The Dune movies remind us how beautiful, mysterious, vast and variable sand dunes can be. For centuries these wondrous landforms have filled people with awe and, in some cases, fear and foreboding because of the obvious remoteness and risks associated with the deserts with which they are synonymous.
That’s what first drew me to researching deserts and dunes more than 40 years ago, and I’ve been studying them ever since. Here are five things I learned that might surprise you:
Not all dunes are made of sand
Ash, snow, and even gypsum can form sand dunes. Dunes develop when small particles are moved by a moderate wind over bare dry surfaces and accumulate where movement is slowed by an obstruction or surface undulation. Where the wind deposits particles, it can create a small mound where other particles also accumulate, eventually forming a dune.
“Sand” is not actually a material; It has a particle size between 0.06 mm and 2 mm in diameter. Sand dunes in deserts and along coasts consist primarily of grains of quartz and feldspar, the most common minerals on Earth.
But in volcanic areas such as the interior of Iceland, dunes may form from ash, while in central Antarctica, the driest and windiest continental world, dunes may form from ice crystals and snow. In New Mexico, USA, very soft and shiny mineral gypsum forms sand dunes; The name of this place is accordingly White Sands.
Sand dunes may record history of climate changes
Sand dunes may appear gentle and variable, but beneath their active surfaces lies old sand that often tells a story of long-term development.
Dune shape is affected by how variable the wind direction is throughout the year: some dunes, such as the crescent-shaped barchan dunes, roll forward under fairly consistent winds, with the sand turning regularly. Others, such as linear and star dunes, develop where wind directions are more variable and accumulate sand up to tens or even hundreds of meters thick.
Using a technique called luminescence dating, we can measure how long dune sand was hidden from sunlight, identifying periods when dunes stopped forming and when soil, now buried under more sand, developed on dune surfaces in wetter climates.
In the Rub’al Khali desert of Arabia, for example, giant linear sand dunes have formed during many dry periods over the last 130,000 years. The dunes may even be much older, because it has not yet been possible to drill to the bottom and determine the full depositional history.
Only one fifth of deserts are covered with sand dunes
Only one-fifth of desert areas have the right conditions to form sand dunes: a supply of fine, loose sediment, sufficient wind energy and a lack of protective vegetation. Other common desert landscape features include mountains, rock slopes, gravel surfaces, and dry lake beds.
Yet we can go beyond today’s deserts and find evidence of more widespread dune landscapes, for example, beneath grasses and woodlands in some savannah regions such as Africa’s Kalahari, and even beneath tropical rainforests in parts of South America. These sand dunes testify to different desert and climate patterns in the past.
Scotland’s ancient sand dunes changed history
In the 1780s, Scottish geologist James Hutton realized that the well-stratified and distinctive red sandstones at Siccar Point on the east coast of Scotland were actually the preserved remains of ancient desert dunes. At this location, the older red sandstone of the Devonian period, as it is now known, suddenly appears above the fine gray mudstones.
Hutton realized that a considerable period of time (we now know more than 65 million years) must have passed between the time the gray rocks were flattened by erosion and the red sands were deposited on top.
His careful theories formed the foundations of modern geology and our understanding that the earth is much older than the date calculated from biblical texts. Further developments in the 20th century enabled us to explain why rocks formed under desert conditions were found in an unexpected Scottish context; We now know that this is caused by movements of the Earth’s crust or plate tectonics.
Coastal dunes provide protection against storms
Sand dunes surround large areas of the world’s coastlines, composed of windblown sand derived from the drying intertidal beach zone and trapped by coastal vegetation. While only 7% of the British coastline is fronted by dunes, 40% of Australia and 60% of Portugal are fronted by dunes.
These dunes play a vital role in protecting low-lying land from tidal surges and storms. But in some areas, sand extraction for human recreation and construction has degraded the dunes, damaging stabilizing vegetation and causing eruptions; Sea level rise has added further risks.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
David Thomas receives funding from NERC and The Leverhulme Trust. He is also affiliated with the University of Witwatersrand in South Africa.