The previous post on Venus described a peculiar type of volcanic construct called a corona, unique to Venus. This blog is read by knowledgeable people with a somewhat critical attitude to authority, while at the same time having a strong respect for experts [For UK readers: see footnote], and this statement was immediately questioned. A structure was uncovered on Earth with showed a remarkable similarity to Venusian coronae: Richat’s structure, also known as the Eye of Africa. Could this be an interplanetary corona? A Venusian refugee hiding in the Sahara?
To deal with the most important point first: the word corona comes from Latin, and means crown, or garland for those crowned with fewer financial resources. I couldn’t take Latin at school, as that was on offer only to the brighter buttons with linguistic ability (my strength was more on the mathematical side). But if you can’t trust a non-expert, who can you trust? As a Latin word, the plural of corona is coronae, unless the word is used in the accusative, when the plural becomes coronas. Thus, you can say coronae are beautiful (nominative) or This post discusses coronas (accusative). In the dative, the plural is coronis: Plumes give coronis their rings. Luckily the English language has lost these distinctions (an echo still remains in the word ‘them’). The grammatical complexity may be why that show is called Games of Thrones rather than Crowd of Crowns. It may be best if readers of this blog don’t go looking for further coronas on Earth: while there is only one (suspected) corona, coronal grammar is a lot simpler. I will use ‘coronae’ for multiple coronas.
(Don’t be fooled by some English words. ‘Conundrum’ is a made-up word, so its plural is ‘conundrums’, not ‘conundrae’. ‘Octopus’ is from Greek, and its proper plural is octopodes (not octopi – it isn’t from Latin) but this is so awkward in English that it makes even ‘octopusses’ sound acceptable.) (And no, I do not know how English lost the plural of sheep.)
Coronae – the story of Venus
Venusian coronae were first seen in 1977, in radar images taken from Earth. Valeri Barsukov gave them their name and first identified them as volcano-tectonic features. Coronae are circular or oval, 75-2000km diameter, with concentric and radial fractures around a central depression with an inner plateau. Their interior can be above the surrounding the plain or below it. Artemis provides a spectacular example: 2100 km across, and criss-crossed with fractures. The numerous fractures may be magma dikes. Several ‘small’ volcanoes occur inside the corona, but don’t be fooled, the largest of these is only a little smaller than Mauna Loa. In between the double ring fracture is a deep trench, reaching 4 km below the surrounding plain. It is a funny feature, almost like a circular continental rift.
The rift-like structure and the large size suggest that Artemis sits above a mantle plume. A model for this was presented by Teras Gerya (Zurich). It assumes that a mantle plume begins to melt into a thick crust and sets up convective cell within the melted crust. This convective cell has an up flow in the centre, and a down flow around the edges. The cell is as wide as the plume head, which can be 1000 km across or more. The upflow pushes up the surface over a very large central area. The down welling gives a depression or trench around the edges. The stress causes extension, and fractures develop, filling with the crustal melt. The main melt stops below the surface, but some shield volcanoes do develop.
Once the plume cools, the central area drops down again. The model does not address the question why more of the magma does not reach the surface: when do you get a corona with some volcanoes inside, and when do you get flood basalt covering half the planet?
The model also can create a structure called a nova. A nova is a star-like pattern on the surface of Venus, with numerous radial cracks extending from a small central rise or depression. In the model, these are the early phases of the formation of a corona. I’ll come back to this later.
Other models have been proposed. Early on, it was thought that coronae were impact craters. Their sizes are similar to the maria on the Moon, for which an impact origin is well determined. But that was very quickly abandoned when better images showed a raised inner plateau, complex tectonics and internal volcanic features. Impact craters also do show at most two concentric rings. An indirect relation to impact craters has also been suggested, where an old large scar has weakened the crust allowing plumes to push through. But impact craters would be randomly distributed on the surface. Coronae are not.
It has been suggested that the outer trench is an arc-like subduction zone, making coronae similar to island arcs on Earth. A Venusian Carribean. But radial fractures often run through the coronal trenches, and that would not happen if they were subduction zones.
Ring-dike intrusions have some similarity to coronae and it has been suggested that coronae are oversized ring dikes. But ring dikes do not show the outer depressions that many coronae show.
Drifting plates and the battle against mantle plumes
This leaves one question unanswered: why coronae are so common on Venus (513 are known) whilst absent on Earth? Mantle plumes happen on both planets, after all, and Earth has some thick crust in the old cratons. Kimberlite plugs form when mantle melt punches directly through the crust, and these are not uncommon on Earth. Are Kimberlite plugs micro-coronae? Or a mini-nova?
