Explore Cornwall's extraordinary geological story with the Cornwall Geologist.
Learn all about the dynamic story of Earth's history locked in the rocks beneath our feet and discover how it has shaped the Cornish landscape.

Cornwall Geologist is an Earth science educator and communicator, whose purpose is to promote, conserve, and educate about Cornwall’s fascinating geological history.

We provide guided walks and tours, talks and workshops, educational workshops and field trips, and consultancy services towards SW England geology and Earth Science education. Find out more about the services we provide below.

Come and explore Cornwall in a way you have never done before! Learn how to read the rocks beneath our feet to tell the story of our geological history, and discover how it has influenced Cornwall's environment, history, and culture.

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Take a walk around Cornwall's world-class geological sites and areas of outstanding natural beauty whilst guided by the Cornwall Geologist!

Delve deeper in to Cornwall's geological past with talks and workshops hosted by Cornwall Geologist.

Develop and enhance your learning and teaching of geology and Earth sciences with the Cornwall Geologist.

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There are some really complicated rocks down at Botallack! This is one of them - a garnet-magnetite skarn.

The Mylor Slate Formation that crops out here underwent low-grade regional metamorphism and was deformed during the Variscan Orogeny in the Carboniferous.

It was subsequently contact metamorphosed (hornfelsed) during emplacement of the adjacent Land’s End Granite in the Permian.

Some of the mafic rocks in the formation contained calcium-bearing minerals, and were locally metasomatised by hydrothermal fluids given off by the granite.

The resultant rock is called a skarn, and this complex metamorphic history has created a diverse range of lithologies and mineral assemblages across the area.

Some of these skarns are locally mineralised and have historically been worked (e.g. Grylls Bunny, Botallack).

At Botallack there appears to be two phases of hydrothermal mineralisation which led to the formation of “tin floors” containing economic levels of cassiterite:

1. Metasomatism of mafic intrusive sills and extrusive volcanics produced stratiform horizons of skarns with a variety of minerals including garnet, magnetite, epidote, axinite, tourmaline and pyroxene. This created a structural / chemical trap.

2. Directly below these are “tin floors. Later fluids get trapped below the skarns, resulting in replacement ore bodies that consist of tourmaline, quartz +/- cassiterite.

The presence of tin in some of the minerals can be as cassiterite (SnO2). However, it can also occur in other minerals such as epidote, and malayaite – a calcium tin silicate mineral (CaSnO[SiO4]), part of the titanite mineral group.

This impacts the economic viability of a tin deposit, due to it being more challenging to process the metal from the mineral.

Wanna find rocks like these and others whilst learning all about them? Then why not book yourself on to the Tin Coast Geowalk coming up this Sunday! Booking link in bio.

#metamorphic #botallack #cornwall #geology #geologist

Nature’s artwork.

This slightly illusional view of the Bude Formation is at Northcott Mouth. Weathering of the rock surface has eroded down through multiple sedimentary layers of mudstone and sandstone to give this beautiful contour effect.

#cornwall #geology #geologyrocks #geolife #geologist

Granite planet. Looking across to Treryn Dinas from Porthcurno.

The sea, sand and granite cliffs amplifying my favourite colours. Absolute heaven. It’s why I chose it as the cover photo for the Cornwall Geologist website.

The granites here make up the southern lobe of the Land’s End Granite, the youngest pluton that makes up the Cornubian Batholith in SW England.

Just around the corner from here at Lamorna the granites have yielded an age of 275 million years old.

This is a classic Land’s End Granite type - coarse-grained, porphyritic, and its predominant accessory mineral is biotite.

There are lots of amazing little details to be found within these granites when you get your close as well. Things like phenocrysts, enclaves, orbicules, and hydrothermal veins all help elucidate the complex and protracted magmatic history of these rocks.

#porthcurno #cornwall #geology #geologyrocks #geologist

Some Cornish slate in the Crackington Formation at its type locality.

