Thursday, 30 July 2020

Where Did Stonehenge's Sarsens Come From?

Where Did Stonehenge's Sarsens Come From?

I came across THIS ARTICLE and then a correspondent sent me the link to the ORIGINAL SOURCE. Stonehenge is a never ending subject. These articles are well worth looking at.

Sarsen stones are post-glacial silcretes - quartz sandstones with a quartz cement and are - almost - 100% silica. The authors discuss the place of origin of the sarsens. Almost all of them have an identical trace element composition and they use this to identify a place with an identical chemical signature. 

This is West Wood, 25 km to the north of Stonehenge. As some of the sarsens weigh about 30 tonnes, this would not have been a trivial undertaking!

Wednesday, 22 July 2020

The East African Rift System and Landslides

The East African Rift System and Landslides

When I think of the East African Rift System, landslides are not the first thing on my mind. THIS ARTICLE from one of my most prolific sources, Earth-logs, may make me change my mind. 

The "Rise to the Rift" was much mentioned many years ago. We now know what causes it - bulging of the continental crust resulting both from its unloading by thinning along the rifts and the buoyancy conferred by high heat flow in the mantle beneath - but the result is landslides.

The East African Rift System (Credit: P.C. Neupane, M.Sc thesis 2011; Fig. 1)

There is enhanced erosion, both into the rift and outwards, especially onto the the Indian Ocean continental shelf. Recent oil exploration work, offshore Tanzania, has illuminated this.

In particular it has shown the presence of a vast submarine landslide, called (intriguingly) the Mafia mega-slide. It is thought that it was caused by a large earthquake and would have caused a large tsunami.

Seismic reflection profile parallel to the Tanzanian coastline with the Mafia mega-slide highlighted in green (Credit: Maselli et al. 2020; Fig. 5)

You can find details of this work HERE. The main thrust of the paper is using the records of offshore sedimentation to fix the dates of the rifting and to note the dangers of future submarine landslides.

Tuesday, 21 July 2020

Active Volcanoes on Venus?

Active Volcanoes on Venus?

THIS ARTICLE in a newspaper tells of an interpretation of ring structures on Venus, found using radar, as being volcanic in origin. They call them coronae structures - the trench surrounding the peaks. The researchers think the coronae are fields of lava flows and major faults.

They suggest that such structures can only be produced by volcanic activity. And many of them were active in the last few million years.

Two coronae on the surface of Venus which are formed when hot material from deep inside the planet rises through the mantle. The black line shows a gap in the imaging data. Photograph: Laurent Montesi/Reuters

The newspaper article is based on a paper in Nature Geoscience, which you can read HERE