Earth s vital statistics Shape: almost spherical Size: 6400km in radius Average density: 5.5gm/cc; surface: 3gm/cc or less; centre may be 10-15gm/cc 15gm/cc Temperature: core: 2200-2750 2750 o c Pressure: core: 3-43 4 million times the atmospheric pressure at sea level Origin and probable age: thought to have condensed and congealed from a nebula of gas, dust, and icy comets about 4.6 billion years ago. The oldest rock: 3.96 billion yrs old: Earth was forming continental crust nearly 400 billion years ago!! As Earth solidified gravity sorted materials by density: heavier substances such as Iron gravitated slowly to the centre and lighter elements such as silica slowly welled upward to the surface to be concentrated on the crust. Result: interior is in concentric circles each of distinct chemical composition and temperature Heat from the interior migrates outward from the centre by conduction and by convection through the different layers some of which are more fluid/ plastic 1
More than a quarter century after humans first set foot on Moon, the deepest boreholes have penetrated barely 12km into Earth s crust: that s less than 1 /500th of the distance to the centre of Earth!! However, it is established that the interior of the Earth is layered! Evidences of the internal structure is obtained by using Earthquake waves, called the Seismic waves The operating Principle: The speed of an earthquake wave is proportional to the density of the materials through which it travels. The denser the material the faster the speed Seismic waves also change direction under certain circumstances through refraction (bending) and reflection Plastic zones simply do not transmit some seismic waves: they absorb them! These distinctive ways help Seismologists to deduce the structure of Earth s interior 2
Since seismic waves spread out from their source just like ripples on a pond, they get weaker the further you get from the earthquake. http://www.seismo.unr.edu/htdocs/links.html http://www.seismo.unr.edu/ftp/pub/louie/class/100/effectskobe.html A Seismograph 3
What do the waves tell us? P waves travel through solid as well as liquid S waves travel only through solid Waves are reflected or refracted according to the density of the material through which they travel P waves are faster than S waves There is a distinct S waves shadow zone There is also a P wave shadow zone where they are significantly weaker P waves appear on the other side of the earth, but arrive much later How the waves travel: the X-Ray act 4
What can we interprete from these? P, S waves do not travel in straight lines inside the Earth P, S waves get curved as they gradually travel inward speed changes with depth indicating changing density Shadow zone: Earth s interior not uniform: Mantle 2900 Km thick Radius of Core: 3470 Km S waves do not travel through liquid: there is a S wave shadow zone: Earth s core is liquid Faint traces of P waves in the antipode shadow zone: the speed increases: refracted 4 times: must have travelled through DENSE MASS: Inner Core is very heavy, dense material: Nickel and Iron Earth s Interior has an onion-like structure! 5
Liquid Outer Core: Thickness: 2250 Km Solid Inner Core: Radius: 1220 Km Composition: Nickel and Iron (NIFE) in SOLID state (Because pressures are enormous: melting point temperatures are higher than prevailing temperatures) Composition: Essentially same as Inner Core but pressures are less: in LIQUID State Density of Inner and Outer Core: 12.5 g/cm3 (accounts for 5.5 g/cm3 for the planet Why IRON?? Almost the right density Composition of the Core: Av. Density of Earth: 5.5 g/ cm3 Av. Density of Crust: 2.7 3 g Av. Density of Mantle: moderately higher than crustal rocks CORE must be VERY DENSE!! In LIQUID state at the estimated temperature and pressure of the Core Abundant on Earth Magnetic qualities 6
Why NICKEL?? Meteorites contain IRON nad NICKEL The Core is actually a little less dense Composition of the Core: Av. Density of Earth: 5.5 g/ cm3 Av. Density of Crust: 2.7 3 g Av. Density of Mantle: moderately higher than crustal rocks CORE must be VERY DENSE!! The INNER Solid Core: Crystalline Iron and Nickel The Outer Liquid Core: Iron, Nickel, some sulphur, silicon Not that well understood!!! It is capable of flowing slowly Composition of Mantle: Lower Mantle: Thickness: 2230 Km State: Solid/ Rigid (Oxides of Iron, Magnesium, Silicon) Upper Mantle: Thickness: 670 Km State: Partially molten/ plastic (different from Lower Mantle) Contains pockets of molten rock: feeds magma chambers The uppermost part is solid. This solid uppermost part and the Crust on top is called LITHOSPHERE 7
So what s the result of the X-Ray act? Crust 30 Km Rocky/ Solid 8
Lithosphere: This is the Earth s outer layer Very important Thickness: of uneven thickness Some places: below 5 km Some places: below 40 km Rigid Plastic Lithosphere can move over Asthenosphere Heat sources deep inside Mantle keep Asthenosphere in motion: causes movements in the crustal layer Continental crust: lowest density: Silica and Aluminium (SIAL) Granite a common rock same density as landmasses Oceanic crust: higher density: Silica and Magnesium (SIMA) Basalt combined density: 3g/ cm3 The Crust: Varies in thickness Has cracks Is fragmented into segments We call these crustal PLATES Continental Plates: largely made of Granite (lighter) Oceanic Plates: largely made of Basalt (heavier) All these plates move over the plastic Asthenosphere 9