The geological recipe for these rocks in North Cornwall includes some Paleozoic mudstone with some added Variscan heat and pressure, producing a delicious plate of slate!

While not as famous as our Celtic neighbours in Wales, Cornwall does produce good quality slate for building and decoration in places.

Slate is a metamorphic rock. It is fine-grained; you seldom see minerals with a hand lens and if you do they are usually very fine white flecks of mica.

Another defining characteristic is a strong metamorphic layering called a foliation or slaty cleavage (giggle). This allows slate to split easily to create thin slithers of rock.

The protolith of slate is a clay-rich mudstone. Clay minerals are shaped like microscopic plates. As the rock is buried, perhaps caught between colliding tectonic plates, it experiences temperatures and pressures of a few hundred °C and a few kilobars.

This causes low-grade regional metamorphism, creating new minerals from the clay like mica, chlorite, and illite. All still platy-shaped.

As the rock is squeezed those platy minerals get aligned into parallelism, oriented perpendicular to the direction of shortening. This microscopic alignment gives slate that characteristic foliation.

After the rocks are relieved of pressure they can expand. The foliation now acts as a plane of weakness along which fractures form.

As well as a handy building material, studying slate tells us a lot about how they form.

This slate in the Carboniferous Crackington Formation formed from muds deposited on an ocean floor. They were quickly buried and deformed by Variscan continental collision resulting in these beautiful slates.

#structuralgeology #tectonics #cornwall #geology #geologyrocks

More geology art from Porthleven, with beautifully folded interbedded mudstones and sandstones.

The sedimentary sequences formed from underwater avalanches on submarine continental slopes, during convergence-related sedimentation in the Gramscatho Basin around 370 million years ago.

As two tectonic plates collided with the onset of the Variscan Orogeny, sediment that eroded from the overriding plate to the south washed down in to the closing ocean.

These sediments were eventually buried to around 10-12 km deep and squished together between these colliding plates. Here they were highly deformed and faulted, leading to some very impressive structural geology.

Whilst the complexity of these structures suggests they have undergone extreme temperatures and pressures, they actually haven’t. They didn’t even make greenschist facies metamorphism, up to around 300°C and 3 kbar of pressure.

However, these conditions and the rheological properties of the rock types were just enough so that when they got caught up between two tectonic plates these beautiful and complex structures formed.

#structuralgeology #tectonics #cornwall #geology #geologyrocks

🔥 Flame structures 🔥

The Bude Formation in north Cornwall is full of incredible sedimentary structures. These were found in a fallen block in the beach at Northcott Mouth.

Flame structures form when a denser sediment gets deposited on less dense unconsolidated sediment.

In this case, the overlying coarse sand pushed down in to the underlying less dense mud creating load casts.

In between the load casts pushing down, fingers of mud inject up into the sand above creating flame-shaped structures.

In some cases the overlying unconsolidated mixture of sediment and fluid may creep downslope and deform the flames, hence preserving the direction of flow at the time of deposition.

All important information in understanding the environment in which these sediments were deposited.

The Bude Formation is part of the Culm Basin. This sedimentary basin initially formed during the Late Devonian and Early Carboniferous as a full graben trending E-W.

With the onset of Variscan convergence in the Carboniferous the basin began to close up causing the depth of the sea to decrease.

Sediment sourced from the north infilled the shallowing basin, transitioning from a deeper marine setting (the Crackington Formation below) to shallow marine and deltaic environments that we see here.

These structures are indicative of a shallow marine setting. There are also examples of fossil wood and localised coal in the area (hence the Culm Basin, culm meaning coal) which is indicative of deltaic deposits.

These structures are also a great indication of which way up a unit is, which is critical when trying to reconstruct any large-scale structures in an area. This is very necessary in the rocks in north Cornwall, that underwent extensive deformation during the Variscan Orogeny.

#sedimentary #cornwall #geology #geologyrocks #geologist

Structural geology heaven at Northcott Mouth at the top of Cornwall.

