Possible eruption at Zvezdnoye Ozero

Image from Google Earth. Zvezdnoye Ozero (Star Lake) Caldera in the middle. Around are volcanic lakes that belong to the Zvezdnoye Volcanic Field. The volcano erupted the Chirinda Ignimbrite 600 000 years ago.

Very little is known about the volcano Zvezdnoye Ozero (Star Lake, 67D26’05’’N 102D24’26’’E) in the Kraznojarsk Kraj in Russia. It is situated in the northern end of the Siberian Traps, but is otherwise unrelated to the famous Igneous Province.

It is believed to have erupted the last time about 600 000 years ago in a caldera forming eruption when it produced the Chirinda Ignimbrites. The volcano was until recently considered to be completely extinct.

In late September a persistant and long (27 days) swarm of earthquakes consisting of more than 4 000 separate earthquakes took place ranging up to 3.1 on the local magnitude scale. Scientists from KVERT under leadership of Dr Igor Kurchatov moved to this very remote area to emplace essential equipment on orders from the GKO (Gosudarstvennyj komitet oborony).

Photograph by Dr Igor Kurchatov. In the backdrop is the village of Chirinda that lies on the edge of the Zvezdnoye Ozero (Star Lake). The village is now believed to be gone with all of its residents and the scientific team under leadership of Dr. Igor Kurchatov.

From the third of October until mid February they measured a Bradyseism causing an uplift of 190cm. Then a second swarm started on the 17^th of February that has been ongoing until yesterday. During the last few days heightened levels of seismic tremor combined with elevated reading of SO2 in the Zvezdnoye Ozero. The lake had on the 30^th of March turned highly acidic and the water had dropped nine meters.

The heightened activity caused President Putin to order the evacuation of the 53 nomads living in the area. See video.

Initial reports say onset of eruption was highly explosive, after the initial report to the Committee of Internal Security of the State, no further report has come from the scientists or the nomads evacuated to the nearby village.

CARL

Edinburgh – Volcanic heart of Scotland

 

Image from Wikipedia. Arthurs Seat, Edinburgh.

Edinburgh – home of the Scottish Parliament, Military Tattoo, Princes Street and gardens, Scott memorial, Murrayfield, Valvona and Crolla’s food emporium, sundry pubs (!), volcanoes…Eh, volcanoes?

Surprising as it may be to some people, Edinburgh plays host to a great variety of igneous rocks. The most obvious and in our case, the most interesting, are the volcano remnants of Arthur’s Seat, and the Castle Rock. These are long extinct and date from the Lower Carboniferous, about 350 million years ago. Arthur’s Seat, the larger of these, is assumed to be the main structure, the other a subsidiary, or satellite vent. These are central pipe vents from pipe conduits, cf linear conduits as associated with fissure eruptions.

The rock of Arthur’s Seat is mainly a vent agglomerate with several crystal phyric microgabbro eruption pipes (Lions Head and Lions Haunch vents so called as at a distance the remnants resemble a lion lying down) – agglomerate being a mix of explosive block and vent collapse debris, ash and lava, whilst crystal phyric refers to the presence of some minerals present with larger crystals than the background rock. The presence of gabbro in the conduit pipes indicates these volcanoes erupted a basaltic lava, gabbro being the intrusive coarse grained equivalent to basalt being of the same mineralogical composition. (Dolerite, or Diabase to some authors, again has the same composition, but has a grain size intermediate between basalt and gabbro and is usually assigned to these rocks when found in dyke and sill intrusions). The combined vent material as mapped, gives an irregular vent of 750-1000 metres across.

Picture of pipe brecciated St Austell granite to illustrate vent agglomerate appearance; a true vent agglomerate has a much larger fragment range in both size and composition however.

Photograph by Alan C who gratiously has bestowed the rights to the Volcano café. Pipe brecciated St Austell granite

The opening picture shows the main vents of the Lions Head (left) and Lions Haunch (right) in the background, with Salisbury Crags in front; these are the quarried remains of a post volcanic episode Teschenite (olivine analcime microgabbro – analcime is a hydrous sodic zeolite mineral comparable to feldspathoids; zeolite minerals usually being associated with vesicle-filling in amygdaloidal basalt flows) sill intruded into sub-volcanic sediments of Lower Carboniferous age.

