Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Geezer on 09/01/2010 00:31:06
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There is an island off the East coast of Scotland called Bass Rock (no kidding Bass). There is a similar island off the West coast called Ailsa Craig, which is where material to make curling stones was quarried, so presumeably, it's largely made of granite.
What produced these strange islands? Were they formed around the same time, and are there similar islands in other parts of the World?
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Well, besides being covered with bird sh1t, Bass Rock is a volcanic island - according to Wikipedia.
That means a volcano spewed out rocks. But you cannot make book on that. Ailsa is billed as "... granite was quarried to make curling stones. "Ailsa" is pronounced "ale-sa", with the first syllable stressed. The now uninhabited island is formed from the volcanic plug of an extinct volcano."
This is not possible. Granite is NOT volcanic rock. It is an intrusive igneous rock with a different mineral composition than extrusive rocks. On the geologic map viewer of the UK - which is quite lacking in usability - shows these two islands in red. Red is an extrusive rock. Going back to the 1094 Geologic map of The British Isles the rocks are identified as Tertiary Basaltic Rocks. As Ailsa is an old volcano, I think I would go with the Tertiary Basaltic Volcanic as the cause for both of these places.
And I must comment on the geologic maps of The British Isles that are available. The British Geologic Survey does NOT put labels or a legend on their on-line map! Since they do not use the WORLDWIDE normally accepted coloring (colouring) for their formation types and age, it makes it using the map only a British "insider" map. It seems the gentry at the survey feel only the British need to know what is going on in their land. SNOBS!
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I thnk you're being a bit unfair there JB. I mean, when they produced it in 1094, there probably were not too many international standards to align with.
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In terms of their attitudes, and making the data freely available, the British OS and BGS sucks in comparison with the USGS.
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I thnk you're being a bit unfair there JB. I mean, when they produced it in 1094, there probably were not too many international standards to align with.
Actually the seal on the wax said 1095
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In terms of their attitudes, and making the data freely available, the British OS and BGS sucks in comparison with the USGS.
This.
BGS charge the Mineral Planning Authority (County & Unitary Councils)an annual licence fee to supply and use "their" data to produce constraint and consultation maps (the idea being that consultation maps can prevent someone building something that sterilises a mineral resource).
BGS then complained when the maps were included in publications produced by the Mineral Planning Authority...
Flipping ridiculous.
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Dr. Ian West has done a great service for his country by putting up a web age on the Geology of the UK. It is the only real source for this information. The maps are form the early part of the last century but they are as detailed as you are going to get.
Plug for:
Dr Ian West
School of Ocean and Earth Science ,
National Oceanography Centre, Southampton
Southampton University
His Home page is http://www.soton.ac.uk/~imw/
The first link in the table below the pictures is to his page on the geology of Great Britain. Maps are based on Victoria and turn of the century geologic maps. They are great in the absence of the UK doing any decent publishing of their own.
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Cool! I'll check it out.
BTW - How did Ailsa Craig come to be made of granite if it's a volcano?
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Heh, while we're on the subject of UK Geology and early geological maps, I think mention ought to be made of William Smith, who more or less established Geology as a science and who produced the first geological map of Britain and Wales. Smith's map has been the basis of just about every geological map of the UK since then.
http://en.wikipedia.org/wiki/William_Smith_(geologist) (http://en.wikipedia.org/wiki/William_Smith_(geologist))
I can recommend Simon Winchester's biography of Smith: 'The Map That Changed the World: William Smith and the Birth of Modern Geology' as a good read if you're interested in geology.
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BTW - How did Ailsa Craig come to be made of granite if it's a volcano?
Perhaps wee need to teach Geezer to read.
Ailsa is billed as "... granite was quarried to make curling stones. ... The now uninhabited island is formed from the volcanic plug of an extinct volcano."
This is not possible. Granite is NOT volcanic rock. It is an intrusive igneous rock with a different mineral composition than extrusive rocks.
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BTW - How did Ailsa Craig come to be made of granite if it's a volcano?
Perhaps wee need to teach Geezer to read.
Ailsa is billed as "... granite was quarried to make curling stones. ... The now uninhabited island is formed from the volcanic plug of an extinct volcano."
This is not possible. Granite is NOT volcanic rock. It is an intrusive igneous rock with a different mineral composition than extrusive rocks.
OK, wize guy, you're saying it's not a volcano. So how was it formed, oh mighty oracle? And don't try tell me it was extruded from a tube of toothpaste or something.