A series of large-scale upright anticline and syncline folds are exposed in the foreshore here, elucidating impressive deformation of these Carboniferous during the Variscan Orogeny.

The fact the trace of bedding appears to create these massive zigzag shapes in the horizontal foreshore platform tells us the fold hinges are plunging.

Because the fold limbs of the anticlines converge and the syncline fold limbs diverge in a westerly (seaward) direction these folds are plunging towards the west.

You can also make out different fold hinge geometries as well meaning there must be different strain rates and rheological contrasts in the rocks as well.

I feel a geology doodle coming on for this place. Keep an eye out for more posts on this amazing location!

#cornwall #geology #structuralgeology #geologist #geologyrocks

A beautifully exposed fold hinge in the Portscatho Formation at Loe Bar.

If you’re a structural geologist, you’d get as excited as I do when you see something like this. Not very often you get the fold hinge exposed just so to make measuring it so much easier!

We use the term hinge for a fold where the two limbs of the fold meet; where the bedding surface has bent if you like. Think of it like the hinge of a door or the spine of a book.

We would measure the compass orientation of this and its vertical angle relative to horizontal. This way we can analyse the shape, movement and stresses they formed under, and make comparisons to other data from across the county.

This is classified as F2 because it’s a second generation fold. There was an earlier deformation episode preserved in these rocks that elsewhere have been deformed by this episode.

They formed during the Variscan Orogeny over 300 million years ago when two continents collided.

#structuralgeology #loebar #cornwall #geology #geologyrocks

A beautiful upright anticline fold at Northcott Mouth at the top of Cornwall.

These layers of sedimentary rock were originally deposited as sediment gravity flows in the Culm Basin around 320 million years ago in the Carboniferous.

Squeezing and buckling of these rocks occurred during Variscan convergence. Cornwall was effectively squeezed together like an accordion in a mountain chain for over 80 million years!

Crustal shortening caused the sedimentary basins to concertina together. Their stratified sedimentary contents got squished and expelled out on to neighbouring basins, in doing so forming these beautiful structures.

This fold is upright. In other words, if you join all the hinge points the line it creates (the fold axial trace) is vertical.

We use the term hinge for where the two fold limbs meet; where the bedding surface has buckled if you like. Think of it like the hinge of a door or the spine of a book.

For folds to form these rocks must have undergone some sort of shear, a type of deformation experienced by a rock mass. There are two types of shear - pure and simple (anyone singing the Hear’Say song now?).

I’ll use the “cloth on a table” analogy to explain.

If you pushed a cloth together equally from both sides it would cause it to buckle as a series of upright folds like here. This is pure shear.

If you move your hand along the top of the cloth it would cause it to “ruck” over. These rucks tilt in the direction of the top sense of shear, the direction you move your hand in. This is simple shear.

Many folds in Cornwall have a top sense of sheer to the north - Variscan convergence had a northward direction.

The difference here in north Cornwall is controlled by the symmetrical, full-graben geometry the Culm Basin in which these sedimentary rocks were originally deposited.

Whilst folds on the northern side of the basin face north, at the southern margin its contents spilled out southwards during inversion. In the middle of the basin like at Northcott Mouth the folds are upright, more akin to pure shear.

A tectonic story well worth seeing!

Hat for scale.

#structuralgeology #tectonics #geology #geologist #cornwall

Hypnotic exposures of thinly bedded mudstones and sandstones just south of Porthleven.

These turbidite sequences are properly impressive. They represent units deposited from turbidity currents (underwater avalanches) on submarine continental slopes.

To imagine how they form think of a dense, cloudy, turbulent mix of sediment and water hugging the seabed as it flows downslope. As the flow slowed it eventually deposited the dense, coarser sediment. These are the light grey sandstone beds.

In between these turbidity flows were periods of quiet, calm conditions, allowing fine particles (known as hemipelagic sediment) to settle out of the water column and deposit on the sea bed. These represent the dark grey mudstone beds.