Salisbury Crags are part of a site dedicated to James Hutton, who has been called the ‘Father of Geology’. It was here that Hutton part formulated his theory of Uniformitarianism, that confounded the existing Neptunism movement that insisted the Earth dated from the biblical Great Flood, by siting the presence of a rock of obvious molten origin being intruded into sediments/volcanic rock, the sediments being just visible below the sill.

Distal from the volcano, mapping and borehole evidence shows there to be up to 250metres thickness of tuffs and lavas adjacent to the vent and in the Midlothian borehole, some 10km to the south east, approximately 70m of volcanic material was found at the horizon of the Arthurs Seat volcanics.

Castle Rock is the erosional core of a relatively small and assumed satellite volcano off Arthurs Seat, composed of microgabbro approximately 150 metres in diameter. The vent again cuts through shallow water marine sediments of Lower Carboniferous – Dinantian – age. These sediments comprise predominantly sandstones and minor shale horizons; but it should be noted that in some texts the sediments erroneously are noted as limestones, a confusion arising from the rocks of this age being assigned to the Carboniferous Limestone division.

Picture from rampantscotland.com showing Castle Rock and vent.

The reference cited here below gives an artists impression of Edinburgh with the two volcanoes superimposed along with an imaginary sea level – with a lot of imagination this image could well have been a Carboniferous equivalent of El Hierro (Arthurs Seat complete with aerial cone) and our Bob (Castle Rock)!

http://www.geo.ed.ac.uk/arthurseat/geology/overlay.html

Image of how Edinburgh would have looked like during the "interesting times".

More recently, during the last Ice Age, ice sheet movement has produced a classic example of a ‘crag-and-tail’ with the Castle Rock – the crag – protecting the bedded Carboniferous sediments of the Royal Mile – the tail – from ice erosion, indicating mass ice movement from the west. More recently, the ice-deepened gouge channel on the north side (Princes Street gardens) has been utilised by the railway as an ideal route through the city!

Image from Google Earth.

And finally;

Image from from http://www.irocks.com/db_pics/pics/d06-238a.jpg Mystery stone...

ALAN C

An introduction to igneous rocks – Part 2

Picture of Mica from geology.about.com

Micas

Of which there are two predominant varieties in igneous rocks – Biotite and Muscovite – are a group of phyllosilicates; the name in allusion to the method and ease of perfect cleavage of the mineral into exceedingly thin laminae. They are characteristic of a wide range of rock type. Biotite, the mafic variety, is found in anything from Basalt to Rhyolite, whilst Muscovite is very unusual in volcanic rock, being more common in intrusive igneous and metamorphic rocks.

Olivine Group

Olivine is the general term applied to a group of minerals of another infinitely variable continuous substitution series, of magnesium and iron silicates, the end members being Forsterite (magnesium Mg2SiO4) and Fayalite (iron Fe2SiO4). Forsterite has a melting point of 1890degC, Fayalite 1205degC, thus the temperature of the melt can be roughly determined by the composition of the olivine in the sample. It may be found that phenocrysts – crystals much larger than the ground mass of the rock – have a higher magnesium content than those of the ground mass and indicates crysyallisation began much earlier than at the time of emplacement. Zonation, where crystals have a Mg rich ‘core’ and have increasing Fe content towards the rim, also occur and is a similar melt condition/change indicator. Olivines are usually found in Basic and Ultrabasic lavas; Peridotites (Peridot an alternative name of olivine), are rocks predominantly composed of olivine and are assumed to be Mantle derived, occurring as rare lavas and as xenoliths in basalts – ie a fragment of the deeper mantle torn off by and ejected with the rising magma.

Oxides

Excess silica in the form of free Quartz (SiO2), primarily occurs in Rhyolite in the acid division and in Rhyodacite and Trachyte, intermediary between Rhyolite and Andesite (Intermediate).

