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menjadi samada kawasan perumahan atau kawasan sawit atau getah. Masalahnya
timbul apabila permukaan yang botak tidak lagi berupaya menghalang hakisan air
hujan! Sebenarnya banyak logam berat terjadi secara semulajadi di atas
permukaan bumi. Berbagai jenis logam berat ditahan dari pada masuk ke dalam sungai
oleh rumput2, pokok pokok dan tumbuh tumbuhan di atas permukaan bumi. Sekiranya
permukaan bumi telah digondolkan, apakah lagi daya yang dapat menahan logam
berat yang memudaratkan kesihatan manusia daripada dihanyut masuk oleh hujan ke
dalam aliran sungai? Sila lah ikuti ...
The Earth
formed from the same cloud of matter that formed the Sun, but the planets
acquired different compositions during the formation
and evolution of the solar system. In turn, the natural history of the Earth caused parts of this
planet to have differing concentrations of the elements. The mass of the Earth
is approximately 5.98×1024 kg. It is composed mostly of iron
(32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2%
consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to
be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%),
sulfur (4.5%), and less than 1% trace elements.[10]
Earth's bulk
(total) elemental abundance
Name
|
Symbol
|
ppm (μg/g)
|
ppb (atoms)
|
An estimate[11] of the elemental abundances in the
total mass of the Earth. Note that numbers are estimates, and they will vary
depending on source and method of estimation. Order of magnitude of data can
roughly be relied upon. ppb (atoms) is parts per billion, meaning that is the
number of atoms of a given element in every billion atoms in the Earth.
Earth's
crustal elemental abundance
Main article: Abundance
of elements in Earth's crust
Abundance (atom fraction) of the chemical
elements in Earth's upper continental crust as a function of atomic number. The
rarest elements in the crust (shown in yellow) are the most dense. They were
further rarefied in the crust by being siderophile (iron-loving) elements, in
the Goldschmidt
classification of elements. Siderophiles were depleted by being
relocated into the Earth's core. Their abundance in meteoroid materials is relatively higher.
Additionally, tellurium and selenium have been depleted from the crust due to
formation of volatile hydrides. This graph illustrates the relative abundance
of the chemical elements in Earth's upper continental crust, which is
relatively accessible for measurements and estimation. Many of the elements
shown in the graph are classified into (partially overlapping) categories:
- rock-forming elements (major elements in green field, and minor
elements in light green field);
- rare earth elements
(lanthanides, La-Lu, and Y; labeled in blue);
- major industrial metals (global production >~3×107
kg/year; labeled in red);
- precious metals
(labeled in purple);
- the nine rarest "metals" — the six platinum group elements plus Au, Re, and Te (a metalloid) — in the yellow field.
Note that there are two breaks
where the unstable elements technetium (atomic number:
43) and promethium (atomic number: 61) would be. These
are both extremely rare, since on Earth they are only produced through the spontaneous fission of very heavy radioactive elements (for example, uranium, thorium, or the trace amounts of plutonium that exist in uranium ores), or by the
interaction of certain other elements with cosmic rays. Both of the first two of these
elements have been identified spectroscopically in the atmospheres of stars,
where they are produced by ongoing nucleosynthetic processes. There are also
breaks where the six noble gases would be,
since they are not chemically bound in the Earth's crust, and they are only
generated by decay chains from radioactive elements and are therefore extremely
rare there. The twelve naturally occurring very rare, highly radioactive
elements (polonium, astatine, francium, radium, actinium, protactinium, neptunium, plutonium, americium, curium, berkelium, and californium) are not included, since any of these
elements that were present at the formation of the Earth have decayed away eons
ago, and their quantity today is negligible and is only produced from the radioactive decay of uranium and thorium.
Oxygen and silicon are notably quite
common elements. They have frequently combined with each other to form common silicate minerals.