The most amazing thing here is the time scales in which these beds formed. The thin sandstone beds would have formed from sediment depositing over a few hours or days.

The mudstones, however, would have accumulated over several hundreds or even possibly thousands or tens of thousands of years! So much geological time recorded in this little outcrop.

These rocks formed during convergence-related sedimentation in the Gramscatho Basin around 370 million years ago.

As two tectonic plates collided with the onset of the Variscan Orogeny, sediment that eroded from the overriding plate to the south washed down in to the closing ocean.

These sediments were eventually buried and squished together between these colliding plates, in what is known as an orogenic wedge or accretionary prism. Here they were highly deformed and faulted, leading to some very impressive structural geology.

#porthleven #sedimentary #cornwall #geology #geologist

Cornwall Geowalks coming up this May…

Cape Cornwall (half-day) - Saturday 2nd May
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/cape-cornwall-guided-geowalk-half-day-saturday-2nd-may-2026

Bosigran (half-day) - Tuesday 5th May
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/bosigran-guided-geowalk-half-day-tuesday-5th-may-2026

Loe Bar (half-day) - Saturday 16th May
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/loe-bar-guided-geowalk-half-day-saturday-16th-may-2026

Tin Coast (half-day) - Sunday 24th May
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/tin-coast-guided-geowalk-half-day-sunday-24th-may-2026

Cornwall Geowalks are guided geology walks around Cornwall’s world-class geological sites and areas of outstanding natural beauty, guided by me! Please see the website for more details. Links in bio.

#geowalks #cornwall #cornwallgeology #geology #geologist

Second phase deformation in rocks just south of Porthleven.

Tectonics are continuous but their dynamics can evolve over time. Changes in direction and intensity of stresses in the crust deform rocks differently, which provide a record of how plate tectonics evolve through Earth’s history.

Here, a moderately inclined S2 crenulation cleavage and F2 folds deforms a bedding-parallel S1 cleavage.

We can tell the relative age from cross-cutting relationships. The S1 foliation has been buckled by strain that created the S2 cleavage.

Also, the earlier cleavage and bedding has undergone pressure dissolution along the S2 cleavage, making it look like layers have been cut out.

Both 1st and 2nd phase deformation in south Cornwall formed during convergence tectonics related to the Variscan Orogeny during the Paleozoic.

Both deformation events have a top sense of shear to the NNW. However, the relationships we see here suggest a time gap between events. There is also a clear change in strain intensity, with D1 being higher strain than D2.

Whilst D1 is ubiquitous in the area (another indication of high strain), D2 structures are more localised. They largely occur around late stage thrust faults, where strain is more likely accommodated.

The fact there are two distinct deformation episodes suggests this transition was not entirely progressive, and that there could have been a hiatus in Variscan tectonics.

This D numbering system applies to the structures we see in Upper Devonian rocks in south Cornwall. To properly apply it requires an extensive structural geology investigation.

Some outcrop show evidence of all deformation episodes, whereas others show some or only one. You need to build the story over large areas.

Elsewhere in the region much younger rocks might not have existed during earlier deformation phases, so first phase structures in them might be D2 structures in older rocks nearby. Complicated!

#structuralgeology #tectonic #cornwall #cornwallgeology #geology

Incredible clouds over the Tin Coast this afternoon.

#cornwall #ukweather #clouds #weather #bbcweatherwatchers

Beautiful little F3 folds in the cliffs just south of Porthleven.

These little structures are not only Nature’s abstract artwork but an insight into tectonic processes these rocks have experienced in their long and turbulent geological history.

F3 refers to the fact these are third generation folds. In other words, the third time these rocks were deformed since they formed.

This is deciphered by cross-cutting relationships - we can see where F3 folds deform earlier structures.

We use structures in the rock to piece together how they relate to large-scale, regional tectonic processes.

F3 folds represent a time when the crust here was being pulled apart following the end of the Variscan mountain building event, around 300 million years ago.