Metal oxides occur mainly in the Basic/Ultra-basic divisions, with Magnetite the most important (the presence of Magnetite in Basic rocks commonly leads to magnetic anomalies affecting compass bearings); titaniferousmagnetite and Chromite are found as density segregations in some very basic and ultra-basic flows, but usually are more economically important in intrusive complexes.

Reaction Series

This brings us to Bowen’s Reaction Series whereby in a magma cooling from say 2000°C, different minerals fractionally crystallise at gradually lower temperatures – comparable in a way to the ‘cat crackers’ used in hydrocarbon refining vessels where different hydrocarbons are distilled off at different temperatures, To simplify, if the differing minerals crystallise they remove from the melt their components and the melt chemistry changes continually, but if these minerals stay available in the melt for resorption, the mineralogy of the melt changes to a different mineral suite with various component minerals coming out of the melt as their solidification temperature is met, until such time as total melt crystallisation occurs, ie the melt solidifies. Two separate convergent lines of melt alteration, one mafic the other felsic occur thus:

First crystallisation       Olivine                                                                                     Bytownite

                                                Mg Pyroxene                                                                   Labradorite

                                                   CaMg Pyroxene                                                        Andesine

                                                        Amphibole                                                         Oligoclase

                                                             Biotite                                                          Albite

                                                                 K Feldspar                                          Muscovite

Last crystallisation                                                           Quartz

It is obvious that in coarse grained rocks the individual minerals are relatively easy to identify, but in fine grain volcanic material a thin section is invariably required. A thin slice is polished on one side, mounted on a microscope slide with canada balsam – a natural resin with the same refractive index as glass – and the slice is lapped down to 30 microns (0.0030mm). The section is then studied under natural and plane polarised light to identify the minerals present by their optical properties.

In conclusion

Image by eoearth.org Igneous classification.

Finally, one for the ladies, a 4.2 carat Peridot gemstone – the common olivine in a better guise!!

From Directorygemstones.org

ALAN C

The proud Author doing something rather Scottish to a lot of mud.

An introduction to igneous rocks – Part 1

Picture by geology.about.com Image showing an Hawaiian basalt.

What is an igneous rock?

It’s hard, may be pale or nearly black, but what’s in it?

This entry is aimed as a brief introduction to igneous mineralogy/petrology to the ‘beginners’ and it may be useful to have available a mineralogy and/or petrology text, see the ‘Books’ below the title bar.

An igneous rock is essentially a collection of potassium-, sodium-, calcium-(ie alkalis), iron- and magnesium-(ie ferromagnesian) silicates and alumino-silicates, free quartz and ferro- and ferro-titanium/chromium (and other metal) oxide and occasionally sulphide, minerals that have solidified from the molten state, ie magma.

These minerals are grouped in several ways according to their relative importance to the rock mass: Essential or Primary are those from which the rock is primarily composed, eg Quartz, Mica and Feldspar in granite, or who’s presence gives name to a specific rock type, eg Reibekite Microgranite (as on Ailsa Craig – used to make the best curling stones!); Accessory may be present but have no bearing on the rock type, eg Zircon, Apatite in granites; Secondary produced by later weathering or hydrothermal alteration of the original essential minerals,eg Kaolin from the alteration of feldspar in granite or Chlorite from the hydrothermal alteration of primary ferromagnesian minerals.

A further classification is based on the silica saturation of the rock; silica saturated Acidic, silica poor Basic; this classification does not refer to the amount of free silica – ie Quartz – in the rock, but to the total silicate in the minerals present. In addition, Intermediate rocks are those showing mixed acid and basic characteristics; Ultra-basic (or Ultra-mafic) are silica depleted and contain rare oxides. Note, the acid-basic categorisation is not that of chemists redox pH divisions.

Examples of these groups are:

Acid : Rhyolite

Intermediate : Andesite. Note Dacite and Trachyte lavas fall between Intermediate and Acid

Basic : Calc-alkaline Basalt, High-alumina Basalt, Tholiitic Basalt

Ultra-basic : Picritic basalt

The minerals are in 6 main groups: Feldspars/Feldspathoids, Amphiboles, Pyroxenes, Micas, Olivines and oxides (of silicon and metals). The lighter coloured minerals are termed Felsic, the darker ferromagnesian, Mafic; the relative proportions roughly determining the colour of the rock; hence acid rocks which have a high felsic content are generally paler than the basic types with higher mafic minerals.

