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Imaggeo on Mondays: Marble outcrops

29 Sep

This week’s Imaggeo on Mondays image was taken by Prof. Konstantinos Kourtidis, in Alykes, along the southern coast of Thassos island, where he photographed the beautifully white marbles that outcrop along the coastline. The Greek Island of Thassos is located in northeastern Greece, close to the coast of Thrace in the Aegean Sea, although geographically it belongs to the Macedonia region. There is geological evidence to suggest that at one time, the island was joined to the mainland.

Marble Outcrops. (Credit: Konstantinos Kourtidis via imaggeo.egu.eu)

Marble Outcrops. (Credit: Konstantinos Kourtidis via imaggeo.egu.eu)

“The island is formed of alternating marbles, gneisses and schists” explains Konstantinos, “in the southern Thassos area, where this image was taken, Palaeozoic (around 400 million years in age) and Mesozoic metamorphosed rocks of the Rhodopi Massif and more recent sedimentary Miocene formations (around 25 million years old) are exposed.” The sediments in this area are dominated by conglomerates, sandstones and argillaceous sands.

Banded iron formations, also known as BIFs, are repeated thin layers of iron-rich material which are alternated with shales and/or silica rich cherts. There are numerous occurrences of BIFs across Thassos island and this is interesting because BIFs are typical sediments of the Precambrian rock record and can indicate the presence of rocks which are in excess of 3 billion years old! It is unusual to find BIFs in the younger rocks record. On Thassos Island their formation is associated with changes in the depositional environment and climate.

During the formation of BIFs, volcano-sedimentary units become heavily mineralised and rich in iron and manganese oxides. In addition the island has dense accumulations of zinc and lead. As a result there is a long mining history on Thassos, dating back to 13,000 BC. The marbles seen in today’s Imaggeo on Mondays image belong to an ancient mine at sea level which was “exploited given the excellent quality of the marbles” states Konstantinos. The stone has been used in art projects, monuments and the building of numerous ancient temples.

Ancient Marble Quarry in Thassos, Eastern Macedonia, Greece. (Credit: Konstantinos Kourtidis via imaggeo.egu.eu)

Ancient marble quarry in Thassos, Eastern Macedonia, Greece. (Credit: Ioannis Daglis via imaggeo.egu.eu)

Given the islands rich archeological and geological heritage the Greek Institute of Geology and Mineral Exploration (IGME) has produced a geological guide for the southern part of the island, which also includes 4 geotrails and is available online.

The known unknowns – the outstanding 49 questions in Earth Sciences (Part II)

19 Sep

Here is the second instalment in our series covering the biggest unknowns in the Geosciences.

Last week we explored what it is about the Earth’s origin that still remains unclear and this week we probe the Earth’s deep interior. Unlike in Jules Verne’s Journey to the Centre of the Earth, there are no volcanic tubes we can climb down which will allow us to discover the inner workings of our Planet. Direct understanding can only be gained from rock samples; our inability to collect those at depths in excess of 12 km means direct observations are restricted to the Crust. How do we know what happens far down below? We have to rely heavily on indirect measurements such as seismic wave tomography, together with geodynamic and petrological modelling. But, really, how confident can we be, considering we can’t really ‘see’ into the centre of the Earth, that our hypotheses are correct? Only this week, new research casts doubts over the long established theory that hot spot volcanism is fed by deep rooted mantle plumes. This highlights just how important it is to continue efforts to push our understanding and find new and innovative ways to journey into the centre of the Earth.