#structuralgeology #tectonics #cornwall #cornwallgeology #geology

Tiny structures telling a big story.

Things like this are what fascinate me most about structural geology. Using tiny structures like these little folds and cleavage development to tell a story on a tectonic scale.

By studying the shape, orientation, distribution, and relative timing of these beautiful little illustrations preserved in the geological record we can understand how giant tectonic plates moved, collided, and pulled apart hundreds of millions of years ago.

Next time you’re out on the Cornish coast, get up close and observe the rocks in detail. Look at those tiny features preserved in the rock, and just appreciate how geological processes influenced how they formed over deep time.

And you can do that with me next Monday! I’m running a Geowalk down to Loe Bar.

Here we traverse the spectacular coastline around Loe Bar and Porthleven in south Cornwall, and explore Cornwall’s dynamic geological story of plate tectonics.

Discover amazing structures in the rock, and learn how they tell us about when Cornwall was once at the centre of a giant mountain range.

You can book now via the following link: https://www.cornwallgeologist.co.uk/event-details-registration/loe-bar-guided-geowalk-full-day-monday-20th-april-2026

#structuralgeology #tectonics #loebar #cornwall #geology

Exotic granite exposed at the Hendra end of Praa Sands.

This is the westernmost exposure of the Tregonning-Godolphin Granite.

This small granite pluton with a double-barrelled name is actually made up of two distinct parts – the older, biotite-rich “Godolphin” granite, and the younger, topaz-rich “Tregonning” granite.

It formed in the Early Permian intruding the Late Devonian Mylor Slate Formation, the boundary with which is exposed in the rocks here at low tide.

It is the Tregonning Granite we see along the coast. This granite has a unique mineralogy and chemistry compared to other granites in the region.

It is classified based on its elevated topaz content and distinctive geochemical fingerprint, containing elevated levels of lithium and fluorine.

It was also very wet – the magma when it intruded contained vast amounts of dissolved H2O molecules. Because of this we see lots of mineral veins crossing the granite and surrounding rock, caused by associated hydrothermal activity.

#granite #praasands #cornwall #cornishgeology #geology

Stunning schists at Housel Bay.

The Lizard peninsula is known for its exotic rocks, largely made of those we see in oceanic crust and upper part of the mantle known as an ophiolite.

However, some of the metamorphic rocks in the Lizard Complex are actually much older than the ophiolite.

In ophiolite sequences these are sometimes referred to as the “metamorphic sole” - faulted sheets of basement rocks obducted alongside the ophiolite.

The Landewednack Amphibolites exposed around Housel Bay are a fine-grained mix of pyroxene, amphibole, and plagioclase, with localised epidote. This assemblage suggests a mafic protolith, e.g. basalt.

It has a strong tectonic foliation which is axial-planar to small, isoclinal folds. In places minerals have stretched creating mineral lineations.

Its grainsize and foliation means it can be called a schist. The name “amphibolite” means a rock full of amphibole; in a metamorphic rock this may indicate it experienced amphibolite facies metamorphism.

Thermobarometric estimates indicate peak temps of c. 600°C and 10 +/-2 kbar of pressure which does equate to amphibolite facies metamorphism.

Work by Nutman et al. (2023) gave an age of c. 480 Ma (Early Ordovician) for the formation of the protolith. Mackay-Champion and others (2024) show peak metamorphism occurred c. 395 Ma (Early Devonian).

Both these ages predate formation of the Lizard Ophiolite, disassociating the Landewednack Amphibolites from the mafic and ultramafic rocks to the north.

As these rocks could have been thrusted tens or hundreds of km, it is really hard to fully understand the tectonic environment in which they formed.

A theory put forward in the Mackay-Champion et al. paper suggests the Landewednack Amphibolites are part of older oceanic crust metamorphosed during an earlier subduction phase.

Slab rollback during this subduction initiated seafloor spreading in a supra-subduction zone, developing the Lizard Ophiolite c. 386 Ma. Both units were then obducted by 380 Ma during the final closure of the Rheic Ocean.

