Geology Matters » Rocks We think Geology Matters, do you? Tue, 21 May 2013 06:54:57 +0000 en hourly 1 The language of the quarryman Thu, 17 May 2012 10:00:33 +0000 simonharris At the same time that scientists were beginning to differentiate and name rock units, the quarrymen working deep in the mines underneath Wren’s Nest would have developed their own naming system for the rocks they encountered. Working by candlelight, their names would have been based on basic features and the look of the rock. Experienced miners would have been able to tell the grade of rock they were mining from one look or perhaps even by feel alone.

Seven Sisters Cavern: The miners have removed as much of the limestone as possible using what was known as the "pillar-and-stall" method. As these pillars have eroded further, they have left the caverns in danger of collapse.

Murchison lists some of the words used by the miners and quarrymen in his Silurian System:

  • “Strong hanging stone”
  • “Top sink”
  • “Pricking”
  • “Half-yard measure”
  • “Strong grey measure”
  • “The flints”
  • “Silks measures”
  • “bavin” and “rotch”
  • “ballstone”

Some of the names we can only begin to guess at the meaning of, but it is not unreasonable to think that perhaps “Half-yard measure” would have been a band of limestone 18 inches thick, and “pricking” is described as a way-board of shale (blasting layer) so would perhaps have been drilled (pricked) with numerous holes which could be filled with black powder to blast layers of rock away from the walls of the mine.

We do know that “ballstone” referred to a dome-shaped mass of pure limestone which would have disrupted the bedding of the rock around it, and could sometimes have been very large, up to 6 metres high by 20 metres wide. We now refer to them as bioherms and we know that they represent the remains of small reefs similar to those found in tropical lagoons today, and here the reef-building organisms such as corals and stromatoporoids formed massive calcareous structures which would later be turned into limestone.

Today the science of lithostratigraphy deals with the naming of rock layers and formal names are assigned to these rock units such as the Upper Quarried Limestone Member.


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Erratic Anvil’s Thu, 29 Dec 2011 09:07:58 +0000 chrisb This is an extract from the Black Country Geological Society newsletter number 210, December 2011. It investigates how glacial erratics have been used as a local resouce in the Black Country.

A Granite Erratic anvil


The story of the erratics


A monument to the Erratics





For further information see:

Written by Mike Williams
BCGS Treasurer





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Rock with laughter Thu, 08 Dec 2011 08:59:26 +0000 chrisb This short but amusing extract entitled ‘Geobabble’ was taken from the newsletter of the Black Country Geological Society number 187, December 2007. It reveals that even minerals can have a sense of humour at this festive time of year.

Essexite by zeesstof via Flickr

Petrology deals with the origin, composition, structure, and alteration of rocks. To do this geologists slice up perfectly good wild rocks into extremely thin sections, thin enough for light to pass through the minerals. Once the sections have been glued to glass slides, they can then observe the mineralogical make-up of various rocks. Igneous rocks in particular can be very spectacular under cross polarised filters within the microscope. When viewed under crossed polars some minerals can be very colourful, for example Olivine can be yellow, red and blues; this is called birefringence. To see the different colours from the minerals you must twist them around on the microscope the change in angle shows different colours. Quartz, which is colourless under crossed polars, becomes dark (or black) at certain angles, this is called extinction. Because this extinction (dark area) moves across the quartz crystal in a wave-like fashion as opposed to the whole mineral becoming dark, it is called an undulose extinction.

Those of you who have studied geology will know the feeling when spending hours at a petrological microscope looking at slide after slide as part of a study. Imagine then the reaction of the geologist when a smiley face presented itself. This is exactly what happened to a researcher looking at a thin-section from the volcanic deposits on Santorini. This accretionary lapilli or coarse ash had this happy olivine crystal. About 2mm across, in a groundmass of rectangular plagioclase laths, larger pyroxene crystals and volcanic glass that appears black on the slide. This erupted onto the cinder cone 54,000 years ago.

