Why are the continents wider at the north and tapering towards the south?
The fundamental unit that makes up the continental land masses is called a craton. Basically a craton is a chunk of crust that is small enough to resist being broken up by tectonic forces, so it has existed since continental crust first formed. The continents we see today are not fundamental units, but instead are made up of lots of cratons that have aggregated. For example Africa is made up of five cratons of essentially random shapes and sizes.
The continents we see today originated when the supercontinent Pangea broke up. The breaks happened at the edges of cratons, but exactly where the breaks happened was effectively random. It would have been impossible for a Paleozoic geologist to predict exactly which cratons were going to end up in which continent.
The point of all this is that the shapes of the continents are random accidents related to exactly how the tectonic plates were moving when Pangea broke up. Even if it's true that all continents are wider at the top then the bottom (which is debatable) this is by chance and there is no deep underlying significance to the observation.
Plate Tectonics is the planetary process responsible for the distribution and shape of continents and ocean basins across the Earth's surface. It is the unifying physical process by which the modern earth sciences explain virtually all observations of continental drift, structural geology, physical geography, seismology, and so on. Because it is the fundamental process explaining physical geology, and because Plate Tectonics is the accepted scientific term, a search for this term should provide many good references.
The Plate Tectonics article on Wikipedia is a good starting point. However, I say that it is now incorrect to refer to Plate Tectonics as a theory. It is more an observed fact now than a theory - because the major plates are now known and have been mapped.
Based on an answer given in a similar questioned posed on the Guardian's reader question submissions, I disagree with the current leading answer which states that there is no significance to the observation. It's very stark when you look at a map, and I don't think it can be reasonably ascribed to chance.
On the Guardian site I linked, a user "palfreyman" gave a great example. If you imagine a previous supercontinent like Gondwanaland, which was roughly circular, and it broke up at a point on the surface, the pieces would have to taper to that point. The easy way to see this is that if you start out with a pie and cut it into slices, then the slices will taper towards the center. In the case of Gondwana land, the pieces broke off at the bottom edge, like half of a pie. The slices taper to the point of separation.
The principle then is that the splitting occurred with the pieces breaking off moving towards the equator. I'm not sure why that happened, but given that it happened it follows that the pieces will be tapered towards the South.
The way how craton formed togheter in the shapes we recognize today as continents , depend on the viscosity of liquified sub layer of the earth, centrifugal and centripetal forces applied and gravity pool from the moon. I hope this helps.
I think it has something do with spinning of Earth on excises which is north and south. When we spin some semi solid mater come near the surface. Moreover the Earth is shaped like pear narrower at bottom.