The Sahara desert is the largest desert on Earth.  The Sahara Desert has a prevailing wind pattern.  The wind almost blows the same way.  The hot wind of the desert cooks the trees at the edge and they die and that area becomes desert.  Once the soil is sterilized, it is done for.  Deserts grow on themselves.  This process is called desertification.  The Sahara desert is 1300 miles North to South.  It grows 4 miles per year.  

The Sahara Desert has a prevailing wind pattern.  The wind almost always blows the
same way.  This causes the desert to grow. This process is called desertification.

Scientists estimate the desert to be about 4000 years old. 
See: Potsdam Institute for Climate Impact Research in Germany. July 15, 1999 Geophysical Research Letters.

If the Earth is billions of years old, why don't we have a bigger desert someplace? 

Why is the biggest desert in the world only 4,000 years old? 
(Ans: The Biblical flood about 4,000 years ago!)

Not enough mud on the sea floor

A relatively small amount of sediment is now on the ocean floor, indicating only a few thousand years of accumulation.  This embarrassing fact is one of the reasons why the continental drift theory is vehemently defended by those who believe in evolution. 

Not enough sodium in the sea

The oceans are getting saltier.  If they are billions of years old, they would be much saltier than they were now. 

Oil underground!

The rock encasing oil deposits could not withstand the pressure for more than a few thousand years.

Erosion

The erosion rate of the continents is such that they would erode to sea level in less than 14,000,000 years, destroying all old fossils.  (American Science Vol 56 p356-374)