Until the 1950s, most scientists believed that the geology of the ocea

Until the 1950s, most scientists believed that the
geology of the ocean floor had remained essentially
unchanged for many millions of years. But this idea
became insupportable as new discoveries were made.
(5) First, scientists noticed that the ocean floor exhibited
odd magnetic variations. Though unexpected, this was
not entirely surprising, because it was known that
basalt—the volcanic rock making up much of the
ocean floor—contains magnetite, a strongly magnetic
(10) mineral that was already known to locally distort
compass readings on land. This distortion is due to the
fact that although some basalt has so-called “normal”
polarity—that is, the magnetite in it has the same
polarity as the earth’s present magnetic field—other
(15) basalt has reversed polarity, an alignment opposite
that of the present field. This occurs because in magma
(molten rock), grains of magnetite—behaving like
little compass needles—align themselves with the
earth’s magnetic field, which has reversed at various
(20) times throughout history. When magma cools to form
solid basalt, the alignment of the magnetite grains is
“locked in,” recording the earth’s polarity at the time
of cooling.

As more of the ocean floor was mapped, the
(25) magnetic variations revealed recognizable patterns,
particularly in the area around the other great oceanic
discovery of the 1950s: the global mid-ocean ridge, an
immense submarine mountain range that winds its way
around the earth much like the seams of a baseball.
(30) Alternating stripes of rock with differing polarities are
laid out in rows on either side of the mid-ocean ridge:
one stripe with normal polarity and the next with
reversed polarity. Scientists theorized that mid-ocean
ridges mark structurally weak zones where the ocean
(35) floor is being pulled apart along the ridge crest. New
magma from deep within the earth rises easily through
these weak zones and eventually erupts along the crest
of the ridges to create new oceanic crust. Over millions
of years, this process, called ocean floor spreading,
(40) built the mid-ocean ridge.

This theory was supported by several lines of
evidence. First, at or near the ridge crest, the rocks are
very young, and they become progressively older
away from the crest. Further, the youngest rocks all
(45) have normal polarity. Finally, because geophysicists
had already determined the ages of continental volcanic
rocks and, by measuring the magnetic orientation of
these same rocks, had assigned ages to the earth’s
recent magnetic reversals, they were able to compare
(50) these known ages of magnetic reversals with the
ocean floor’s magnetic striping pattern, enabling scientists
to show that, if we assume that the ocean floor moved
away from the spreading center at a rate of several
centimeters per year, there is a remarkable correlation
(55) between the ages of the earth’s magnetic reversals
and the striping pattern.

1. Which one of the following most accurately expresses the main idea of the passage?

(A) In the 1950s, scientists refined their theories concerning the process by which the ocean floor was formed many millions of years ago.
(B) The discovery of basalt’s magnetic properties in the 1950s led scientists to formulate a new theory to account for the magnetic striping on the ocean floor.
(C) In the 1950s, two significant discoveries led to the transformation of scientific views about the geology of the oceans.
(D) Local distortions to compass readings are caused, scientists have discovered, by magma that rises through weak zones in the ocean floor to create new oceanic crust.
(E) The discovery of the ocean floor’s magnetic variations convinced scientists of the need to map the entire ocean floor, which in turn led to the discovery of the global mid-ocean ridge.


2. The author characterizes the correlation mentioned in the last sentence of the passage as “remarkable” in order to suggest that the correlation

(A) indicates that ocean floor spreading occurs at an extremely slow rate
(B) explains the existence of the global mid-ocean ridge
(C) demonstrates that the earth’s magnetic field is considerably stronger than previously believed
(D) provides strong confirmation of the ocean floor spreading theory
(E) reveals that the earth’s magnetic reversals have occurred at very regular intervals


3. According to the passage, which one of the following is true of magnetite grains?

(A) In the youngest basalt, they are aligned with the earth’s current polarity.
(B) In magma, most but not all of them align themselves with the earth’s magnetic field.
(C) They are not found in other types of rock besides basalt.
(D) They are about the size of typical grains of sand.
(E) They are too small to be visible to the naked eye.


4. If the time intervals between the earth’s magnetic field reversals fluctuate greatly, then, based on the passage, which one of the following is most likely to be true?

(A) Compass readings are most likely to be distorted near the peaks of the mid-ocean ridge.
(B) It is this fluctuation that causes the ridge to wind around the earth like the seams on a baseball.
(C) Some of the magnetic stripes of basalt on the ocean floor are much wider than others.
(D) Continental rock is a more reliable indicator of the earth’s magnetic field reversals than is oceanic rock.
(E) Within any given magnetic stripe on the ocean floor, the age of the basalt does not vary.


5. Which one of the following would, if true, most help to support the ocean floor spreading theory?

(A) There are types of rock other than basalt that are known to distort compass readings.
(B) The ages of the earth’s magnetic reversals have been verified by means other than examining magnetite grains in rock.
(C) Pieces of basalt similar to the type found on the mid-ocean ridge have been found on the continents.
(D) Along its length, the peak of the mid-ocean ridge varies greatly in height above the ocean floor.
(E) Basalt is the only type of volcanic rock found in portions of the ocean floor nearest to the continents.


6. Which one of the following is most strongly supported by the passage?

(A) Submarine basalt found near the continents is likely to be some of the oldest rock on the ocean floor.
(B) The older a sample of basalt is, the more times it has reversed its polarity.
(C) Compass readings are more likely to become distorted at sea than on land.
(D) The magnetic fields surrounding magnetite grains gradually weaken over millions of years on the ocean floor.
(E) Any rock that exhibits present-day magnetic polarity was formed after the latest reversal of the earth’s magnetic field.