It may be relevant here to digress to the effects of decreasing silica content on mineralogy as mentioned earlier. With reference to the potassic and sodic feldspar/feldspathoids, the silica saturated end members, feldspar, are Orthoclase and Albite and by the removal – ie silica depletion – of one SiO2 molecule, two stages of felspathoids are produced thus, (ie feldspathoids being silica poor feldspar):

Orthoclase – KAlSi3O8                      Albite – NaAlSi3O8

Leucite – KAlSi2O6                           Jadeite – NaAlSi2O6

Kalsilite – KAlSiO4                            Nepheline – NaAlSiO4

Picture from: mii.org Feldspar microcline.

Feldspars

Feldspars and Feldspathoids comprise the bulk of the felsic minerals, their relationships mentioned above, but feldspars are the larger rock-forming group and are subdivided into potassic (K feldspar) and sodic-Na and calcic-Ca (combined Na and Ca form the Plagioclase sub-group) varieties. They are generally pale coloured, whites, greys to pinks and almost colourless.

K feldspars, predominantly Orthoclase and Sanidine, are characteristic of the more acidic rocks – dacite, trachyte and rhyolites

The Plagioclase group are a chemical continuous substitution series of 6 recognised minerals between the two end members Albite (Na end) and Anorthite (Ca end). The more sodic members are associated, in general, with more acid rocks, calcic with basic. The minerals of the 6 divisions are identified by name and analysis notation of the Albite (Ab):Anorthite(An) ratio thus:

Albite          Ab100An0 to Ab90An10

Oligoclase   Ab90An10 to Ab70An30

Andesine     Ab70An30 to Ab50An50

Labradorite  Ab50An50 to Ab30An70

Bytownite    Ab30An70 to Ab10An90

Anorthite     Ab10An90 to Ab0An100

Pyroxenes and Amphiboles combined, are the main mafic rock forming mineral groups in volcanic rocks and as in the feldspars, both exhibit chemical substitution series between end-members in their respective groups.

Pyroxenes are a large complex group of chain silicates – so called from the molecular strucure of the minerals – and they are subdivided on a crystallographic basis into 2 sub-groups, Ortho- and Clino-pyroxenes (of the Orthorhombic and Monoclinic crystal groups respectively. There are 7 crystallographic groups: Cubic, Tetragonal, Orthorhombic, Monoclinic, Triclinic, Hexagonal and Trigonal; the differences being according to the crystal symetry ie the relative positions of the crystal rotation axes).

The main orthopyroxenes having Enstatite -En – (MgSiO3) and Ferrosilite – Fs – (FeSiO3) as end members, the intervening mineral Hypersthene in older texts is now also referred as Orthopyroxene. The minerals are identified by their En:Fs ratio.

The Clinopyroxenes, again Mg and Fe silicates, but in some minerals with Ca, Al or Na. Augite, Diopside, Pigeonite and Aegrine are the main minerals.

The main Pyroxenes are

Augite           Most common pyroxene in basalt, andesite; contains Al and Ca

Diopside        in Basic rocks; contains Ca

Pigeonite       As Augite; contains Al

Aegrine         Alkali pyroxene, in more acid rocks; contains Na and Fe 

Hypersthene  In Intermediate and Basic rocks

Enstatite       In Intermediate and Basic rocks

The Amphiboles are another large common group of rock forming minerals, chemically comparable with the pyroxenes, the main differences being in the crystal structure with Amphiboles arranged as a double chain and the presence of an hydroxyl radical (OH) in the molecule thus for example an orthorhombic equivalent member of each:

Amphibole   Anthophyllite Mg7Si8O22(OH)2

Pyroxene     Enstatite MgSiO3

Again, the minerals crystallise in the orthorhombic and monoclinic groups and substitution series are between the end-members.

By far the most important Amphibole in igneous rocks is Hornblende (a CaMgFeAl silicate), in the more acid divisions from acid Andesite to Rhyolite; the other members being more commonly associated with metamorphic rocks.

ALAN C

Author in full Scottish action!