An Olivine mineral smiles back

Written by Bill Groves and Chris Broughton, picture and information from David Miller.
BCGS Members


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Ore inspiring Thu, 20 Oct 2011 09:01:15 +0000 chrisb Tin, Dudley Museum Service

Native Tin, 007494

This is an extract from ‘The Dudley Bug’ section in the Black Country Geological Society newsletter 209: October 2011.

Today we use metal for practically everything, over the last 50 years uses and demand for metals has increased due to their properties. The most common metals we use in today’s modern society include copper, aluminium, iron and steel. Less abundant metals include gold, titanium, platinum and silver.

Metals are found naturally occurring within rocks but are only economically viable when the geological processes which formed the rock have caused the metals to concentrate. These concentrations are known as ore deposits. They can be found within the earth formed through magmatic processes, or on the surface formed through weathering and deposition.

In order to work out the processes which have formed the ore deposit we use SADE: Source Agent Deposition Energy. The source of metal is where it is found on a long term basis such as the crust or the oceans. The agents of transport indicate how they are removed such as in magma or water. This can depend on the solubility of the metal which can be altered by temperature, pH and other substances around. In order for a metal to be deposited a suitable site and the correct conditions are required. Finally the energy to move the substance is required, such as convection or radioactive decay. The three main ways in which ores are deposited are through igneous, surface and hydrothermal processes.

Magmatic processes form three types of deposits: magmatic segregation deposits, pegmatite deposits and porphyry copper deposits. Magmatic segregation deposits can form through fractional crystallisation within a magma chamber. This occurs when different minerals cool and crystallise at different times. As it cools this separates, and may sink to the bottom of the magma chamber and form layers at the base. The layers concentrated the different minerals in order of freezing point. Another method is liquid immiscibility, which occurs when two magma fractions separate from a homogeneous magma. They may be chemically similar or one of the fractions may be enriched in a particular mineral. This means that this remaining magma could be saturated with ore minerals such as sulphides.

Pegmatite deposits are formed as pegmatite rock which is a very coarse igneous rock associated with granite plutons. They form with the final part of the magma chamber that is still un-crystallised. Because this part of the magma usually has high water content, the crystals which form as it cools are very large, for example the Bikita Pegmatite deposit in Zimbabwe.

Malachite, Copper Ore

Malachite, Geology temp 41

Porphyry copper deposits are associated with porphyritic felsic intrusions which form large but very low grade deposits. These deposits are formed as rising magma degasses and the fluid is left carrying soluble metals known as chlorite complexes. As the magma continues to rise it boils causing the surrounding rocks to fracture. The fluid quickly fills the gaps created by the fracturing and begins to alter the rocks. As it cools the metals within the fluid cool and form vein deposits which can be large enough to form ore deposits. Because of the location of these deposits within rock, it isn’t economically viable to extract the ore deposit on a small scale. The main metals which are sourced from these deposits include Cu (copper), Sn (tin), Au (silver) and Ag (gold). One of the largest and best known examples of a Porphyry Copper Deposit is the Bingham Deposit in Utah, USA. This deposit measures 2.5km x 1.7km and is 1.0km deep.

Surface process deposits form from processes which include chemical leeching where a substance is removed from a rock by fluids which run through the rock. Others include secondary enrichment deposits and chemical transport. The latter method forms banded iron ore deposits.

The stability of a metal ore depends on how reactive the metal is. Very reactive metals such as aluminium react to form stable oxides. Less reactive metals such as iron react to form less stable oxides. So the reactivity of the metal will dictate its extraction method. Different methods are used to extract metals.

Alison Roberts
BCGS Member


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A Quick Guide to the Isle of Skye, Scotland Thu, 22 Sep 2011 09:00:57 +0000 chrisb This is an extract from the newsletter of The ‘Black Country Geological Society‘, No. 197 October 2009. This was part of the regular section entitled ‘The Dudley Bug’ written by members Alison Roberts and Chris Broughton.