The Earth’s Interior

Tectonic plates (Credit: USGS, Department of the Interior/USGS)

Tectonic plates (Credit: USGS, Department of the Interior/USGS)

  1. What are the chemical composition and mechanical properties of rocks in the Earth’s mantle at the extreme pressure and temperature they undergo? As planets age and cool off, their internal and surface processes coevolve, chemically and mechanically, shaping in turn the atmospheric composition. Therefore this question has direct implications for our understanding of the environmental evolution of the Earth. (Kerr, 2005, Science)
  2. What are the dynamic processes in the Earth interior that accommodate and fuel plate tectonics? As seismometers spread more evenly over the planet’s surface, the seismic imaging of the interior will rapidly improve, providing a detailed distribution of seismic wave velocity. Simultaneously, lab-based mineral physics must better constrain what these mechanical wave velocities tell us about the hot, deep rocks of uncertain composition in the mantle. Only then will computer models be able to test the currently proposed geodynamic models by trying to quantitatively fit those data with other geophysical observations such as gravity variations. (Kerr, 2005, Science)
  3. How does the Earth’s magnetic field work? Sedimentary and volcanic rocks have recorded changes of the magnetic field throughout the evolution of the Earth. What causes the sudden reversals of the paleomagnetic field? What caused the long periods (more than 10 Myr) with no magnetic inversions (superchrons)? How does the geomagnetic field link to the iron convection properties at the deep Earth? Or inversely, what can we learn about the mechanical behavior of the materials at those depths from the geomagnetic field? (see this nature paper for further context). Are the magnetic reversals too fast to be related to core dynamics? (Biggin et al., 2012, Nature Geoscience). Could their frequency be related to the distribution of tectonic plates? (Pétrélis et al., 2011, GRL). What causes superchrons (periods without reversals)? Something internal within the core, or is it induced externally by the mantle/subducting slabs? Was the geomagnetic field always dipolar, or was it more asymmetric in the past, as suggested in by Biggin et al., (2012)?

    Magnetosphere rendition (Credit: Wikimedia Commons user NASA  http://sec.gsfc.nasa.gov/popscise.jpg)

    Magnetosphere rendition (Credit: Wikimedia Commons user NASA http://sec.gsfc.nasa.gov/popscise.jpg)

  4. What makes the magnetic field change between reversals? What is the history of and what controls the excursions of the rotation pole relative to the surface geography, known as true polar wander? (Creveling et al., 2012, Nature).
  5. How do hotspots come about? Are intraplate hotspots really made by deep sources of uprising materials (mantle plumes) coming from the deepest Earth’s mantle? Or can they be explained by shallower convection? (e.g Morgan, 1971, Nature and Fouch, 2012, Geology)
  6. What are the properties of deep rocks? How can we translate the heterogeneity in density, seismic wave velocity, and electromagnetic resistivity presently observed in the mantle and the lithosphere into variations of the mineralogical composition? And how do these measures relate to the dynamics of the Earth and to key mechanical properties such as the viscosity?  (Faccenna & Becker, 210, Nature)]
  7. What are the causes for and massive flood basalts such as the Columbia River Basalts?

    Distribution of the World’s Large Igneous Provinces (Source: Wikimedia Commons, user: USGS)

    Distribution of the World’s Large Igneous Provinces (Source: Wikimedia Commons, user: USGS)

Do you think these seven questions cover the fundamental issues yet to be addressed when it comes to the Earth’s deep interior? If you’d like to add to this discussion, please feel free to voice your opinions using the comments section below, we really would love to hear from you!

Enjoyed these seven questions? Why not explore the first post in the series?

Coming up next: Plate tectonics and deformation.

 

By Laura Roberts Artal, EGU Communications Officer, based on the article previously posted on RetosTerricolas by Daniel Garcia-Castellanos, researcher at CSIC, Barcelona

Imaggeo on Mondays: Spectacular splatter – the marvels of a mud volcano

28 Jul

Mud volcanoes, unlike many others, do not extrude lava. Instead, they release glutinous bubbling brown slurry of mineral-rich water and sediment. They range in size from several kilometres across, to less than a metre – the little ones are known as mud pots, reflecting their diminutive nature. The world’s largest, though, is Lusi: a mud volcano in East Java that released an astonishing 180,000 cubic metres of fluid each day during the peak of its 2006 eruption. It’s likely to continue erupting for another 26 years!