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As the Oracle has already stated ...
As Ailsa is an old volcano, I think I would go with the Tertiary Basaltic Volcanic as the cause for both of these places.
Geezer, please get a reading instructor.
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The now uninhabited island is formed from the volcanic plug of an extinct volcano."
This is not possible. Granite is NOT volcanic rock. It is an intrusive igneous rock with a different mineral composition than extrusive rocks.
Sorry Jim Bob, despite your reputation here you can sometimes be wrong.
1. The mineral composition of rhyolites differs in no major way from that of granites.
2. While granite is not extrusive you are presuming that all volcanic rocks are extrusive and are quite ignoring the solidified magma chambers and conduits to surface that are eqaully expressions of volcanic activity.
3. Ailsa Craig constitutes the latter. It is a micro-granite, reflecting its proximity to surface at the time of crystallisation.
If my memory serves me correctly it is one of the Tertiary volcanic centres on the West Coast of Scotland associated with the opening of the Atlantic Ocean.
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The now uninhabited island is formed from the volcanic plug of an extinct volcano."
This is not possible. Granite is NOT volcanic rock. It is an intrusive igneous rock with a different mineral composition than extrusive rocks.
Sorry Jim Bob, despite your reputation here you can sometimes be wrong.
1. The mineral composition of rhyolites differs in no major way from that of granites.
2. While granite is not extrusive you are presuming that all volcanic rocks are extrusive and are quite ignoring the solidified magma chambers and conduits to surface that are eqaully expressions of volcanic activity.
3. Ailsa Craig constitutes the latter. It is a micro-granite, reflecting its proximity to surface at the time of crystallisation.
If my memory serves me correctly it is one of the Tertiary volcanic centres on the West Coast of Scotland associated with the opening of the Atlantic Ocean.
LOL!
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OK, I made a mistake in the chemistry, neglecting rhyolites. They are extrusive rocks. My bad.
Now, to pick nits and correct misconceptions:
Volcanic -
1. Characteristic of, pertaining to, situated in or upon, formed in, or derived from volcanoes. See also: extrusive
Granite -
1. Coarse-grained igneous rock dominated by light-colored minerals, consisting of about 50 percent orthoclase, 25 percent quartz, and balance of plagioclase feldspars and ferromagnesian silicates. Granites and granodiorites comprise 95 percent of all intrusive rocks.
Source: Leet, L. Don. 1982. Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall
2. a. A plutonic rock in which quartz constitutes 10% to 50% of the felsic components and in which the alkali feldspar/total feldspar ratio is generally restricted to the range of 65% to 90%. Rocks in this range of composition are scarce, and sentiment has been growing to expand the definition to include rocks designated as adamellite or quartz monzonite, which are abundant in the United States. AGI
b. Broadly applied, any holocrystalline, quartz-bearing plutonic rock. Syn: granitic rock
c. Commercial granite. AGI
Source: Dictionary of Mining, Mineral, and Related Terms
By definition (above) all volcanic rocks are associated with volcanoes, excessivly, predominantly extrusive with MINOR, very shallow non-extrusive rocks. There is a vast difference between extrusive and intrusive. Granite, no matter how fine grained, is NOT extrusive. It cannot be volcanic by any stretch of the imagination. All volcanoes need a magma chamber beneath them BUT not all magma chambers are granites. Magma chambers, when they become solids, become intrusive igneous rocks.
Igneous rocks begin as magma. Intrusive igneous rocks, like granite, form when magma cools inside the Earth. Extrusive igneous rocks, like the basalt lava flow in this photo, form at the Earth's surface. Volcanic rocks are extrusive igneous rocks. - http://volcano.oregonstate.edu/vwdocs/vwlessons/rocks.html
Felsic rocks (rhyolites, andesites, granites, etc.) are almost always associated with subduction zones, not rifting.
Ailsa Craig cannot be an extrusive granite. It can be an extrusive rhyolite.
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http://maps.bgs.ac.uk/geologyviewer/
1:625,000 Geology details
UNNAMED IGNEOUS INTRUSION, PALAEOGENE - FELSIC-ROCK
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OK - I found the references needed:
First a picture came upin the search - "http://www.scran.ac.uk/database/record.php?usi=000-000-522-095-C" The caption is
"Title: Riebeckite granite; Ailsa Craig, Firth of Clyde, Ayrshire
Scran ID: 000-000-522-095-C
Resource Rights Holder: British Geological Survey / NERC. All rights reserved."
