Between a Rock and a Hard Place

Science snap (#31): Mammatus clouds

Elspeth Robertson July 21, 2014

After all the thunderous weather this weekend and being British, I thought I’d do a weather themed science snap. Don’t bolt yet; it’s a volcanic-weather themed!

Volcanic mammatus clouds forming after the eruption at Mount St. Helens. Copyright: Douglass Miller

This is a picture of mammatus clouds following the eruption of Mount St. Helens in 1980. These clouds are pretty rare, unusual and distinctive. Formally, the Glossary of Meteorology defines mammatus clouds as “hanging protuberances, like pouches, on the undersurface of a cloud”. The definition is aptly descriptive, but in essence mammatus are a series of bulges at the base of clouds, often under large thunderous cumulonimbus clouds. There are many different types of mammatus clouds, each with distinct properties and occurring under various cloud types, and mammatus in volcanic clouds is just one subcategory. There are relatively few documented occurrences of Mammatus under volcanic clouds. Apart from Mount St. Helen’s, they’ve been observed at during the eruption of Mount St. Augustine on 27–31 March 1986 and Mount Redoubt on 21 April 1990. No generally accepted formation for these mechanism exists, however it is clear that a sharp temperature gradient and a wind shear across the cloud-air boundary is needed to create these clouds. Did anyone spot Mammatus clouds in the UK this weekend?

Science Snap (#29): African Fairy Circles

Elspeth Robertson July 14, 2014

Mysterious Fairy Circles dotting the Namibian grasslands. Credit: Neurgens

If you’re wandering among the arid desert that stretches from Angola to South Africa, you may notice the ground pot-marked by millions of circular barren patches. These striking features are known as “Fairy circles”, and can grow up to 15 meters in diameter. Tall grasses often surround these circles, further accentuating these miniature crop circles. How these Fairy Circles form is hotly debated. Theories have to account for their non-random location, and a lifespan 30-60 years where they grow in size borefore grassland eventually invades them again.

Oral myths of the Himba people attribute the circles to gods and spirits and traditionally they are thought to have spiritual and magical powers. My favourite myth, however, is that these Fairy Circles are scars in the landscape where a dragon breathed out toxic gases.

Unfortunately, a scientific explanation likely exists. In 2013, Juergens published an article supporting a popular theory that sand termites are responsible. The paper states that sand termites form the circles by eating grassroots to expose the soil. Once exposed, the soil more easily absorbs, which ultimately helps maintain the grasslands in extremely dry conditions. However, the sand termite theory has been critiqued for assuming correlation with causation, as Juergens suspects that termites are responsible because they’re the only species consistently observed at the circles. Thus, sand termites were identified by a process of species elimination.

In contention to the termite theory, Cramer and Barger (2013) believe that Fairy Circles are a consequence of natural competition in grasses. They suggest that landscapes with a mixture of grasses form “self organizing” circles due to underground competition for water resources. Both hypotheses have aspects that remain inconclusive, thus no theory currently prevails. They currently struggle to explain why circles appear across a variety of regions, soil and vegetation types and furthermore, no one has ever observed termites gnaw out a circle. Perhaps there is still room for a supernatural cause after all.

Science Snap (#28): The Eye of the Sahara

Elspeth Robertson July 7, 2014

The Eye of the Sahara. Image credit: NASA

Surrounded by thousands of square miles of ubiquitous desert, the “Eye of the Sahara” peers out from the Earth’s surface and at nearly 50 km wide, its easily visible from space too. The “Eye of the Sahara” is known as a Richat Structure, a geological feature consisting of a series of alternating circular layers of sedimentary, igneous and metamorphic rock, exposed by erosion.

The “Eye of the Sahara” is located in central Mauritiana and is also known as Guelb er Richat. The sheer size of the Eye meant it wasn’t discovered until space exploration took off. So here’s a challenge, find it on Google Earth.

The “Eye of the Sahara” was formed by a magmatic intrusion, which forced its way up and warped the overlying rock layers into a dome shape. The intrusion initially never reached the surface, but now erosion has effectively sliced dome’s top off, exposing its inner structure.

The Eye is extremely symmetrical, a striking feature that led scientists to interpret it as an impact crater. This idea was dismissed, however, when scientists began researching its structure. Nevertheless, scientists still can’t explain exactly why the Eye is so symmetrical.

The layers of rock inside the eye are visually distinct as each varies in colour, composition, and resistance to erosion. Inside the Eye there is a rich variety of geological rocks, including rhyolites, gabbros, carbonatites and kimberlites. Quartzite layers are highly resistant, but breccias and volcanic rocks are more prone to weathering and erosion. Intrusive kimberlite plugs beneath the Eye suggest the presence of deep and large alkaline magmatic intrusions and was likely responsible for uplifting the Eye.

Science Snap (#28): Brandberg Massif, Namibia

Elspeth Robertson June 26, 2014

The 120 million year old Brandberg Massif, Namibia. Image credit: NASA

Brandberg Massif is Namibia’s highest mountain, but if you look from above, you’ll notice it’s no ordinary one. Brandberg is a single mass of granite that pierced its way through the Earth’s crust into the Namib Desert. Looking at the Landsat 7 image, Brandberg is a circular dark and steep-sided mountain, imposing itself over the desert below. It reaches height of 2.5 km and stretches across 31 km.

Nowadays, the landscape is geologically quiet but the Brandberg intrusion formed over 120 million years ago and marks a period in the Earth’s history where volcanism was rife due to the break up of the supercontinent Gondwana. The majority of the Massif is composed of homogeneous medium grained biotite-hornblende granite. However, to the west there is a 2 km diameter pyroxene-bearing monzonite and in the south it is crosscut with arfvedsonite granite dykes and sills. As the Massif protrudes from the landscape, it influences local climate by drawing in the rains. The rain then percolates through the granites and washes out through springs.

Apparently, if you reach the granite for sunrise or sunset, you’ll see it glow red under the suns rays. Appropriately, the locals call it Daures, “the burning mountain”.