The Earth Story

Ol Doinyo Lengai

The Maasai call this volcano “Mountain of God” and it’s certainly an awesome landmark in a spectacular landscape. Looking like a child’s drawing of a perfect conical volcano, it rears up from the Great Rift Valley floor to an altitude of 2740m (8987ft) above sea level and 99 miles away from Kilimanjaro. In Tanzania, the walls of the Northern Rift are studded with huge volcanoes that are mostly inactive now. Lengai reminds us of what once was. Its steep sides have been eroded by rainwater run-off into deep fissures and canyons that, from a distance, form beautiful abstract patterns of light and shade. At the top, there’s an active crater divided from an inactive one by a high, sheer wall.

Lengai is a typical stratovolcano, formed from layers of lava and pyroclasts with a central vent. Its last major eruption was in 1966, but compared to the huge volcanic eruptions in Indonesia and the Andes that was a small though impressive affair. Since then, there has been minor activity, including explosions of pale grey ash from January to March 1983 and August to November 2007. Between those dates, there has been much excitement with black lava bubbling, oozing, and trickling in the active crater. Emerging at a temperature of 600C (1100F), the black lava is cool compared with that of other volcanoes. It is also rich in sodium carbonate, which turns greyish white as it cools, giving the top of the mountain an unusual “dirty snow” appearance.

In addition, the active crater floor has changed a lot over the last 20 years or so. Rounded, hollow domes of cool lava and ash cones as high as 7m (23ft) or more have risen and then collapsed. At night, some of these ash cones glow red in an eerie, otherworldly way. Steam jets smoking on the rim are the result of rainwater trickling down through the cracks in the rock and turning to steam as it reaches a heat source. Then it expands and, since it’s under pressure, it’s forced upwards again, escaping through small cracks in the surface.

~ JM

Image Credit: Ol Doinyo Lengai crater. Accessed from http://bit.ly/2exIKiT on 21/10/16

More info: YouTube Short Video: “Ol Doinyo Lengai Volcano Eastern Africa” http://bit.ly/2evhbYd

Marine minerals

Like a clump of sea urchins lurking in a coral cavelet, beautiful sprays of the zeolite mineral natrolite have grown within an old gas bubble cavity (known as vesicles) from the Deccan trapp basalts of India (see http://on.fb.me/1ByYtnX). Named in 1803 after the ancient name for sodium and the Greek lithos (for stone), it is a hydrated sodium aluminium silicate that forms as the cooling basalts stew in a mixture of their own juices and mineralised groundwater. For obvious reasons its colloquial name is needle stone. It can be melted in a candle flame, and will turn it yellow due to its sodium content. We shared another amazing Deccan geode with zeolite minerals at http://on.fb.me/1VqevbG).

Loz

Image credit:: Christian Brodmann

Benitoite on natrolite While we have covered California's state gemstone before (see http://on.fb.me/1KSaNnm), we just couldn't resist sharing this wonderful 8x8x5cm slab of deep blue crystals on the zeolite mineral natrolite. Their formation is complex, involving hydrothermal fluids passing through two separate rock formations lower down in the sedimentary stack, mobilising a variety of elementsalong the way and precipitating them as a rare assemblage of minerals. It is ultimately a child of tectonic metamorphism that occurred when chunks of crust and upper mantle were sucked down with the subducting oceanic plates off the west coast of America and bouncing back up to the surface due to their buoyancy. Loz Image credit: Exceptional Minerals

Marine mineral Like a sea urchin lurking in a coral cavelet, a beautiful spray of the zeolite mineral natrolite has grown on a bed of quartz within a geode cavity from the Deccan trapp basalts of India (see http://tinyurl.com/n2g8wt4). Named in 1803 after the ancient name for sodium and the Greek lithos (for stone), it is a hydrated sodium aluminium silicate that forms as the cooling basalts stewin a mixture of their own juices and mineralised groundwater. For obvious reasons its colloquial name is needle stone. It can be melted in a candle flame, and will turn it yellow due to its sodium content. The geode measures 9 centimetres across. We recently shared another amazing Deccan geode with zeolite minerals athttp://tinyurl.com/pd7ptgv. Loz Image credit: Joe Budd/Rob Lavinsky/iRocks.com http://www.mindat.org/min-2947.html