Strange and fascinating rock formations on Earth

Eye of Sahara

Eye of Sahara (Source: NASA)

The “Eye of Sahara”, formally known as the Richat Structure or the Guelb er Richât is an eroded circular structure located in the Sahara’s Adrar Plateau in Mauritania. The geological structure spans ~45 km (~28 mi) in diameter can be seen from space and has always captured the attention of astronauts. The “Eye of Sahara” is classified as a domed anticline (i.e., rocks at the “bullseye” or the centre of the structure are older than those on the flanks of the structure) made up of concentric ridges of Proterozoic to Paleozoic sedimentary rocks (sandstone and limestones) from the Taudeni Basin which have been intruded by igneous rocks (carbonatites, gabbros, rhyolite, and kimberlites)1,2. Though the genesis of the “Eye of Sahara” structure is largely accepted as a product of gradual erosion of a volcanic dome 2, earlier models suggest that the structure is a product of a meteorite impact3.

Giant Crystal Cave

Giant Crystal Cave with giant gypsum crystals (Source: Juan Manuel Garcia-Ruiz)

The Giant Crystal Cave is a 109 m (~360 ft)-long horseshoe-shaped cavity within limestone country rock about 300m (~985 ft) below the surface at Naica mine, Chihuahua State, Mexico. The cave was discovered in April 2000 by brothers Juan and Pedro Sanchez while working underground. The Giant Crystal Cave contains large (up to 11 m or 36 ft long) gypsum crystals which weigh up to 56 tons. The giant gypsum crystals are proposed to have formed in the following stages: (i) interaction of groundwater and a sulphur-rich magma in a magma chamber beneath the Naica mine, (ii) percolation of this sulphur-rich water through faults and the ultimate mixing of this sulphur-rich water with oxygenated surface groundwater to form anhydrite (calcium sulphate, CaSO4), and (iii) the conversion of anhydrite to gypsum (hydrous calcium sulphate, CaSO4.H2O) when temperatures dropped to or below the anhydrite-to-gypsum conversion temperature of 58oC (136oF)4. The temperatures are proposed to have remained just below 58oC (136oF) where gypsum continued to grow at very slow rates (0.000014 nanometres per second) over the course of at most 500,000 years5.

Goblin Valley “mushrooms”

Goblin Valley “mushrooms” at Goblin Valley Park (Source: Roig61/Shutterstock)

Goblin Valley Park is located in Utah State Park, United States. The park is known for mushroom-shaped rocks or hoodoos locally referred as “goblins” due to their ghostly appearance. The “goblins” are a result of differential weathering of sandstones overlying softer rocks (siltstones and shales) of the Entrada sandstones deposited in tidal flats during the middle Jurassic (~170 million years ago)6.


References

1. Matton, G. La structure du Richat (Mauritanie), un processus alcalin peri-Atlantique. (Universite de Quebec a Montreal, 2008).

2. Matton, G. & Jebrak, M. The ‘eye of Africa’ (Richat dome, Mauritania): an isolated Creataceous alkaline-hydrothermal complex. J. African Earth Sci. 97, 109–124 (2014).

3. Cailleux, A., Guillumaut, A. & Pomerol, C. Presence de coesite, indice de hautes pressions, dans I’accident circular des Richat (Adrar Mauritanien). C R Hebd Seanxc Acad Sci 258, 5488–5490 (1964).

4. Van Driessche, A. E. S., Canals, A., Ossorio, M., Reyes, R. C. & Garcia Ruiz, J. M. Unravelling the sulphate sources of (giant) gypsum crystals using gypsum isotope fractionation factors. J. Geol. 124, 235–245 (2016).

5. Carreno-Marquez, I. J. A. et al. Evolution of the astonishing Naica giant crystals in Chihuahua, Mexico. Minerals 11, (2021).

6. Milligan, M. The Geology of Goblin Valley State Park. (1999).


Comments

  1. ChakkiarJune 6, 2022 at 7:22 PM

    eye of Sahara was a zink hole which developed by drain out early Ocean there.

    Mushroom rocks formation NOT by wind (weathering). its by salty ocean water once filled surround.


    ReplyDelete

Post a Comment

Popular posts from this blog

Controversies and knowledge gaps in Geology (Part 1): Formation of ‘Archean granitoids’ (TTGs)

Image
What are TTGs? A suite of granitoids comprising tonalites , trondhjemite and granodiorites collectively referred to as tonalite-trondhjemite-granodiorite (TTG) granitoids are the main constituents of Archean terranes 1 . TTGs were mostly emplaced in the Archean and are not generated in modern-day subduction zones 1 . Thus, these rocks have invariably undergone multiple episodes of metamorphism and usually occur as strongly deformed gneisses (Fig. 1). Figure 1. TTG specimen (Source: Wikipidea) Geochemically, TTGs are sodic (K 2 O/Na 2 O<0.5), contain high contents of light rare earth elements (LREE) and low contents of high rare earth elements (HREE) resulting in strongly fractionated REE patterns (average La/Yb N of 32.4), exhibit negative Nb-Ti anomalies, and lack pronounced Eu and Sr anomalies 2 . The fractionated REE patterns and the Nb-Ti anomalies are interpreted to be indicative of the presence of garnet, amphibole and/or rutile in the melt residue 3 , whereas the Sr and
Read more

Adding crushed rocks to soils absorbs atmospheric carbon and curb global warming

Image
Summary Enhanced Rock Weathering involves the application of pulverized rocks to soils. It is one of the proposed ways of reducing effects of global warming. The Paris Agreement on climate change adopted the limiting of the increase in global warming to well below 2 o C. Quick and efficient removal of carbon dioxide (CO 2 ) is thus an urgent requirement to achieve the set targets. The natural chemical weathering of rocks (a process involving the reaction of atmospheric CO 2 with water to form a weak carbonic acid (H 2 CO 3 ), and the attack of silicate and carbonate minerals by the carbonic acid) normally absorbs only 0.3% of the atmospheric CO 2 . Enhanced rock weathering (ERW) - the modification by acceleration of the removal of atmospheric CO 2 by the application of pulverized magnesium-rich silicate rocks (mostly basalts) and calcium-rich rocks (carbonates) - is one of the cost-effective methods of extracting CO 2 from the atmosphere.  A recent study has shown that ERW has the
Read more

New study proposes ‘Goldilocks’ temperature zones as factories for generation of mineral deposits

Image
Current understanding of the formation of porphyry deposits Porphyry deposits are a major source of copper (Cu), gold (Au), and molybdenum (Mo) (Fig. 1). These deposits contribute 75%, 50%, and 20% of the Cu-, Mo-, Au produced in the world, respectively 1 . Figure 1. Molybdenite (dark grey molybdenum-sulphide mineral) hosted in quartz (grey) veins in a porphyry deposit (Source 2 ) Typically, porphyry deposits form in magmatic arcs (oceanic and continental) above subduction zones in a series of processes beginning with (i) the expulsion of metal-enriched fluids from the subducting slab into the mantle wedge resulting in partial melting of the mantle wedge, followed by, (ii) the migration and stalling of such magmas in the magma chambers in the mid to lower crust, and subsequently, (iii) the migration of metal-rich fluids produced during the crystallisation of magmas in magma chambers into the upper crust where they form pipe- and dyke-like porphyry deposits  3 , 4 (Fig. 2). Figur
Read more