InternalStructure of the EarthWhat is the evidence for our knowledgeof the internal structure of the Earth?As we enter the twenty first century weare beginning to learn more and more about the composition of the Earth. Early predictions have thrown up some rather strange and peculiar thoughtsas to what is making up our Earth, but now day?¦s scientistscan be confident that the Earth is made up of what they think.
As fromexperiments and other sources of information a picture to what is reallydown there is becoming much clearer. So how do these scientists know that theEarth?¦s sections are made up of different compositions, andhow do we know that the physical state of each layer is what it is?The outmost layer of the Earth is the crust,this is what we stand on and covers the earth entirely. It is made up ofmany different rocks and minerals, we know that the composition of theEarth?¦s crust is generally the same due to the mines andboreholes that humans have made down into it. Mines that have been duggo down and still bring up valuable minerals that can be found just asclose to the Earth?¦s surface. The deepest goes down around3km into the earth, and the temperature is 70?XC, the only way forminers to work is because of the air conditioning, and still the type ofrock looks the same all around.
Also boreholes that have been drilled asfar as half way into the Earth?¦s crust bring up rocks thatlook very similar to the ones on the surface. So scientists can safelysay that the Earth has a crust which is very similar in composition allthe way down until the mantle is reached. When earthquakes happen they produce twotypes of waves P-waves and S-waves. Primary waves (p-waves) are the fastestwaves, they travel away from a seismic event. Primary waves are longitudinal,they can travel through solids, liquids and gases.
The secondary waves(s-waves) travel slower than the primary waves, and are traverse waves. This type of wave can only travel through solids. Measuring these wavesis called seismology. Scientists have known for a long time thatthe lava, which comes out from volcanoes when they erupt, was from themantle. The asthenosphere is the probable source of much basaltic magma,this is because the velocity in S-waves is slowed down and partially absorbedin the asthenosphere. This gives the characteristics that the waves arepassing through a solid, which the mantle is, but that contains a smallamount of liquid.
Also when the volcano erupt occasionally they shoot outsolid nodules that have come from the solid rock in the mantle, the so-calledplumbing of the mantle. These rocks have been broken of and carried outwith the flow of the lava, this type of rock is called peridotite and iswhat mostly makes up the mantle. It is a fairly recent discovery to provethat the mantle of the earth is not the only part of the interior. Seismologyis a new discovery this century that enables observations of natural groundvibration signals, basically the study of earthquakes. It can also be thestudy of artificially generated seismic signals.
Scientists then started to record thesesignals from earthquakes using seismographs, which are set in stationsaround the world to record the signals. In all some 125 stations existaround the world. They noticed that the P and S-waves did not travel ina straight line through the Earth, they came to the presumption that theEarth?¦s mantle was made up of many different materials. Thiscaused the P and S-waves to travel at different speeds, because of theway the materials conduct the waves at different velocities.
The wavesalso bent as they went from layer to layer, this caused their path throughthe interior of the earth to be curved. This was then put into practice, so afteran earthquake happened in a country the P and S-waves were sent out. Furtheraround the globe the waves were expected to be received after a coupleof minutes. Which they would, first the P-waves came followed by the S-waves,with an interval time in the middle. This time could then be scaled upto give the results of what they though would be the times for the P andthen the S-waves to arrive at other destinations.
This theory was correct,further away from the point of the epicentre, first the P-waves arrivedfollowed by the S-waves a few minutes later. So then they thought thatthis would be the case for all over the globe, but they found out somethingelse. They tried to predict the time they expected the waves to reach adestination on exactly on the other side of the globe, so they scaled upthe time interval between the two waves arriving. First the P-wave arrived,on time as they expected but the S-wave didn?¦t, this wasbecause the P-waves can travel through any physical state. However S-wavescan only travel through solids which is why they can pass through the mantle,so a change of physical state must happen in the middle of the mantle somewhere.
The area where the S-waves enter and do not come back out is called theshadow zone. The P-waves also have a shadow zone. This would be from about105?X to 142?X marked from the Focus of the earthquake. Thisis because when the P-waves enter the core they are bent downwards, theyare then bent down again when they leave the core-mantle boundary. So nowaves can emerge at the surface before 142?X. From these results scientists are convincedthat inside the mantle there is a molten core that must be blocking outthe S-waves.
So scientists have very good evidence toprove what they believe to be inside of the mantle. They even have theirways to prove what they believe the core to be made up of, they think thereare two layers, an inner core surrounded by the outer core. The outer coreis believed to be made up of liquid iron and the actual centre of the coreis made of solid iron. They have numerous reasons to back up thistheory.
Scientists can work out how big the massof the earth is, not by trying to weigh it because that is merely impossible. Instead they used the gravity on the earth to help them. We know the velocityat which objects fall to the earth, so from this scientists were able towork out the mass of the earth. So they found the total mass of the earth,and compared that to a mass made up of just the crust and the mantle.
Theycould get this mass reading because they know the density of the crustand the mantle. However even after working this mass out the total amountwas well short, compared with the mass they found from using the velocityat which objects fall to the earth. So they were convinced by these resultsthat the mantle was not the only thing down there, they put the lack ofmass to believing that something heavier and denser was in the middle. When the earth formed it originally condensed,by gravitational attraction of cosmic dust and gas. The continuing contractionof these materials caused them to heat, as did some of the radioactivityof some of the heavier elements. As this progressed the earth became veryhot and it began to melt.
This caused the different layers to form in theearth, because all the lighter materials moved up to the surface to producethe crust. So the heavier materials like the metals iron and nickel sankto form the core, the materials in the mantle were made form the silicatesthat didn?¦t sink or float. When meteorites reach the surface of ourplanet before they are entirely consumed, they provide us with valuableinformation. Meteorites are believed to be fragments from other planets,formed some 4. 6 billion years ago. Around the same time as the earth wasbeen formed.
These meteorites are mostly made up of iron, this is whatexcites scientists. As they believe that this is proof that the middlecore of our earth is made from solid iron. Thinking that meteorites arepart of planets that have broken up and sent fragments flying out intospace. So they know that the inner core is solidiron but why when the outer core is a liquid iron.
Well as the depth increasesin the earth then so does the pressure. So scientists put the solid middledown the fact that the pressure becomes too much for the liquid iron, sothe pressure solidifies the core. Another point of evidence is the earth?¦smagnetic field, again suggesting an iron core, because iron is a metalthat can be magnetised. The magnetic field is thought to be in the liquidouter core, because of the readily movement, and iron been a good conductor. This is what may be required for a dynamo with the capacity to generateenough current to produce the earth?¦s magnetic field. Theliquid iron is thought be stirred in a motion by heat from the core.
Thisaction is thought to produce an electric current and therefore the magneticfield. This is another explanation as to the outer core been liquid, becauseif it wasn?¦t then where would the magnetic field come from. As permanent magnetism cannot be kept with temperatures exceeding 500?XC,however the outer core may well produce electric currents because of thefree movement. The inner core would not be able to do this, as it is asolid.
