Assessing the potential for low-enthalpy geothermal energy generation in South Africa.

South Africa is the largest producer of greenhouse gasses on the African continent, and one of the largest producers worldwide. This is largely due to the country's dependence on coal to produce energy, with over 80% of electricity in South Africa being produced from coal burning power stations. Aware of the problems associated with greenhouse gas production, the nation has set itself the ambitious target of generating 40% of its energy needs from renewable sources, such as wind, solar, or hydroelectric plants, by 2030. This target is made harder by rising demand for electricity, with a rising population and many populations denied electricity under the former apartheid regime now wanting to be connected. Geothermal energy, energy generated by geological processes, is not usually considered as a viable option in South Africa, as the nation lies far from any active volcanic field, the most obvious source of geothermal energy, but the country could still potentially generate some of its power from low-energy geothermal sources, which rely on temperature gradients within deep rocks, rather than active heat eruption at the surface.

In a paper published in the South African Journal of Science on 29 December 2017, Taufeeq Dhansay of the South African Council for Geoscience and the Africa Earth Observatory Network at Nelson Mandela University, Chiedza Musekiwa, Thakane Ntholi, Luc Chevallier, and Doug Cole, also of the South African Council for Geoscience, and Maarten de Wit, also of the Africa Earth Observatory Network at Nelson Mandela University, present the results of a study into the viability of low enthalpy geothermal energy production in South Africa.

Low enthalpy geothermal energy production relies on temperature differences within geological formations rather than direct heating by magma or hydrothermal sources, and can produce useful amounts of electricity at temperature gradients as low as 40°C per km. This does not necessarily require the presence of plutonic intrusions from the Earth's interior, in many cases the presence of radiogenic elements such as uranium within the rock strata produces sufficient heating for low enthalpy geothermal plants to operate..

Schematic illustration of a binary fluid enhanced geothermal system related to surrounding fracture-controlled geological features. Dhansay et al. (2017).

Much of South Africa is underlain by the Kaapvaal Craton, an ancient subcontinental mass of lithospheric material that averages 40-50 km in thickness, but is 250 km deep at its thickest. This ancient mass at first seems to have rather poor potential for geothermal energy production of any kind, but the craton is in fact made up of several large blocks of more ancient material that were fused together during the Archean Eon (4.0-2.5 billion years ago). Thus, while the majority of the Kaapvaal Craton is highly thermally stable, something which has discouraged investigation of the potential for geothermal energy in the area, it is cross-cut by a number of ancient geological sutures, with rather different properties.

These sutures are the remains of ancient orogenic belts, bands of volcanic activity similar to that seem around the Pacific Rim today. The geological process associated with such structures tend to concentrate heavier elements within certain strata. This has led to the high concentrations of gold and other precious metals that drive the South African mineral industry, and also higher concentrations of radiogenic metals such as uranium, which at high enough densities can raise the temperatures of the deposits that host them.

Within the Kapvaal Craton these include the Cape Granite Suite (part of the Cape Fold Belt, which formed during the assemblage of the supercontinent of Gondwana) which contains uranium concentrations of up to 34 parts per million, the Namaqua-Natal Belt that formed during the assemblage of the supercontinent of Rodinia, and has uranium concentrations of between 10 and 54 parts per million, the Thabazimbi-Murchison Lineament, which marks the boundary between the ancient Kaapvaal Greenstone Belt and the slightly younger Limpopo Mobile Belt, with uranium concentrations of up to 30 parts per million, and traces of older Archean granite-gneisses around Mombela and Johannesburg, with uranium concentrations of 20-28 parts per million.

Overview of the major tectonic structures and zones across South Africa with the locations of significant earthquake focal mechanisms and inferred structures related to these events. Locations of the various data sources used within this study (e.g. hot springs and temperature measurement points) and high heat producing plutonic rocks are also highlighted. Note that the Namaqua-Natal Belt probably continues beneath the Cape Fold Belt as far as the offshore Agulhas Fracture Zone. Dhansay et al. (2017).

Dhansay et al. obtained measurements of temperature in South African rocks at depths of between 2 and 5 km from previous studies, as well as inferring deeper temperatures from hot springs at the surface, and the presence of deep faults from Earthquake data, and used this data to build up a map of temperature gradients across the country.

Graphical overview of the calculated geothermal gradients across South Africa. Map includes major tectonic contacts and structures, seismic activity and earthquake focal mechanisms and hot spring locations. Dhansay et al. (2017).

Using this data Dhansay et al. were able to identify a number of regions with geothermal temperature gradients high enough to support potential low-enthalpy geothermal energy generation, most notable along the the Colesberg, Thabazimbi-Murchison and Makonjwa Lineaments, ancient tectonic zones associated with early stages of the assemblage of the Kaapvaal Craton, which are also associated with a number of recent seismic events, and a number of hot springs, which may be due to reactivation of ancient faults with a northeast to southwest orientation.

