago, forming the northern Atlantic Ocean.
(20)Some of the lithospheric plates carry ocean floor and others carry land masses or a combination of the two types. The movement of the lithospheric plates is responsible
for earthquakes, volcanoes, and the Earth's largest mountain ranges. Current understanding of the interaction between different plates explains why these occur
where they do. For example, the edge of the Pacific Ocean has been called the “Ring
(25)of Fire” because so many volcanic eruptions and earthquakes happen there. Before the 1960's, geologists could not explain why active volcanoes and strong earthquakes were concentrated in that region. The theory of plate tectonics gave them an answer.
9. With which of the following topics is the passage mainly concerned?
(A) The contributions of the theory of plate tectonics to geological knowledge
(B) The mineral composition of the Earth's crust
(C) The location of the Earth's major plates
(D) The methods used by scientists to measure plate movement
10. According to the passage, the lithospheric plates are given support by the
(A) upper mantle
(B) ocean floor
(C) crust
(D) asthenosphere
11. The author compares the relationship between the lithosphere and the
asthenosphere to which of the following?
(A) Lava flowing from a volcano
(B) A boat floating on the water
(C) A fish swimming in a pond
(D) The erosion of rocks by running water
12. The word“one”in line 16 refers to
(A) movements
(B) masses
(C) sea
(D) depression
13. According to the passage, the northern Atlantic Ocean was formed when
(A) Pangaea was created
(B) plate movement ceased
(C) Gondwanaland collided with Pangaea
(D) parts of Laurasia separated from each other
14. The word “carry” in line 20 could best be replaced by
(A) damage
(B) squeeze
(C) connect
(D) support
15. In line 27, the word “concentrated” is closest in meaning to which of the following?
(A) Allowed
(B) Clustered
(C) Exploded
(D) Strengthened
16. Which of the following can be inferred about the theory of plate tectonics?
(A) It is no longer of great interest to geologists.
(B) It was first proposed in the 1960's.
(C) It fails to explain why earthquakes occur.
(D) It refutes the theory of the existence of a supercontinent.
17. The paragraph following the passage most probably discusses
(A) why certain geological events happen where they do
(B) how geological occurrences have changed over the years
(C) the most unusual geological developments in the Earth's history
(D) the latest innovations in geological measurement
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