(c) Now, let\'s put in some numbers: consider a spreading rift on the sea floor
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(c) Now, let's put in some numbers: consider a spreading rift on the sea floor which is 1 cm of new crust per year to either side of it. If the rift is 1000 km long, what is the total area of new crust created by the rift in 200 million years (in square kilometers)? What creating i fraction of the Earth's crust's overall area is this? (d) Explain how the magnetic fields "frozen" into rocks of different ages can be used to determine the orientation of Earth's magnetic field at different times in the past.Explanation / Answer
c) crust formation 1c.m/yr on each side of rift
so total crust formation on both sides=2c.m/yr
length of rift=1000k.m=10^8c.m
total area of crust formed per year =2*10^8 c.m^2
time period covered=200 mys
so total crust area in 200 mys=200*10^6*(2*10^8)=4*10^16 c.m^2=4*10^6 k.m^2
earth's area =4*3.14*(6371)^2
the estimated area is propertion of earth area= (estimated area/earth's area)*100=0.78%
d)The crust is formed by cooling and crystallisation of magma . The magma contains minerals which are various types of magnetic substances but when the present as the magma form due to curie tempeature the minerals don't have any magnetisation . but with cooling and crystallization the minerals regain their magnetization below the curie temperature . So the magnetisation is stored in the minerals in orientation of the then earth's magnetic field .These magnetism remains as thermal remenant magnetism .
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