(a) Potentially viable low-enthalpy geothermal investigation regions (1–5); based on (b) high heat producing plutonic rocks and overlying volcanosedimentary rocks; and (c) approximate groundwater yield. (d) Regional seismicity. Dhansay et al. (2017).

See also...

Follow Sciency Thoughts on Facebook.

Magnitude 5.1 Earthquake in the Eastern Macedonia and Thrace Region of Greece.

The United States Geological Survey recorded a Magnitude 5.1 Earthquake at a depth of 6 km, in the Eastern Macedonia and Thrace Region of Greece, slightly before 6.25 am local time (slightly before 4.25 am GMT) on Tuesday 2 January 2018. No damage or injuries have been reported following this event, but people have reported feeling the event from much of northeastern Greece, eastern Macedonia and southwest and Bulgaria.

 

The approximate location of the 2 January 2018 Greek Earthquake. USGS.

 

The Aegean Sea is underlain by the Aegean Sea Plate, a small tectonic plate caught between the African Plate to the south, the Anatolian Plate to the east and the Eurasian Plate to the northwest. The Anatolian Plate is being pushed to the west by the northward movement of the Arabian Plate to the east, pushing the Aegean Plate south and west into collision with the northward moving African Plate. Part of the African Plate is being subducted beneath the Aegean Plate along the Hellenic Trench, which runs to the south of the Greek Islands from Cyprus to Crete then northwest under the Ionian sea parallel to the Peloponnesian Coast. This is not a smooth process, as the plates frequently stick together then break apart once the pressure has built up sufficiently, leading to (fairly frequent) Earthquakes.

 

 Simplified map of the plate movements of the eastern Mediterranean. Univeriteit Utrecht.

 

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

 

See also...

 

Follow Sciency Thoughts on Facebook.

Micrometerites from Late Cretaceous Chalk depostits from southern England.

Micrometeorites are tiny (less than 2 mm) fragments of asteroidal (or occasionally cometary) material that survive the descent through the Earth's atmosphere, and which are collected by planetary scientists from sites where input from terrestrial sedimentary sources is minimal, such as Antarctic ice- and snow-fields and oceanic basins. Some micrometeorites survive the journey to the Earth relatively impact, but most are melted by the temperatures caused by friction with the Earths atmosphere (which is greater than that caused by simply falling due to the high relative speeds at which these bodies are travelling prior to encountering the Earth), often reaching temperatures in excess of 2000°C,  which causes them to melt, forming spherical droplets due to surface tension, which recrystallise to form circular bodies called spherules.

These spherules do not retain the same chemistry as their parent bodies; lighter elements, such as sodium, sulphur, phosphorus, chlorine, and manganese tend to evaporate completely, while heavier elements such as iron and nickel separate internally, forming discrete layers. As these liquid spherules descend further they are quench cooled through contact with the denser lower atmosphere, causing them dendritic crystals to form. About 95% of such spherules have a silicate dominated composition, while about 4% are iron dominated and about 1% have a mixed composition. Silicate dominated spherules can have different mineralogies, which is thought to relate to the temperature to which they were heated, rather than their original composition, with porphyritic spherules forming at the lowest temperatures, then barred olivine spherules, then cryptocrystalline spherules, and finally vitreous (glassy) spherules at the highest temperatures. Iron dominated spherules are divided into metal-bearing spherules, which contain an iron-nickel bead surrounded by a layer of wüstite (an iron-oxide mineral), and oxidised sperules, which are composed of a mixture of wüstite and magnetite (another iron-oxide mineral).

Spherules, unlike unaltered micrometeorites, are distinctive enough to be recognised in the rock record and recovered from ancient sediments, with examples having been recovered from a wide range of sedimentary rocks, from Archean limestones in Australia to Eocene marine sediments in Barbados, as well as ancient granites in China. However, such spherules do not simply enter the rock record and remain unchanged waiting to be discovered, they are altered by chemical processes going on within the rock, causing changes referred to as taphonomy. This taphonomy can take a number of forms, including alteration of minerals, hydration or dissolution of anhydrous minerals and metals, and encrustation with halite (salt), calcite (carbonate) or other materials.

In a paper published in the journal Earth and Planetary Science Letters on 1 September 2017, Martin Suttle and Mathew Genge of the Impacts and Astromaterials Research Centre at Imperial College London and the Department of Earth Science at the Natural History Museum, describe the discovery of spherule micrometeorites in Late Cretaceous chalk deposits obtained from a road cutting at Ranmore Common in Surrey, southern England.