An interesting suggestion is that on Earth, continental drift stops coronae from forming. The idea is that a corona needs a long time to form, perhaps 100 million years. In that time, Earth’s continents move and the same point does not stay above the plume. Only Africa is currently near stationary, and that has only been since the last 30 million years. Instead, on Earth a mantle plume develops into a hot spot trail. Put the plume underneath the same bit of continental crust for 100 million years, and perhaps you would get a corona. If only these continents could stay put long enough.
Are coronae really unique to Venus? The New Horizon images of Pluto have shown an amazing scenery, but the images of its moon Charon have not received as much attention. Right at the rim of the main image of Charon, a deep long chasm is seen, perhaps 10 km deep (it is hard to tell) and 700 km long. It may well be round: it curves behind the horizon. Could this be a corona? It has the right size, and the visible part has the right structure. Too much is hidden to be sure, though.
The Eye of Africa
Richatt’s structure, 40 km diameter, is located in the sand deserts of Mauritania, in the western Sahara. It was discovered from space in the 1960’s. The ring is too large to be easily visible from the ground: you can stand inside it and not see it. It is hard to recognize that a low, 10 meter wide ridge is a circle if you can only see a small part, and it stretches beyond the horizon. The largest impact crater in Germany was also only discovered from above, although there is a town in it.
The 100-million year old structure consists of three rings, dipping outward. The main rocks here are sedimentary (the blueish tint is salt). Two of the rings are mafic ring dikes, which seem to have formed from the same magma chamber. The inner mafic ring has a gap, caused by a maar explosion which ripped through it. The outer ring has a slight offset in the north due to a fault. There are volcanic rocks, but the main structure is not volcanic and the volcanic activity was only near the centre. The remnants of two craters are here, each about 2 km diameter, on either side of the centre. The craters are largely buried under the sand. Both are considered maars; they formed after the main structure. The rocks in the centre show that they have been extensively affected by hydrothermal activity. To the north, just outside the outer gabbro ring, is a kimberlite sill. It has the same age as the main structure (give or take a few million year) but there is no clear relation. The central area has a large number of fairly short, radial dikes, containing carbonitite.
The structure is considered an uplifted but eroded dome. It formed 100 million year ago, around the time the Atlantic Ocean began to open. The area has multiple sedimentary layers, mostly marine from the continental shelf but some formed above water. The layers are vertically offset near the centre, indicating a collapse has occurred there.
So what happened? About 100 million year ago, heat from below causing doming, causing the various sedimentary layers to bulge up. Two ring dikes formed, filled with mafic mantle magma, supplemented with some crustal melt; the ring dikes stayed underground and these did not break the surface. The radial carbonitite dikes probably formed next, from melted sediment mixing in with the magma, but these also remained below ground. Water percolated through and a phase of hydrothermal activity began. The heated water dissolved much of the sediment, especially the lime stone, and caused collapse of the central area. The two maars also dated from this time, and this may have been the first volcanic activity on the surface. Later, the remnant of the dome underwent deep erosion. The harder layers survived better and now stand above the softer sedimentary layers. Sand filled in the craters, but left some (but not all) of the rings visible. It is almost perfectly circular because the dome was beautifully symmetric, and the sedimentary layers were undisturbed and horizontal, a beneficiary of Africa’ solidness even while the Atlantic was opening. The role of the kimberlite sill is a big unknown in the process: was this related, or was its appearance at roughly the same time a coincidence?
The evidence for doming in the Eye of Africa seems to set it apart from the coronae. But it is akin to another type of volcanic feature on Venus, closely related to coronae. Novae are large domes with numerous radial fractures, and a central depression. About 60 are known on Venus. In contrast to coronae, they lack concentric rings. Typical sizes are 50-100 km but some are larger.
Novae are considered an early form of a corona, something that may continue to develop into a full-blown corona, or it may form when the plume is smaller or the crust thicker, preventing the formation of a full corona. Novae don’t look like Richat’s structure because of the lack of rings. But take a nova and apply erosion, something that does not happen on Venus, and the result could be revealing. The erosion could remove some layers, leave others, causing the tilted layers to form concentric rings. This is what a nova on Venus may have looked like had the planet kept its water. Thus, Richat’s stucture may be a nova on Earth: Terrae Nova.
Is Richat’s structure a corona? No. It is too small, and it formed mostly from sediments, ring dikes, and erosion. There are similarities and both did form from volcanic heat from below which failed to reach the surface, but Richat’s structure never developed into a corona. Continental drift may be to blame. The structure is more akin to a Venusian nova, a dome which failed to become a corona.It is a beautiful feature, the biggest eye on Earth. But it ain’t Venus.
 Footnote for UK readers: exactly the opposite to Michael Gove, who favours authority but told the electorate they shouldn’t believe experts. In his defence, he was minister for education.