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30. GWD-30-Q11-Q13 最老的火山岩石和发现它的科学意义
First identified in 1969, komatiites
are Earth's oldest known volcanic
rocks and contain three times as much
Line magnesium as do most volcanic rocks.
(5) This chemical composition suggests
that komatiites formed from the hottest
lava known ever to have erupted: a
high concentration of magnesium
changes the physical properties of
(10) lava so that unusually high tempera-
tures would be required for the lava
to exist as a liquid.
Komatiites' discovery was surpris-
ing in light of then-current geological
(15) theories about magmas, molten rock
that forms in the Earth's mantle (the
layer beneath the crust) and composes
volcanic lava eruptions. Prior to 1960,
geologists Bowen and Hess disagreed
(20) over whether or not the very high tem-
peratures needed to produce magmas
rich in magnesium could have existed
on Earth. Hess suggested that the
presence of water, probably released
(25) from minerals decomposing in the
Earth's mantle, might have meant that
a high-magnesium magma could have
existed at a lower temperature. But
Bowen showed experimentally that the
(30) high temperatures were indeed nec-
essary. By 1960, it was generally
accepted that volcanic rocks with such
high levels of magnesium could not
exist, and thus the discovery of koma-
(35) tiites changed geologists' assumptions
about the characteristics of the Earth's
mantle around the time of the formation
of komatiites, between 2.5 and 4 billion
years ago.
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