Much of the gas that bubbles up through these muddy pools is methane, though the exact mix of gasses varies from site to site and is tied to other geological activity in the region, with those close to igneous volcanoes often releasing less methane than those associated with clathrate deposits. Small bubbles of gas can coalesce to form a much larger one, which, on reaching the surface, bursts and sends flecks of clayey fluid asunder, just as they do here:

The sediment-rich spatter from a bubbling mud volcano. (Credit: Tobias Heckmann via imaggeo.egu.eu)

The sediment-rich spatter from a bubbling mud volcano. (Credit: Tobias Heckmann via imaggeo.egu.eu)

By Sara Mynott, EGU Communications Officer

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

 

Geosciences Column: The Toba eruption probably did have a global effect after all

25 Jul

Almost everyone has heard of the Toba super-eruption, which took place on the island of Sumatra roughly 74,000 years ago, but the only evidence of tephra or tuff (volcanic fragments) from the eruption is in Asia, with nothing definite further afield. It has sometimes been thought that this huge eruption may have led to a volcanic winter, a period of at least several years of low temperatures following a large eruption. This is caused by the effect of enormous quantities of volcanic ash entering the atmosphere and reducing the sun’s penetration. Sulphides also help to reduce solar energy penetration and increase the Earth’s albedo (increasing the reflection of solar radiation and leading to cooler temperatures).

In a 2013 study, Anders Svensson and colleagues suggest that a link has now been made between the onset of Greenland Interstadials (GI) 19 and 20 and Antarctic Isotope Maxima (AIM) 19 and 20 for the Toba eruption. Stadials are periods of low temperatures, lasting less than a thousand years, during the warmer periods between ice ages. Conversely, interstadials are warmer periods lasting up to ten thousand years, occurring during an ice age but not lasting long enough to qualify as interglacial periods. The GI periods and AIMs are numbered according to a scale based on Dansgaard-Oeschger events. These are relatively short-lived climate fluctuations, which occurred 25 times during the last glacial period (covering 110,000 to 12,000 years ago) and are used to help date events.

The study’s claim is based on their matching of volcanic acidity spikes at both poles produced by increased sulphur compounds in the atmosphere following the Toba eruption, which have been matched to the existing dates based on Asian tephra records and the bipolar seesaw hypothesis. This hypothesis explains the thousand-year offset between temperature changes over the two poles during the last glacial period as being caused by a seesaw mechanism involving heat redistribution by the Atlantic Meridional Overturning Circulation (AMOC) system, whereby warm water moves north from southern waters, mixes with cooler water in Arctic regions, sinks, and returns south as deep bottom water.

The fluctuations in temperature associated with Dansgaard-Oeschger events over a few decades are reflected in Greenland ice cores but Antarctic ice cores show a picture of slower changes over hundreds to thousands of years, running out of phase with the Greenland records. A direct link was made by Carlo Barbante and colleagues in a 2006 paper. They used methane records from a Northern Greenland ice core and oxygen isotope records from the Antarctic Dronning Maud Land ice core to show direct coupling between warm events in the Antarctic and the duration of cold events in Greenland indicating their probable common origin in a reduction in AMOC.  An increase in freshwater entering the North Atlantic during warming would slow heat transport towards the north. This would lead to cooler surface air temperatures there and warmer temperatures in the southern waters and vice versa – taking several decades to pass from one hemisphere to the other, hence the name bipolar seesaw effect.

When volcanic gases, especially sulphurous gases, travel round the world in the atmosphere they form aerosols, which are trapped with air in precipitation at the poles in the form of bubbles. The bubbles are entombed at the depth at which firn (a type of rock-hard snow that looks like wet sugar) is compacted into ice. Succeeding precipitation and compaction over thousands of years provides evidence of atmospheric composition at different times, in trapped air bubbles, which can be analysed in ice cores. But the ages of both ice and gases are offset by 100-1000 years, depending on factors such as thickness of the firn, temperature and the presence of impurities in the ice. Ice cores are a bit like tree rings in that their thickness and other properties reflect climatic conditions at the time and differences can be counted as annual rings for the younger cores, before compaction makes it impossible to distinguish them.

