新托福TPO27阅读原文及译文(二)

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TPO27

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(

二

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：

The Formation of Volcanic Islands

TPO27-2

：

The Formation of Volcanic Islands

Earth’s surface is

not

made up of a single sheet

of rock that forms a crust but

rather a number

of “tectonic plates”that fit closely, like the pieces of a giant jigsaw

puzzle.

Some

plates

carry

islands

or

continents

others

form

the

seafloor.

All

are

slowly

moving

because

the

plates

float

on

a

denser

semi- liquid

mantle,

the

layer

between the crust an

d Earth’s core. The plates have edges that are spreading ridges

(where

two

plates

are

moving

apart

and

new

seafloor

is

being

created),

subduction

zones

(where

two

plates

collide

and

one

plunges

beneath

the

other),

or

transform

faults

(where

two

plates

neither

converge

nor

diverge

but

merely

move

past

one

another).

It

is

at

the

boundaries

between

plates

that

most

of

Earth’s

volcanism

and

earthquake activity occur.

Generally speaking, the interiors of plates are geologically uneventful. However,

there

are

exceptions.

A

glance

at

a

map

of

the

Pacific

Ocean

reveals

that

there

are

many islands far out at sea that are actually volcanoes ----many no longer active, some

overgrown

with

coral----that

originated

from

activity

at

points

in

the

interior

of

the

Pacific Plate that forms the Pacific seafloor.

How

can

volcanic

activity

occur

so

far

from

a

plate

boundary?

The

Hawaiian

Islands provide a very instructive answer. Like many other island groups, they form a

chain. The Hawaiian Islands Chain extends northwest from the island of Hawaii. In

the 1840s American geologist James Daly observed that the different Hawaii islands

seem

to

share

a

similar

geologic

evolution

but

are

progressively

more

eroded,

and

therefore probable older, toward the northwest. Then in 1963, in the early days of the

development

of

the

theory

of

plate

tectonics.

Canadian

geophysicist

Tuzo

Wilson

realized that this age progression could result if the islands were formed on a surface

plate moving over a fixed volcanic source in the interior. Wilson suggested that the

long

chain

of

volcanoes

stretching

northwest

from

Hawaii

is

simply

the

surface

expression

of

a

long-lived

volcanic

source

located

beneath

the

tectonic

plate

in

the

mantle. Today’s most northwest island would have been the first to form.

Then as the

plate

moved

slowly

northwest,

new

volcanic

islands

would

have

forms

as

the

plate

moved over the volcanic source. The most recent island, Hawaii, would be at the end

of the chain and is now over the volcanic source.

Although

this

idea

was

not

immediately

accepted,

the

dating

of

lavas

in

the

Hawaii

(and

other)

chains

showed

that

their

ages

increase

away

from

the

presently

active volcano, just as Daly had suggested. Wilson’s analysis of these data is now a

central part of plate tectonics. Most volcanoes that occur in the interiors of plates are

believed

to

be

produced

by

mantle

plumes,

columns

of

molten

rock

that

rise

from

deep within the mantle. A volcano remains an active “hot spot”as long as it is over the

plume.

The

plumes

apparently

originate

at

great

depths,

perhaps

as

deep

as

the

boundary between the core and the mantle, and many have been active for a very long

time. The oldest volcanoes in the Hawaii hot-spot trail have ages close to 80 million

years. Other islands, including Tahiti and Easter Islands in the pacific, Reunion and

Mauritius

in

the

India

Ocean,

and

indeed

most

of

the

large

islands

in

the

world’s

oceans, owe their existence to mantle plumes.

The oceanic volcanic islands and their hot-spot trails are thus especially useful

for geologist because they record the past locations of the plate over a fixed source.

They

therefore

permit

the

reconstruction

of

the

process

of

seafloor

spreading,

and

consequently

of

the

geography

of

continents

and

of

ocean

basins

in

the

past.

For

example, given the current position of the Pacific Plate, Hawaii is above the Pacific

Ocean

hot

spot.

So

the

position

of

The

Pacific

Plate

50

million

years

ago

can

be

determined by moving it such that a 50-million-year-old volcano in the hot-spot trail

sits

at

the

location

of

Hawaii

today.

However

because

the

ocean

basins

really

are

short-

lived features on geologic times scale, reconstruction the world’s geography by

backtracking

along

the

hot- spot

trail

works

only

for

the

last

5

percent

or

so

of

geologic time.

TPO27-2

译文：火山岛的形成

地球的外壳并不是由单块岩石形成 的，

而是许多的

“

构造板块

严密的组合在

一起的，

就像是一个巨大的拼图。

一些板块承载着岛屿或是大陆，

其它的 则形成