Rare-earth
elemental abundance
"Rare" earth
elements is a historical misnomer. The persistence of the term reflects
unfamiliarity rather than true rarity. The more abundant rare earth elements
are each similar in crustal concentration to commonplace industrial metals such
as chromium, nickel, copper, zinc, molybdenum, tin, tungsten, or lead. The two
least abundant rare earth elements (thulium and lutetium) are nearly 200 times more common than
gold. However, in contrast to the ordinary base and precious metals, rare earth
elements have very little tendency to become concentrated in exploitable ore
deposits. Consequently, most of the world's supply of rare earth elements comes
from only a handful of sources. Furthermore, the rare earth metals are all
quite chemically similar to each other, and they are thus quite difficult to
separate into quantities of the pure elements.
Differences in abundances of
individual rare earth elements in the upper continental crust of the Earth
represent the superposition of two effects, one nuclear and one geochemical.
First, the rare earth elements with even atomic numbers (58Ce, 60Nd,
...) have greater cosmic and terrestrial abundances than the adjacent rare
earth elements with odd atomic numbers (57La, 59Pr, ...).
Second, the lighter rare earth elements are more incompatible (because they have
larger ionic radii) and therefore more strongly concentrated in the continental
crust than the heavier rare earth elements. In most rare earth ore deposits,
the first four rare earth elements – lanthanum, cerium, praseodymium, and neodymium – constitute 80% to 99% of the total
amount of rare earth metal that can be found in the ore.
See also: Earth#Chemical composition
Main article: Earth science
Further information: Earth
physical characteristics tables
Earth is a terrestrial planet,
meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar
terrestrial planets in size and mass. Of these four planets, Earth also has the
highest density, the highest surface gravity, the strongest magnetic field,
and fastest rotation,[63] and is probably the only one with
active plate tectonics.[64]
Chemical composition of the crust[74]
|
||||
Compound
|
Formula
|
Composition
|
||
Continental
|
Oceanic
|
|||
SiO2
|
60.2%
|
48.6%
|
||
Al2O3
|
15.2%
|
16.5%
|
||
CaO
|
5.5%
|
12.3%
|
||
MgO
|
3.1%
|
6.8%
|
||
FeO
|
3.8%
|
6.2%
|
||
Na2O
|
3.0%
|
2.6%
|
||
K2O
|
2.8%
|
0.4%
|
||
Fe2O3
|
2.5%
|
2.3%
|
||
H2O
|
1.4%
|
1.1%
|
||
CO2
|
1.2%
|
1.4%
|
||
TiO2
|
0.7%
|
1.4%
|
||
P2O5
|
0.2%
|
0.3%
|
||
Total
|
99.6%
|
99.9%
|
||
Chemical
composition
See also: Abundance of
elements on Earth
The mass of the Earth is approximately
5.98×1024 kg. It is composed mostly of
iron
(32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2%
consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to
be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%),
sulfur (4.5%), and less than 1% trace elements.[75]
The geochemist F. W. Clarke
calculated that a little more than 47% of the Earth's crust consists of oxygen.
The more common rock constituents of the Earth's crust are nearly all oxides;
chlorine, sulfur and fluorine are the only important exceptions to this and
their total amount in any rock is usually much less than 1%. The principal
oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The
silica functions principally as an acid, forming silicates, and all the
commonest minerals of igneous rocks are of this nature. From a computation
based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were
composed of 11 oxides (see the table at right), with the other constituents
occurring in minute quantities.[76]
Internal
structure
Main article: Structure of the Earth
The interior of the Earth,
like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other
terrestrial planets, it has a distinct outer and inner core. The outer layer of
the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from
the mantle by the Mohorovičić
discontinuity, and the thickness of the crust varies: averaging 6 km (kilometers) under the oceans and 30-50 km on
the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that
the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer
on which the lithosphere rides. Important changes in crystal structure within
the mantle occur at 410 and 660 km below the surface,
spanning a transition zone
that separates the upper and lower mantle. Beneath the mantle, an extremely low
viscosity liquid outer core lies above a
solid inner core.[77] The inner core may rotate at a
slightly higher angular velocity
than the remainder of the planet, advancing by 0.1–0.5° per year.[78]
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