璧勬枡澶?綘鎵劇偣甯歌瘑鎬х殑鍚?
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.
Mir space station and Earth's limb 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 olivene 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:
South America by Galileo
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 Earth's Plate Boundaries delineated by earthquake epicenters South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
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.