Suttle and Genge were able to obtain 76 spherules from their rock sample, including 60 iron oxide spherules, 13 iron/silica spherules and 3 silica spherules, ranging in size from 10 to 165 μm. The majority of the iron oxide spherules were comprised of magnetite, with small amounts of aluminium, silicon and manganese, and trace amounts of other minerals but no nickel, while one was comprised of a mixture of wüstite and magnetite, with a significant proportion of nickel. The iron/silica spherules also lacked any nickel, but did contain small amounts of chromium and manganese. The silica spherules contained small amounts of iron, aluminium and manganese.

External and internal textures of Fe-oxide spherules, interpreted as fossilised cosmic spherules. Particle (D) (C16-0003) is the single unaltered nickel-bearing iron oxide spherule, composed of wüstite, while the remaining spherules are composed of maganese-bearing magnetite. Surface dendrites and residual chalk sediment, including fragmented coccolithophore tests can be seen coating external surfaces of spherules (A)–(C). In spherule (D) and (F) sub-circular cavities are present, representing the loss of an iron–nickel metal bead by corrosion during residence on the Cretaceous seafloor, these spherules can therefore, be identified as metal-bearing iron oxide spherules. In contrast, spherule (G) contains isolated irregular small cavities, representing vesicles formed by residual gas trapped during inward crystallisation and is therefore an oxidised iron spherule. Suttle & Genge (2017).

The iron oxide and iron silica spherules were either homogeneous throughout or showed dendritic crystal formation, with many of the spherules with dendritic crystals also having cavities within, probably indicative of dissolution of minerals. One of the silica spherules has an olivine mineralogy, with the other two being porphyritic.

External and internal textures of iron-silicide spherules, most probably composed of suessite. These spherules are interpreted as fossilised cosmic spherules. Replacement by silicides imperfectly pseudomorphs the original texture, leading to changes in volume, accounting for the presence of micron sized voids seen in (F) and protrusions (D), protecting from the particle’s surface. Despite preservation artifacts, original textures can be discerned, allowing their identification as cosmic spherules. Dendritic crystals are observable in all particles and attest to a rapid cooling history as molten droplets. Particle C16-0009 (A) and (B) preserves only a single phase (most likely wüstite) while particle C16-0010 (D)–(F) preserves both the original magnetite and wüstite as different silicide minerals. Cavities in (B) are a result of volatile gases released during atmospheric entry. Suttle & Genge (2017).

Four of the spherules, the three silica spherules and the nickel-bearing wüstite spherule, are essentially identical to modern spherules obtained from Antarctic snowfields, leaving little doubt as to their meteoric origin. The presence of dendritic crystals in many of the magnetite spherules suggests being heated to a temperature of over 1350°C. The cavities within many of the spherules are probably due to the dissolution of soluble crystals as the spherules lay upon the sea-floor.

Sectioned images of silicate cosmic spherules. Spherules (A) and (B) are ancient, unaltered chondritic silica-type spherules, classified as micro-porphyritic (A) and barred olivine (B) subtypes. For comparison particle (C) is a modern, barred olivine spherule recovered from Larkman Nunatak, Antarctica. Suttle & Genge (2017).

The presence of manganese in many of the spherules requires more explanation, as manganese is extremely rare in modern micrometeorites. Suttle and Genge suggest that this is probably not indicative of the original composition of the meteorites, but rather re-crystallisation of the spherules on the seafloor. They suggest that the spherules probably had a nickle-bearing wüstite composition, which was recrystallised to magnetite on the seafloor, in the process losing their nickel content, but gaining manganese (which is often abundant in marine sediments).

See also...

Follow Sciency Thoughts on Facebook.

Bifiguratus adelaidae: A new species of soil Fungus from North Carolina.

Fungi are one of the most diverse groups of organisms on Earth, and play a major role in almost all terrestrial ecosystems. There are thought to be about 5.1 million extant species of Fungi on Earth, though only about 100 000 species have been described to date, with most of these being either visible Macrofungi (Mushrooms etc.) or pathogens of Humans, crops or domestic animals. This means that many major groups of Fungi have hardly been studies at all, particularly early diverging terrestrial Fungi (Fungal groups that diversified away from other Fungi soon after the group colonised the land, around a billion years ago).

In a paper published in the journal Mycologia on 6 September 2017, Terry Torres-Cruz, Terri Billingsley Tobias, and Maryam Almatruk of the Department of Biological Sciences at Western Illinois University, Cedar Hesse and Cheryl Kuske, of the Bioscience Division at the Los Alamos National Laboratory, Alessandro Desirò, Gian Maria Niccolò Benucci, and Gregory Bonito of the Department of Plant, Soil and Microbial Sciences at Michigan State University, Jason Stajich of the Department of Plant Pathology and Microbiology at the University of California, Riverside, Christopher Dunlap of the National Center for Agricultural Utilization Research at the U.S. Department of Agriculture, Elizabeth Arnold of the School of Plant Sciences and Department of Ecology and Evolutionary Biology at the University of Arizona, and Andrea Porras-Alfaro, also of the Department of Biological Sciences at Western Illinois University, describe a new species of soil Fungus from a Pine plantation in North Carolina.

