The sinking of the ocean floor
Movements of the Earth's crust
Unequal heating of the atmosphere
Giant waves casued by the pull of the moon
Plant stems surround and squeeze rocks until they break
Plant roots grow into cracks in rocks, forcing them apart
Plant leaves protect the rocks from extreme temperatures
Plants fall into rocks and release their powerful chemicals
Deforestation of trees
Overcultivation of crops
Above-average rainfall and flash flooding
Subtle changes in the orbital path of Earth over vast periods of time
Rusting of minerals
Human factors have contributed to desertification over time.
Plate tectonics has shifted the location of the region over time.
No evidence for ancient deserts has been preserved in the rock record.
Global wind and ocean current cycles have evolved dramatically over time.
Pebbles and cobbles
Coarse grains of sand
Here's an interesting quiz for you.