Hellisheidahraun Lava

Photographer: Eggert Nordahl, all rights reserved. Notice the pipelines up to the left, and the SIL sticking out ontop of the further crater rim.

Hello Everyone!

I wish to congratulate the winners of this weeks competition. I had done a lot to the image by Eggert Nordahl. I croped it to remove the Hellisheidarvirkjun pipelines on the left of the picture, and I also croped off the Hellisheidar-SIL. Then I inverted it to further bungle up any Google Image Searches.

Map showing where the mystery lava image was taken. As you all notice it is slap bang between the various bore-holes.

So, Alan caught the surface lavas, Irpsit nicked the Grámosi and the Hengill points, and finally Spica had the Hellisheidi eruption (in Icelandic Hellisheidahraun).

While coasting around for info on Hengill I found a drill core that had been analyzed. It seems like Hengill has had not less then 5 different magma-zones. I think the weirdest would be the chloro-epidote stage. If I understand it correctly chloro-epidotes are constructed by forsterite (olivine) is quenched into a salt-brine. This phase would of course be from the time when Hengill was rather suffering from being sub-aquatic (before it rose out of the sea). I guess that Alan will let me know if I got this all wrong and in no uncertain terms.

So, to round it off, one could say that the image is from the Hellisheidarvirkjun Powerplant.

CARL

Icelands forgotten Volcanoes

Eldgigur seen from the Hágöngur volcano.

Sometimes a bit of honest digging will pay off in the oddest ways. Trying to utilize an unexpected free day I decided to find an alternative explanation for the uplift associated with the Hamarinn volcano. A long shot as good as any other to pursue on a rather grey day.

While on the prowl for this unknown central volcano I got a lead, and started to dig. To my surprise I found an old (1952) geological report from a Danish survey that seemed to have something to do with my suspected culprit. To my utter surprise it was about another equally unknown volcano. And as if this was not enough, it also named a few other volcanoes I had never heard about.

Image taken from the linked paper below. It shows very well the Grimsvötn fissure swarm as it goes to the SSW through Thordharhyrna volcano.

As you can see on the image the top most volcano is one of the more infamous on Iceland, Grimsvötn. And that one does not merit a lot place here, more than the obvious mentioning of its southern fissure swarm that ends up in Laki. Most of you know already that Grimsvötn was the responsible parent volcano for the Laki rifting fissure eruption, and that Thordharhyrna erupted simultaneously with Grimsvötn and Laki.

But hand on the heart, how many of you knew about Háabunga central volcano? Well, some of the more volcanoholic of the readers of this blog probably do know about it. I did at least. Also Thordharhyrna is well known, but there the fun probably ends for the readers of the blog.

SSW of Thordharhyrna resides a volcano I never had heard of before, Geirvörtur. About this one I cannot say much, at least more than that it resides on top of the Grimsvötn fissure swarm, and that probably is a remote sub feature of Grimsvötn.

SSW to Geirvörtur we have the 1200 meter high volcano of Hágöngur. And know it is starting to get really exiting. Close by to Hágöngur is the SE is the 854 meter high post-glacial volcano of Eldgigur. It resides on the Grimsvötn fissure swarm, but it is doubtful that it is magmatic subset. This is due to the rather odd nature of the lava.

Eldgigur seen from the other side.

Technically this is a very large scoriae cone, at the top resides two small and one large crater. Only the larger crater has produced a minute lava flow. The rest of the volcano is built up of 3 layers of different lavas showing as 3 concentric slag walls.

The cone is built up by very fine grained material filled in with bombs. The first type of lava is a black plagioclase containing phenocrysts of clinopyroxene. The second layer is a grey more evolved and crystallized lava that is translucent. It is a type of plagioclase-porphyric lava. The third lava is named as bytownite, be that as it may, the content of iron is high in the red lava with 15 percent by volume. Inside all 3 types of lava are found layers of clear colorless olivine (forsterite).

The size of the grain is very small, 2 millimeters and downwards. Inside of this are lava bombs prolific. Especially the red lava and black lava seems to have produced a lot of lava bombs during eruptions. The grey translucent lava and the forsterite seems to have produced significantly less lava bombs.

The lavas point to a totally atypical form of eruption for the Grimsvötn line. The eruptions seems to have been very explosive for the size of this volcano. Noteworthy is that Eldgigur shows no sign of being affected by ice, nor water, so all 3 of the eruptions has happened after the end of the latest ice age. Ice would have affected the shape of the volcano due to its loose lavas, and water would have weathered the olivine.

Iceland is famous for having 27 active volcanoes. This is the believed number of volcanoes that has erupted after the last ice age, and has the ability to erupt again. Clearly Eldgigur has had 3 eruptions after the last ice age, and due to the lack of weathering of the olivine, the last of them should have been during the last 2000 years, probably a lot later than that. So, I think I can safely say that Iceland just got a number 28 to worry about.

With weathering of olivine I mean that pulverized forsterite will decay from a member of the olivine family into a member of the carbomagnesian family due to magnesiums very reactive capabilities as it gets into contact with water and CO2. A centimeter thick layer of powdered forsterite will decay completely within 3 to 5 years if it is left in the open air. So, you can actually date it.

http://2dgf.dk/xpdf/bull-1952-12-2-222-226.pdf

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!