老托福阅读真题及答案：PASSAGE 13

老托福阅读真题及答案：

PASSAGE 13

Any rock that has cooled and solidified from a molten state is an igneous rock.

Therefore,

if

the

Earth

began

as

a

superheated

sphere

in

space,

all

the

rocks

making

up its crust may well have been igneous and thus the ancestors of all other rocks.

Even

today,

approximately

95

percent

of

the

entire

crust

is

igneous.

Periodically,

molten material wells out of the Earth's interior to invade the surface layers or

to

flow

onto

the

surface

itself.

This

material

cools

into

a

wide

variety

of

igneous

rocks.

In

the

molten

state,

it

is

called

magma

as

it

pushes

into

the

crust

and

lava

when it runs out onto the surface.

All magma consists basically of a variety of silicate minerals (high in

silicon-oxygen

compounds),

but

the

chemical

composition

of

any

given

flow

may

differ

radically from that of any other. The resulting igneous rocks will reflect these

differences.

Igneous

rocks

also

vary

in

texture

as

well

as

chemistry.

Granite,

for

instance, is a coarse- grained igneous rock whose individual mineral crystals have

formed to a size easily seen by the naked eye. A slow rate of cooling has allowed

the crystals to reach this size. Normally, slow cooling occurs when the crust is

invaded by magma that remains buried well below the surface. Granite may be found

on the surface of the contemporary landscape, but from its coarse texture we know

that it must have formed through slow cooling at a great depth and later been laid

bare

by

erosion.

Igneous

rocks

with

this

coarse- grained

texture

that

formed

at

depth

are called plutonic.

On

the

other

hand,

if

the

same

magma

flows

onto

the

surface

and

is

quickly

cooled

by

the

atmosphere,

the

resulting

rock

will

be

fine-grained

and

appear

quite

different

from

granite,

although

the

chemical

composition

will

be

identical.

This

kind

of

rock

is called rhyolite. The most finely grained igneous rock is volcanic glass or

obsidian,

which

has

no

crystals.

Some

researchers

believe

this

is

because

of

rapid

cooling; others believe it is because of a lack of water vapor and other gases in

the lava. The black obsidian cliffs

of Yellowstone National

Park

are the result

of

a lava flow of basalt running head on into a glacier. Some of the glacier melted

on contact, but suddenly there also appeared a huge black mass of glassy stone.

1. In the first paragraph, the author mentions that 95% of the Earth's crust

is composed of igneous rock to support the idea that

(A) the Earth began as a molten mass

(B) a thin layer of magma flows beneath the Earth's crust

(C) the minerals found in igneous rock are very common

(D) igneous rock is continually being formed

2. The word

(A) move into

(B) neutralize

(C) cover

(D) deposit

3. The word

(A) vast

(B) natural

(C) existing

(D) uneven

4. The word

(A) granite

(B) surface

(C) landscape