Tag Archives: feldspathoids

The Kerguelen Hypervolcano™

Below the Clouds Stair-case by Swedish architects at Stockholm-based TAF Architect Office.

OK, so what in Gódabunga’s name do Swedish stairs and volcanoes have in common! Apart from the fact they can do you a real mischief if you fall down, a staircase in Swedish is trappa and this gives the name to the extensive flood basalt flows of the Traps volcanic provinces from the stair-like appearance of the flows!

Kerguelen

A little known, but very extensive trap province exists in the southern Indian Ocean, some 4000km west of Australia and 1500km north of Antarctica – the Kerguelen Plateau that has developed over the Kerguelen mantle plume.

The Kerguelen Plateau – the second largest submarine plateau -  lies at approximately 1-2000 metres depth, in an abyssal depth of 3-4000 metres, and has three small island groups, Kerguelen, Heard Island and Mcdonald Island as surface expressions. The plateau extends north-westwards for c2200km covering an area of about 2.2m sq km.

Geologically, the plateau has had a colourful history, being classed as a ‘micro-continent’, it is a remnant of the break-up of the Gondwanaland super-continent and is located over the Kerguelen hot-spot. Deep water geological information is from the JOIDES ODP (ocean drilling programme) and seismic interpretation of oil prospecting data; the plateau is shown to be constructed on a general base of Cretaceous terrestrial and/or shallow water sediments – including coal horizons for at about 40m years. Volcanism began during the middle/late Cretaceous (c120m years ago) with emplacement of trachytes and basalts and continued on a large scale into the Miocene/Oligocene and continues up to the present on Mcdonald Island. Recovered ODP samples of felsic and metamorphic rock indicate the possible presence of a crystalline basement at least in part below the Cretaceous deposits. The total volume of the Kerguelen volcanic province is estimated to be in the order of 25million cu km giving an average of 0.2cu km/year. Submergence of the whole plateau was around 20m years ago.

The references below are superb!

http://www.ga.gov.au/energy/province-sedimentary-basin-geology/petroleum/offshore-southern-australia/kerguelen-plateau.html

http://petrology.oxfordjournals.org/content/43/7/1121.full.pdf

Kerguelen plateau, from Wikipedia: Kerguelen plateau topography.

The island groups involved here, are the tiny yellow dots near the north-west end on the elongate NW-SE pale blue area, Antarctica is the orange-red area at the bottom.

Kerguelen Island is the largest of the island groups surfacing above the Kerguelen Plateau; administered under the French Southern and Antarctic Terretories; covers an area of about 3400sq km and rises to 1850m at Mt Ross, the youngest volcanic expression of Plio/Pleistocene lavas – brown on the map below.

Simplified geological map of the Kerguelen Islands from Wikipedia.

The majority of the island is composed of flood basalts, in grey above, along with minor amounts of trachyte, pinkish, and the plutonic complexes (buff-grey) of Foch -north centre – and Rallier du Baty – sw bottom and the small Mt Crozier intrusion – northern of the two eastern promontories. Volcanism, related to the Kerguelen hotspot, began c40m years ago and continued until about 100,000 BP.

Heard & McDonald Islands

Heard Island and the stratovolcano Big Ben
(photo by A. J. Graff, Australian Antarctic Division)

The Heard and McDonald Islands (colloquially the HIMI) are administered by Australia and as such are home to Australia’s only active volcanoes.

Heard Island, apart from having the highest point on Australian territory at 2745m on Big Ben (9006 ft), has two main volcanoes in Big Ben, in part a 5-6km diameter, glacier covered caldera and the smaller Mt Dixon, plus small scoria cones. Big Ben, approximately 18km in diameter, is mainly of basalt/trachytic composition.

Heard Island shows 3 distinct stages of development, the oldest being the deposition of Miocene limestones 40-50my ago being found over much of the Kerguelen Plateau. These carbonates were followed around 9my ago, by 300-350m of volcaniclastic sediments and pillow lavas of the Drygalski Formation. A period of peneplanation of the Drygalski deposits preceeded the present volcanism, starting about 1my ago.

Satellite image from July 2000, showing an active two kilometre long (and 50-90 metre wide) lava flow trending south-west from the summit of Big Ben.
Photo: Thermal Alert Team, University of Hawai'i

The McDonald Island group lies about 27 miles west of Heard Island and is home to the second of Australia’s most recently active volcanoes and the whole total about 1sq mile in area, rising to 212m at Maxwell Hill. McDonald Island burst into action in 1992 after a 75,000year sleep and has been sporadically active since in late 1995-early1996, 2000-2001 and lastly in 2005 from Samarang Hill. The effect these eruptions had on the island was to almost double the size and increase the height by about100m!

The island is composed mainly of interbedded ,viscous phonolitic tuffs and lavas; phonolite being named after the resounding ‘ring’ when struck, is tough, pale coloured with a high felsic content of predominant feldspathoids over feldspar and is characteristic where a mantle plume is overlain by a thick continental crust.

2004 satellite image of McDonald Island showing island's extent in 1980 (striped).

ALAN C

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 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!

 

Ol Doinyo Lengai – The Mountain of God

Picture from Wikipedia. The 1966 eruption.

Ol Doinyo Lengai in Tanzania, approximately 20miles (30km) to the south of Lake Natron, is one of the most peculiar volcanoes.

Whereas all other volcanoes eject silicate based lava at a temperature of 1000°c plus, this gorgeous monster, at 2980metres (9780ft), ejects molten natrocarbonate at around 5-600°C. and is the only known active example of the type.

Picture taken from Volcanodiscovery. Erupting hornito at night.

Natrocarbonate is a mixure of sodium and potassium carbonates along with a minor quantity of silicate minerals and this type of lava and ejecta so produced are called Carbonatites. The silicate minerals are silica-undersaturated suites, as feldspathoids and nepheline along with olivine and pyroxenes ie minerals with ultra-basic affinities.

When newly erupted, these rocks are dark grey to black in colour and quickly weather to a pale grey to white. Unlike silicate high temperature lavas that are incandescent in daylight, the relatively low carbonatite temperatures give a lava that is black, glowing dull red at night.

Ol Doinyo is one of the volcanoes along the line of the African Rift Valley system. This rift system extends from the Mozambique coast opposite Madagascar in the south, northwards through Africa into the Red Sea by the Horn of Africa and extends past Sinai into the Dead Sea area. This rift system is the present day activity associated with the contunued break-up of the Gondwanaland super-continent and continental drift.

Picture by Nasa

An excellent suite of photos of Ol Doinyo volcano and landforms are found here:

http://www.photovolcanica.com/VolcanoInfo/Oldoinyo%20Lengai/Oldoinyo%20Lengai.html

Carbonatite intrusives are widespread, but very uncommon and include dykes, sills, pipes and veins.

Photograph taken by Eurico Zimbres. Jacupiranga, Brazil intrusive carbonatite: Calcite, Magnetite and Olivine.

Although rare, the presence of free carbon (rather than carbonates) is not unusual in igneous rocks, as evidenced by Diamonds in widespread, but rare, Kimberlite pipe intrusions, graphite in doleritic intrusions – eg the intrusion in Borrowdale (English Lake District, the original graphite source for the Cumberland Pencil Co. in Keswick) and in extra-terrestrial carbonaceous chondrite meteorites.

Diamond in Blue Ground Kimberlite.

Picture taken from http://www.eoearth.org/article/Diamond

Alan C