A handdrawn geological map (by A. Roberts)

The Isle of Skye is the largest of the Inner Hebridean Islands with an area of 3000Km2. Portree is the largest town on the island. The northern part of the island is formed from volcanic activity which is represented by the lava flows that dominate the landscape. Basaltic columns are a popular feature here and can be seen at Kilt Rock. Macleod’s Tables display the flat hilltop topography formed by the lava flows. In some places these dip slightly, particularly at Quirang. Skye’s volcanic activity occurred during the Cenozoic Period (previously known as the Tertiary Period) and is associated with the opening of the Atlantic Ocean 60 million years ago. Central Skye is known as the ‘Skye Central Complex’ and is home to the world famous Cuillins which are formed from two very different igneous units. Britain’s youngest mountains, the ‘Black Cuillins’ are predominantly made from gabbro and peridotite whereas the ‘Red Cuillins’ consist mainly of granite.

The southern part of the island displays the sedimentary units, some dating back to the Precambrian with the youngest types being glacial deposits from the two glaciations which affected Skye between 13,000 and 26,000 years ago. The limestones of the Durness Group display some of the best limestone pavements and related features in the UK. These can be found in the Strath Suardal area, immediately south of the Red Cuillins. The majority of the dykes trend in a general NW-SE direction.

Black Cuillin, Bla Blaven (by A. Roberts)

The oldest rock found on Skye is the Lewisian Gneiss which dates back to the Precambrian when it was formed from the metamorphism of older rocks. The oldest sedimentary rock is the Torridonian Sandstone. Due to the vast amount of igneous rocks there has been a lot of metamorphism altering the sedimentary rocks. The most distinct unit to have been metamorphosed is the limestone from the Strath Suardal Formation of the Durness Group which is Ordovician in age. This has been metamorphosed not only by the volcanic activity of the Red Cuillins but also by the nearby Beinn an Dubhaich Granite. This has formed the famous Skye Marble which is currently being quarried at the Torrin Marble quarry near Broadford, the second largest town on the island. The Moine Thrust which is one of the most important faults in Northwest Scotland runs from the Shetland Islands to the Sleat Peninsula on the south tip of Skye.

Many of the geological units which crop out on Skye can be seen in the Northwest Highlands. These include the Applecross Formation and Diabaig Formation of the Torridon Group, the False-bedded Quartzite Member and the Pipe Rock Member of the Eriboll sandstone Formation and finally the Fucoid Beds Member, the Salterella Grit Member and the Ghrudaidh Formation of the An t-Sron Formation.

Gillen, C. 2003. Geology and Landscapes of Scotland. Terra Publishing: England. 10, 60-61, 152, 155, 194, 204 pp.
Roberts, J. L. 2004. The Highland Geology Trail. Bell and Bain Limited: Glasgow. 69-78 pp.
Stephenson, D. 1994. Skye: A Landscape Fashioned by Geology. Scottish Natural Heritage and British Geological Society: Edinburgh. 2-3 pp.

Alison Roberts
Wallheath, West Midlands


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What is an igneous rock? Thu, 31 Mar 2011 09:00:51 +0000 alisonr Granite and Microgranite

Granite and Microgranite from Scotland

Igneous rocks are hot molten rocks which rise towards the surface. The term igneous originated from the Latin term ignis which means ‘fire’. In order for them to begin to melt temperatures can be anything from 700°C.

They begin to melt down within the deep crust or upper mantle and because the molten rock has a lower density (is lighter) than the surrounding solid rock it begins to rise through cracks within the solid rock.

Once they are near the surface they either erupt through a fissure or volcano (such as in Hawaii or Iceland) or stop just below the surface within the crust where they crystallise. They are either intrusive (solidify quickly within the crust) or extrusive (solidify slowly on the surface). The most common extrusive igneous rock is basalt which is erupted through fissures or volcanoes and the most common intrusive rock is Granite.

Igneous rocks are likely to contain one of the common minerals associated with igneous  rocks; these include quartz, mica, feldspar, olivine, pyroxene and amphibole.