#metamorphic #lizardpoint #cornwall #geology #cornwallgeology

Happy Easter!

Here’s an Easter-themed photo of the Isle of Eigg in the Inner Hebrides, taken from the top of Askival on the Isle of Rum.

Whilst an anglicised interpretation of the name does appear to have Easter connotations, the name Eigg is more likely derived from either Gaelic or Norse describing the island’s unique shape.

That distinctive topography is controlled by Paleogene basalt lava flows that erupted on top of Mesozoic sedimentary rocks when the North Atlantic started to unzip around 60 million years ago.

With adjacent islands called Eigg and Rum you feel that the next island along should be called Eggnog, but you Canna do that! Sorry, terrible joke!

#hebrides #scotland #geology #easter #eigg

Cornwall Geowalks coming up this April…

Tin Coast (full-day) - Saturday 11th April
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/tin-coast-guided-geowalk-full-day-saturday-11th-april-2026

Praa Sands (half-day) - Monday 13th April
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/praa-sands-guided-geowalk-half-day-monday-13th-april-2026

Coverack (half-day) - Saturday 18th April
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/coverack-guided-geowalk-half-day-saturday-18th-april-2026

Loe Bar (full-day) - Monday 20th April
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/loe-bar-guided-geowalk-full-day-monday-20th-april-2026

St Ives (half-day) - Monday 27th April
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/st-ives-guided-geowalk-half-day-monday-27th-april-2026

Cornwall Geowalks are guided geology walks around Cornwall’s world-class geological sites and areas of outstanding natural beauty, guided by me! Please see the website for more details. Links in bio.

#geowalks #cornwall #cornwallgeology #geology #geologist

Beautiful patterns in sandstones at Widemouth Bay.

This type of pattern is often referred to as honeycomb weathering, or tafoni, depending on scale.

It typically in occurs coarse-grained clastic sedimentary rocks such as sandstones that are exposed to salty environments like along the coast.

In reading up on them it seems there is still little consensus in exactly how they form. Various hypotheses have been put forward based on underlying mineralogical and structural controls, and whether it is mechanical or chemical weathering, or a combination of the two.

These exposures at Widemouth Bay are a bit more anthropogenic though - these are actually prehistoric rock carvings! Can you spot the deer? Archaeological work suggests these date back to at least the Paleolithic, some of the earliest human activity on record in Cornwall.

I love the name of this type of weathering as well, “honeycomb”. I fancy a Crunchie now!

Oh, and happy 1st of April…

#sandstone #weathering #cornwall #cornishgeology #geology

This little pebble found on the beach down next to Lizard point is a piece of the oldest rock in Cornwall!

It’s called the Man of War Gneiss, named after a small islet that makes up the rocky reef just offshore where it crops out in situ.

Radiometric dating of minerals within it have yielded an age of 500 million years old, making these rocks the oldest in Cornwall, in fact, SW England.

Gneiss (pronounced “nice”) is a coarse-grained, banded metamorphic rock. They can form deep in the Earth’s crust, experiencing immense temperatures and pressures that can eventually cause partial melting.

The original rocks (protoliths) of the Man of War Gneiss before they underwent metamorphism were igneous. However, they range in composition from felsic (tonalite) to mafic (gabbro).

Minerals within this rock include hornblende, plagioclase, and quartz, with a little bit of biotite, chlorite, actinolite, apatite, magnetite, titanite, garnet, and zircon.

In places there is a strong metamorphic fabric caused by compositional layering, as well as lineations where individual minerals have been stretched out.

These rocks occur structurally below the Devonian mantle peridotites that crop out just to the north. However, their origins are uncertain.

One theory is that these are basement rocks - old, crystalline rocks on top of which younger rocks form. When a fragment of the Lizard oceanic crust was ripped up and dumped on top of the continent during the Variscan Orogeny it could have dragged a fragment of basement with it, which is why they are out of position.