Isotope data for Greenland and Antarctic ice cores over the past 140,000 years. (Credit: Leland McInnes)

Isotope data for Greenland and Antarctic ice cores over the past 140,000 years. (Credit: Leland McInnes)

Oxygen isotope and atmospheric methane signals in ice cores were available to link two separate records (NGRIP and EDML) for the period covering 80-123 thousand years ago.

Different gases can be used to date a particular part of an ice core and, as can be seen above, δ2H in Antarctica and δ18O in Greenland were used for dating purposes. 10Beryllium is found in the atmosphere and its levels change with solar activity and the Earth’s magnetic field. It is only found in the atmosphere for one or two years at a time so it can be used to help synchronise data from different ice cores more closely. A dating method using cosmogenic 10beryllium signals to match both Greenland and Antarctic ice cores to the Laschamp geomagnetic event (a short reversal of the Earth’s magnetic field). This helps pinpoint a particular date at around 41 thousand years ago, and provides a direct link between the ice-core horizons so that the data could be matched and signs of the eruption detected at the known eruption time.  With markers for 41,000 and 80,000 years ago synchronised for the two polar regions, the ice cores were examined for signs of sulphate acidity spikes indicating a major eruption.  The time scales were then combined for datasets from a number of ice cores (NGRIP, EDC, EDML and Vostok) to produce consistent scales for ice and gas records.

From the Greenland (NGRIP) and Antarctic (EDML) ice-core data, evidence of the Toba eruption was synchronised between them for approximately 2000 years around the known eruption time, using a pattern of bipolar volcanic spikes and the Greenland Ice Core Chronology 2005 defined for the NGRIP annual layer count, to compare with Antarctic data. They found evidence of large quantities of atmospheric sulphates (usually linked to volcanic eruptions) in both sets of data in the form of acidity spikes and linked these to the Toba eruption.

In fact, there are four bipolar acidity spikes within a few hundred years of the presumed Toba event, suggesting that there may have been several events, also confirmed by argon dating. Moreover, the authors found that the Toba eruption was linked to up to 4 acidity spikes occurring between 74.1 and 74.5 thousand years ago. These Toba events occurred at a time of rapid climate change from warm interstadial to cold stadial periods in Greenland and the equivalent Antarctic warming within 100 years, which perfectly agrees with the bipolar seesaw hypothesis.

Interestingly, another more recent study, showed that Lake Prespa in southeast Europe reached it’s lowest recorded level, or lowstand, at the time of the Toba eruption. The timing of the lowstand is dated at 73.6 ± 7.7 thousand years ago based on Electron Spin Resonance dating of shells. This short-lived lowstand also coincides with the onset of Greenland Stadial GS-20, with a possible link to the Toba eruption.

This new data and more accurate dating described in these studies, does tend to show that the Toba eruption (or series of eruptions) did, in fact have global impact.

By Gill Ewing, Freelance Science Writer

References:

EPICA Community Members: One-to-One coupling of glacial climate variability in Greenland and Antarctica, Nature, 444, 195-198, doi. 10.1038/nature05301, 2006.

Svensson, A., Bigler, M., Blunier, T., Clausen, H.B., Dahl-Jensen, D., Fischer, H., Fujita, S., Goto-Azuma, K., Johnsen, S.J., Kawamura, K., Kipfstuhl, S., Kohno, M., Perrenin, F., Popp, T., Rasmussen, S.O., Schwander, J., Seierstad, I., Severi, M., Steffensen, J.P., Udisti, R., Uemura, R., Vallelonga, P., Vinther, B.M., Wegner, A., Wilhelms, F. Winstrup, M.: Direct linking of Greenland and Antarctic ice cores at the Toba eruption (74 KaBP), Clim. Past, 9, 749-766, doi. 10.5194/cp-9-749-2013, 2013.

Wagner, B., Leng, M.J., Wilke, T., Böhhm, A., Panagiotopoulos, K., Vogel, H., Lacey, J.H., Zanchetta, G., Sulpizio, R.: Distinct lake level lowstand in Lake Prespa (SE Europe) at the time of the 74 (75) ka Toba eruption, Clim. Past, 10,  261-267, doi.10.5194/cp-10-261-2014, 2014.

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