What fraction of the worlds total land mass is south America?

North America comprises 4.8% of the earth's surface, about 24 million square km. (24,221,000). It is 16.5% of earth's land area. The average elevation of North America is slightly less than the average of the seven continents, so its mass would be a little less than 16.5% of the world's total.

The average elevation North America is 720 meters. The continental plate extends into the earth below sea level, but what I am shooting for here is a decent approximation for the total mass.

The average density of compacted sand is 2080 Kg/m3. Sand is predominantly silicon oxide. There are both lighter and denser materials forming the land mass, but this figure should get us easily within an order of magnitude. Solid basalt is about 3011 Kg/m3, and granite is 2691 Kg/m3. Much of North America rests on basalt, but I'll use the density of granite for my average.

Now all we need do is the dimensional analysis. 24,221,000,000,000 m2 * 720 m/2691 Kg/m3 = 6.5 trillion Kg. North America weighs roughly 7 billion short tons.