Another theory is that they represent a fragment of the upper plate that was thrust on top of Cornwall from the south during the Variscan Orogeny.

An important piece of the jigsaw to help us understand our tectonic history!

#metamorphic #lizardpeninsula #cornwall #cornwallgeology #geology

Even under fabulous spring sunshine the west Penwith coast still looks dramatic!

Taken from Pendeen Watch looking southwest over the Enys, with Levant Mine in the distance.

#pendeen #levant #penwith #cornwall #ukcoast

A message from the mantle down on the Helford in south Cornwall.

On the north side of the Helford near Mawnan a strange brown rock appears in the cliff.

This is a lamprophyre: an ultramafic, ultrapotassic igneous rock (low in silica, rich in Mg, K, Na) that comes from partial melting of the upper mantle.

It’s a common misconception that the mantle is liquid; it is actually mostly solid. However, plate tectonic processes can cause melting of the upper mantle, and has a major control on magmatism in the crust.

These lamprophyres aren’t related to the mantle rocks we find in the Lizard Complex to the south. In fact, they‘re about 100 million years younger. They are actually more related to the granites.

After continental collision thickened the crust during the Variscan Orogeny, it began to stretch and thin rapidly, from around 300 million years ago.

This caused decompression melting of the mantle - when overlying pressure is released, hot solid rock begins to melt.

The granites in Cornwall come from melting of sedimentary rocks in the middle and lower crust. This was triggered by the additional heat of these intruding mantle-derived magmas in excess of 1200°C.

The presence of the lamprophyres is evidence that the mantle melted as well, giving us the additional heat source required to cause vast amounts of melting of the crust to produce the granites.

This lamprophyre is an intrusive sheet, but elsewhere in the region there are extrusive lavas as well meaning some did eventually erupt from volcanoes during the Permian.

#mantle #helford #cornwall #geology #geologist

Crosscutting veins at Praa Sands.

The earlier (left-right) quartz vein is actually quite narrow, only a few mm wide. What we see is an alteration halo where the surrounding granite has been replaced by quartz, chlorite, and sulphide minerals.

The diagonal vein consists of banded quartz with a vuggy centre and euhedral crystals growing inwards. It crosscuts and sinistrally offsets the earlier quartz-chlorite-sulphide vein.

Hydrothermal mineralisation can occur in many geological settings across a range of temperatures and pressures where aqueous fluids or volatiles get heated up and mobilised.

If a rock fractures fluids rush in to re-equilibrate the pressure, altering their ability to carry certain elements. Elements then bond together and precipitate minerals that fill the void to form a mineralised vein.

Chemical and thermodynamic diversity of hydrothermal fluids leads to a complex mineral paragenesis and distinct geospatial and temporal variations in mineralisation across Cornwall.

Repeated fracturing can create multiple episodes of veins. This, plus any alteration zones containing ore minerals is called a lode.

Most mineralisation in Cornwall formed as a consequence of the Permian granites.

A Vast amount of water is dissolved in granitic melt. As those magmas crystallise those waters eventually exsolve from the melt, known as magmatic-hydrothermal fluids.

Heat from the magma also radiated through the host rock, mobilising connate and meteoric waters with different chemistries and causes them to circulate through the surrounding rocks and mix with the magmatic-hydrothermal fluids. This is likely what produced the quartz-chlorite-sulphide vein here.

Long after the granite magmas cooled and crystallised they were still producing heat, originating from the decay of certain radioactive minerals.

This radiogenic heat mobilised salty brines in surrounding sedimentary basins during the Triassic, producing low-temperature, epithermal mineralisation locally known as crosscourse. This is the vuggy, chalcedonic quartz vein here.

#mineralisation #cornwall #geology #geologist #praasands

🟰🟰 Gool Peran Lowen onen hag oll! Happy St Piran’s Day one and all!

St Piran is patron saint of Cornwall and tin miners. St Piran’s Day is celebrated every year in Cornwall on 6th March.