Obsidian (Volcanic Glass)

Today igneous rocks are still forming at locations such as Iceland and Hawaii or around the Pacific Ocean ‘Ring of Fire.’ Iceland is on a constructive plate boundary where two plates are pulling apart along the Mid Atlantic Ridge, this causes magma to rise. Iceland has formed by the build up of lava to the point it has risen above the sea. Hawaii is not on a plate boundary but is within the middle of a plate boundary which is known as an inter-plate hot spot. Both Iceland and Hawaii are believed to be above a mantle plume, which is a large molten body of magma which is slowly rising and causing basaltic volcanism at the surface.


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What is a metamorphic rock? Thu, 24 Mar 2011 09:00:53 +0000 alisonr Mica Schist

Mica Schist from Ireland

A metamorphic rock is a rock that has been altered. The word metamorphic comes from the Greek terms meta which means change and morphe which means form, so to change form. The process can alter metamorphic rocks, igneous rocks and sedimentary rocks. It is important to note that the temperatures at which metamorphism occurs is lower than that of which a rock would melt. Usually it ranges between 250°C and 700°C.

Metamorphism causes the parent rocks to alter and re-crystallise. Some metamorphic rocks such as gneiss can be identified as being metamorphosed because the minerals within the rock have realigned. This gives the rock a layered appearance, the best example of this is a schist or gneiss.

There are two types of metamorphic rock; regional metamorphism and contact metamorphism.



Regional metamorphism occurs, as its name suggests over a large area due to high temperatures and pressure. This commonly happens during mountain building when two plates are colliding. Examples of this today are the Himalayas, one of the youngest mountain chains in the world, which have formed through the collision of the Indo-Australian Plate and the Eurasian Plate. Contact metamorphism occurs through high temperatures within a smaller area, around an igneous intrusion (magma chamber).

A common example of metamorphic rocks in the UK is slate which is common in Wales but metamorphic rocks are found across the UK especially in the Lake District.


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What is a sedimentary rock? Thu, 03 Mar 2011 12:24:45 +0000 alisonr Sandstone

Layered Sandstone

Sedimentary rocks are formed by the breakdown older rocks into smaller grains to form sediment through surface processes. Over time these are deposited mainly with the use of water and build up in layers to form sedimentary rocks. In dry, arid environments such as the desert wind blown grains of sand build up and form sand dunes.

In time these are buried and form the sedimentary rock through the process of lithification. This process involves the sediment becoming cemented and then compacted to form the sedimentary rock.

They are commonly layered and contain sedimentary structures such as ripples, laminations (bedding) and grading. These structures help in reconstructing the environment in which the sediment would have been deposited. Not all environments with sediment in are preserved. A good example of are beach environments, which are hardly ever preserved.

Common sedimentary rocks include sandstone, limestone and mudstone.


Halite (Rock Salt) from Cheshire

Fossils are preserved within sedimentary rocks and may even make up a limestone. They can form limestone such as the Portland Limestone which consists of the remains of small microfossils contained within ooids.

Evaporite rocks such as halite and gypsum are classed as sedimentary rocks and have formed by the evaporation of sea water, leaving behind large deposits. Examples of this can be seen in Cheshire and south-east Spain. The deposits in Spain formed when the Mediterranean Sea dried up due to the water evaporating.


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What are the differences between rocks and minerals? Thu, 17 Feb 2011 09:00:47 +0000 alisonr
Milky Quartz

Milky Quartz from Huaron Mine Peru

The differences between rocks and minerals are that a mineral is a single constituent with a particular colour, atomic and crystalline structure. A rock however is an aggregate of different minerals which does not have a set composition or structure. So without minerals you wouldn’t get rocks.

Orthoclase Feldspar xenolith

Orthoclase Feldspar crystal from Hay Tor

Minerals are often single crystals with a particular shape which helps with identifying them, whereas rocks don’t have any particular shape of crystal. Rocks may contain fossils whereas you don’t find them within minerals.

A well formed crystal would have formed over a long period of time which allows it to develop its structure and shape specific to its mineral form. Minerals within a rock are different, particularly in igneous rocks because they cool quickly so are often small irregular crystals.


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