Why are parts of Africa and the middle east hot, dry, and covered in sand while areas further north and south have lots of trees and flowering shrubs?

Magma is outside the volcano  and lava is inside the volcano. 

Humid continental climate is found in the interior of continents between the 30 and 60 degrees latitude, but usually above the 40 degree line.   40 degrees north of the equator means less direct sunlight and therefore less warmth.   this climate is mainly found in the northern hemisphere since there are no major landmasses in the southern hemisphere.  if you live in northern indiana, you know all about humid continental because that is the climate type you have!