Over time, starting early in Earth history, temperatures in the outer part of the mantle occasionally got hot enough to cause the rock to melt. Rather than melting totally, however, only some of the minerals in the mantle melted. As this process is repeated gradually very light materials accumulate as crust. When plates collide these very lightest rocks are not dragged down but are scraped off. This causes the crust to thicken.
Thicker crust stands higher than thinner crust and makes continents.
By burning coal, gasoline, gas, etc. we have put more CO 2 into the atmosphere than existed before. In this sense the increase in CO 2 may be considered pollution. Ozone is a relatively short-lived molecule. We can create it but what is created will not last long enough for us to transport it to the stratosphere. Ozone in the stratosphere is created as the energy of sunlight breaks down the O 2 molecules.