The new species is named Bifiguratus adelaidae, where 'Bifiguratus' means 'having two morphologies' and 'adelaidae' honours the Costa Rican ecologist Adelaida Chaverri Polini (1947-2003). The species was isolated from a Pine plantation in Duke Forest, North Carolina, but has also been detected in soil samples from the Bartlett Experimental Forest in New Hampshire, the Cedar Creek Ecosystem Science Reserve in Minnesota, and Coronado National Forest in Arizona, as well as samples from Italy, Japan, Sweden and South Korea. The Fungus initially forms white or light ivory colonies with a wrinkled surface and lobbed margin, however as it grows it becomes flattened and covered in mucus, and forms associations with the Bacteria Bacillus licheniformis and Stenotrophomonas sp. A genetic analysis of the species places it within the Mucoromycotina, an enigmatic group of Fungi of uncertain affinities, that also includes a variety of other soil Fungi and Pin Molds.

Bifiguratus adelaidae colony morphology on different media and temperatures. Scale bars are 10 mm. Torres-Cruz et al. (2017).

See also...

Follow Sciency Thoughts on Facebook.

Workers at Zimbabwe graphite mine strike over unpaid salaries.

Workers at the Zimbabwe German Graphite Mine (also known as the Lynx Mine) in Karoi District, Mashonaland West Province, have begun industrial action after their salaries have gone unpaid for 13  months. The action began on Friday 29 December, with workers downing tools and blocking trucks from collecting graphite from the mine compound. The miners also claim that the mine is now without electricity due to unpaid debts, leaving them without light and forcing them to take water from a mine tailing pond (pond used to store sediment-laden waters from mines; such waters typically contain a high proportion of fine silt and clay particles, which take time to settle out of the water, and may contain other pollutants, typically acids, either generated by the local geology or used in the mining process), but that managers at the mine have continued to collect their salaries.

The Zimbabwe German Graphite Mine in Karoi District, Mashonaland West Province. The Sunday Mail.

Graphite is a form of naturally occurring carbon,  most commonly found in metamorphic rocks of sedimentary origin, though it can be made artificially. It is used as a solid lubricant, forms the 'lead' in modern pencils, and is used in batteries and the manufacturing of steel.

The Zimbabwe German Graphite Mine was founded in 1982 as a joint enterprise between the Zimbabwe Mining Development Corporation and Graphit Kropfmhül Gmbh of Germany. It was the only graphite mine in Africa until the Balama Graphite Mine in Cabo Delgado Province, northern Mozambique, opened in 2015. The combination of this new competition in the region with a global decline in graphite prices appears to have undermined the profitability of the Zimbabwe German Graphite Mine, resulting in its current financial crisis.

See also...

Follow Sciency Thoughts on Facebook.

Fireball meteor over northern England.

The UK Meteor Observation Network has received reports of a bright fireball meteor being seen over much of the UK and parts of Ireland slightly after 5.30 pm GMT on Sunday 31 December 2017. The fireball has been described as being greenish in colour, which may indicate it was caused by the explosion of a small meteorite with a high iron or magnesium content, and was seen across most of England, as well as southern and eastern Scotland and parts of Ireland. A fireball is defined as a meteor (shooting star) brighter than the planet Venus. These are typically caused by pieces of rock burning up in the atmosphere, but can be the result of man-made space-junk burning up on re-entry.

The 31 December 2017 northern England meteor. American Meteor Society.

The object was seen moving from east to west over northern England, apparently entering the atmosphere somewhere over the North Sea and exploding in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) above Carlisle. 

Map showing areas where sightings of the meteor were reported, and the route of the object (blue arrow). American Meteor Society.

Objects of this size probably enter the Earth's atmosphere several times a year, though unless they do so over populated areas they are unlikely to be noticed. They are officially described as fireballs if they produce a light brighter than the planet Venus. The brightness of a meteor is caused by friction with the Earth's atmosphere, which is typically far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).

 

 The 31 December 2017 northern England meteor seen from East Barnet in North London. Jim Rowe/UK Meteor Network.

  

These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists are able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services.

Witness reports can help astronomers to understand these events. If you witness a fireball-type meteor over the UK you can report it to the UK Meteor Observation Network here.

See also...

Follow Sciency Thoughts on Facebook.