Few places on earth outside of the polar latitudes are as harsh and desolate as the Sahara desert. Covering about 10 million square kilometres, the desert is the similar in size to the United States or China. It consists of a diverse range of (extremely dry) landscapes, including sand dunes, bare rock, and gravel. Very few people live in the interior of the desert.
Despite the extremity of the desert, like most landscapes on Earth, it is an ephemeral feature on a geologic timescale. In the not-too-distant geologic past, the desert didn’t even exist. In this article, we’ll take a look at the fascinating debate over the age and origins of the Sahara.
By analyzing ancient pollens deposited in a permanent desert lake and plant leaf waxes deposited on the floor of the Atlantic by ancient winds, scientists have determined that the desert experienced a short period of relatively wet conditions for a few thousand years after the end of the most recent ice age. This period of higher moisture owed its existence to a strong monsoon season, which in turn was caused by periodic changes in Earth’s orbit combined with climatic effects related to the melting of the ice sheets. The region wouldn’t have been a verdant forest by any means – instead, it would have resembled the savannahs of modern East Africa.
This period of moisture, although important for ancient humans, represents a tiny anomaly in the history of North Africa. Geologic evidence suggests that, for the past several million years, the Sahara was typically very dry – sometimes even drier than it is today.
Although scientists know that the Sahara is very old, they still debate when exactly it formed. Abundant geologic evidence suggests that the desert has existed for the majority of the past 3 million years. However, the discovery of a 7 million year old fossil sand dune has raised the possibility that the desert is much older. To assess the likelihood of a widespread desert existing in North Africa 7 million years ago, a team led by Dr.Zhongshi Zhang used a computer model to simulate the past climate of the region.
The results suggested that the initial formation of the Sahara seems to have coincided with the closing of the Tethys sea, an ancient ocean between Europe and Africa that continues to shrink to this day (it has become divided into the Mediterranean, Black, and Caspian seas). The shrinking of the sea may have had an impact on the monsoonal climate of the region, cutting off essential moisture supplies.
Although the model results seem to explain the origins of the desert, many geologists won’t be satisfied until the model is backed up by direct geologic evidence collected in the field.