Earth 地球(英)

Earth

地球

(

英

)

Earth is the third planet from the Sun and the fifth largest:

orbit: 149,600,000 km (1.00 AU) from Sun

diameter: 12,756.3 km

mass: 5.972e24 kg

Earth is the only planet whose English name does not derive from Greek/Roman

mythology. The name derives from Old English and Germanic. There are, of course,

hundreds of other names for the planet in other languages. In Roman Mythology, the

goddess

of

the

Earth

was

Tellus

-

the

fertile

soil

(Greek:

Gaia,

terra

mater

-

Mother

Earth).

It was not until the time of Copernicus (the sixteenth century) that it was

understood that the Earth is just another planet.

Earth,

of

course,

can

be

studied

without

the

aid

of

spacecraft.

Nevertheless

it

was not until the twentieth century that we had maps of the entire planet. Pictures

of

the

planet

taken

from

space

are

of

considerable

importance;

for

example,

they

are

an enormous help in weather prediction and especially in tracking and predicting

hurricanes. And they are extraordinarily beautiful.

The

Earth

is

divided

into

several

layers

which

have

distinct

chemical

and

seismic

properties (depths in km):

0- 40 Crust

40- 400 Upper mantle

400- 650 Transition region

650-2700 Lower mantle

2700-2890 D layer

2890-5150 Outer core

5150-6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans,

thicker

under

the

continents.

The

inner

core

and

crust

are

solid;

the

outer

core

and

mantle

layers

are

plastic

or

semi-fluid.

The

various

layers

are

separated

by

discontinuities which are evident in seismic data; the best known of these is the

Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core;

the

part

we

inhabit

is

a

tiny

fraction

of

the

whole

(values

below

x10^24

kilograms):

atmosphere = 0.0000051

oceans = 0.0014

crust = 0.026

mantle = 4.043

outer core = 1.835

inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is

possible

that

some

lighter

elements

may

be

present,

too.

Temperatures

at

the

center

of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower

mantle

is

probably

mostly

silicon,

magnesium

and

oxygen

with

some

iron,

calcium

and

aluminum.

The

upper

mantle

is

mostly

olivine

and

pyroxene

(iron/magnesium

silicates),

calcium

and

aluminum.

We

know

most

of

this

only

from

seismic

techniques;

samples

from

the upper mantle arrive at the surface as lava from volcanoes but the majority of

the

Earth

is

inaccessible.

The

crust

is

primarily

quartz

(silicon

dioxide)

and

other

silicates

like

feldspar.

Taken

as

a

whole,

the

Earth's

chemical

composition

(by

mass)

is:

34.6% Iron

29.5% Oxygen

15.2% Silicon

12.7% Magnesium

2.4% Nickel

1.9% Sulfur

0.05% Titanium

The Earth is the densest major body in the solar system.

The

other

terrestrial

planets

probably

have

similar

structures

and

compositions

with

some

differences:

the

Moon

has

at

most

a

small

core;

Mercury

has

an

extra

large

core (relative to its diameter); the mantles of Mars and the Moon are much thicker;

the Moon and Mercury may not have chemically distinct crusts; Earth may be the only

one

with distinct

inner

and

outer

cores.

Note,

however,

that our knowledge of

planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several

separate

solid

plates

which

float

around

independently

on

top

of

the

hot

mantle

below.

The theory that describes this is known as plate tectonics. It is characterized by

two

major

processes:

spreading

and

subduction.

Spreading

occurs

when

two

plates

move

away

from

each

other

and

new

crust

is

created

by

upwelling

magma

from

below.

Subduction

occurs when two plates collide and the edge of one dives beneath the other and ends

up being destroyed in the mantle. There is also transverse motion at some plate

boundaries

(i.e.

the

San

Andreas

Fault

in

California)

and

collisions

between

continental

plates

(i.e.

India/Eurasia).

There

are

(at

present)

eight

major

plates:

North American Plate - North America, western North Atlantic and Greenland

South American Plate - South America and western South Atlantic

Antarctic Plate - Antarctica and the

Eurasian Plate - eastern North Atlantic, Europe and Asia except for India

African Plate - Africa, eastern South Atlantic and western Indian Ocean

Indian-Australian

Plate

-

India,

Australia,

New

Zealand

and

most

of

Indian

Ocean

Nazca Plate - eastern Pacific Ocean adjacent to South America

Pacific

Plate

-

most

of

the

Pacific

Ocean

(and

the

southern

coast

of

California!)

There are also twenty or more small plates such as the Arabian, Cocos, and

Philippine

Plates.

Earthquakes

are

much

more

common

at

the

plate

boundaries.

Plotting

their locations makes it easy to see the plate boundaries.

The Earth's surface is very young. In the relatively short (by astronomical

standards)

period

of

500,000,000

years

or

so

erosion

and

tectonic

processes

destroy

and recreate

most of the Earth's

surface

and thereby

eliminate almost all traces

of

earlier

geologic

surface

history

(such

as

impact

craters).

Thus

the

very

early

history