OK - Now this interests me as riebeckite is a rather unusual component of granite. I have a piece of riebeckite on my finger - see bottom of this post
Wikipedia - Riebeckite "The riebeckite granite known as ailsite, found on the island of Ailsa Craig in western Scotland, is prized for its use in the manufacture of curling stones.
NOW - as riebeckite granite is found all over Scotland how did it get there? The Answer:
Abstract
"Basin structure and Tertiary magmatism on Skye, NW Scotland
R. W. H. BUTLER1 & D. H. W. HUTTON2
1 Department of Earth Sciences, The University, Leeds LS2 9JT, UK
2 Department of Geological Sciences, The University, Science Site, South Road, Durham DH1 3LE, UK
"The emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic, Mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW–SE-trending faults that transfered Mesozoic displacements between the principal fault strands.
"A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation."
The island of Ailsa Craig is a product of inner-catatonic granitic emplacement.
My riebeckite ring:
This is a ring I inherited from my father. He made this from its original components. He cut and fashioned the stone and made the setting from bulk silver using wax to make the mold and poring the silver into the mold and setting the stone. It may not be the most precious ring in the world but it is one of the few things I really treasure.
The blue stone is riebeckite. The brown is a streak of what is commonly known as tiger's eye agate. The riebeckite is called blue tiger's eye. Riebeckite is a blue, sodium rich amphibole. - Na2(Fe,Mg)5Si8O22(OH)2.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi38.photobucket.com%2Falbums%2Fe111%2Fgeezer69%2FIMG_0504a.jpg&hash=af118d130a3bcd7f146232c781d6d925)
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Igneous rocks begin as magma. Intrusive igneous rocks, like granite, form when magma cools inside the Earth. Extrusive igneous rocks, like the basalt lava flow in this photo, form at the Earth's surface. Volcanic rocks are extrusive igneous rocks. - http://volcano.oregonstate.edu/vwdocs/vwlessons/rocks.html
Felsic rocks (rhyolites, andesites, granites, etc.) are almost always associated with subduction zones, not rifting.
Ailsa Craig cannot be an extrusive granite. It can be an extrusive rhyolite.
Technically, intrusive- but barely so. These rocks cooled in the vent of the volcano. Just to really mess up the terminology, I would call them subvolcanic.
From what I can gather, both Bass Rock and Aisla Craig are volcanic plugs, igneous rock that rose up into the volcano, cooled just below the vent, but never actually made it to the surface. The similarity ends there.
Ailsa Craig is a young (Tertiary) riebeckite granite. Riebeckite is a sodium rich amphibole, and JimBob displays a fine sample of it on his ring. This volcano erupted in a basalt province during a rifting event.
My new favorite place, Bass Rock, is quite a bit older (say 200 million years older), and is made up of phonolite (a silica-poor igneous rock). This volcano formed from partial melting of aluminum rich crustal rocks during a compressive event (subduction). Probably the most famous phonolite volcanic plug is Devil's Tower in Wyoming- the focal point of the movie "Close Encounters"
In mineral exploration, sub-volcanic is usually a good thing.
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Hey! That really, er, rocks Bass.
Would the volcanoes that formed these islands have formed over a common hot spot, if that's the right term, or is impossible to know?
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the amount of times iv been through that area of the world and never seen it gutted
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so a volcanic plug is a mass of rock that was used in the forming of volcainos. volcanic rack is rock that got spewed out of the volcaino?
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Hey! That really, er, rocks Bass.
Would the volcanoes that formed these islands have formed over a common hot spot, if that's the right term, or is impossible to know?
Wouldn't be a common hot spot, since these volcanic plugs formed by different processes and there is such a large gap in the times of eruption (~200 million years)
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so a volcanic plug is a mass of rock that was used in the forming of volcainos. volcanic rack is rock that got spewed out of the volcaino?
Right.
Think of the plug as the molten rock that never quite got to the surface, but hardened in very shallow subsurface in the volcanic conduit.
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"I just love hard rock geologist," he said sarcastically, "They come up to a limitation in their terminology and create new terms, akin to the term 'sort of pregnant.' "
I will wager dollars to donuts that some geologist even invent terms - say, oh, for breccias. [;D]
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Hey! I resemble that remark! [::)]
(JB, you old softy, jealousy really doesn't become you) [:D]
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I have a "crusty" personality to maintain - don't give it away fer crip's sake!!!!!!
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Yep. As I often say, JB is just a sedimental softy (or was it a soft sediment?)