The story of St Piran is still a great one, and it has its roots in geology!

Legend has it St Piran rediscovered tin smelting when he arrived in Cornwall in the 5th Century.

Whether he did or not is up for debate, but we do know that tin mining and the technology for smelting tin has existed in Cornwall since the Bronze Age.

What is known of him is that he was a 5th Century abbot from Ireland. After being banished from his homeland for preaching Christianity, he was reputedly thrown over a cliff tied to a millstone into a stormy sea.

By means unbeknownst to science the millstone floated (perhaps the millstone was made of pumice!) and the sea went calm.

St Piran drifted across the Irish Sea and he landed at Perranporth beach, although it was likely called something different back then as it is now named after him!

After building an oratory in the sand dunes above the beach (the remains of which can still be seen today), St Piran noticed the black rocks he used for hearthstones around the fire began to ooze a white metallic liquid when hot.

This is, of course, molten tin that is being smelted out of rock containing tin ore called cassiterite that has a black to dark brown colour.

It is these colours that are used for Cornwall’s flag today. The black background represents tin ore and the white cross represents tin metal. Therefore, the Cornish flag has geological origins!

St Piran’s Day celebrates Cornwall and its special culture and heritage. Join in the festivities by eating some saffron cake or a pasty, and most importantly spending some time to appreciate this amazing place!

⚫️⚫️⚫️⚫️⚫️⚪️⚪️⚫️⚫️⚫️⚫️⚫️
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⚫️⚫️⚫️⚫️⚫️⚪️⚪️⚫️⚫️⚫️⚫️⚫️
⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️
⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️⚪️
⚫️⚫️⚫️⚫️⚫️⚪️⚪️⚫️⚫️⚫️⚫️⚫️
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#stpiran #stpiransday #tin #cornwall #geology

Cornwall Geowalks coming up this March…

Praa Sands and Rinsey (full-day) - Saturday 7th March
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/praa-sands-and-rinsey-guided-geowalk-full-day-saturday-7th-march-2026

St Ives (half-day) - Monday 9th March
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/st-ives-guided-geowalk-half-day-monday-9th-march-2026

Lizard Point (half-day) - Friday 20th March
Book now: https://www.cornwallgeologist.co.uk/event-details-registration/lizard-point-guided-geowalk-half-day-friday-20th-march-2026

Cornwall Geowalks are guided geology walks around Cornwall’s world-class geological sites and areas of outstanding natural beauty, guided by me! Please see the website for more details. Links in bio.

#geowalks #cornwall #geology #cornishgeology #geologist

A ductile shear zone in gabbro from the Moho transition zone at Coverack.

Taking a trip Coverack is also a journey deep inside the Earth! Here you can traverse the boundary between the Earth’s oceanic crust and mantle.

This transition between the Earth’s crust and mantle is called the Mohorovičić Discontinuity (Moho for short). It is seen as a seismic reflection caused by different physical properties of rocks either side.

The transition zone here is about 200 m wide, consisting of mafic and ultramafic sheets, ranging from cm to several m thick.

As these superheated gabbroic magmas intruded into mantle peridotites and began to cool, continued stresses caused ductile, plastic deformation of the minerals as they formed.

You could call this rock a migmatite - a high-grade metamorphic rock where it gets so hot the rock begins to partially melt, evidence of which can be seen nearby.

You could also call this rock an augen gneiss - note how the more rigid dark green pyroxene minerals have kept their shape more than the white plagioclase minerals, more susceptible to being stretched. The pyroxenes look like eyes, hence the term “augen”.

These rocks are part of an ophiolite sequence that make up the Lizard Complex. An ophiolite is a fragment of oceanic crust and top bit of the mantle.

Oceanic crust is typically 5-7 km thick, compared to continental crust which is several 10s of km thick. It is also denser than continents, due to higher iron and magnesium and lower silica content.

The Lizard ophiolite formed 400 million years ago, when Cornwall was in the southern hemisphere on the margin of a vast ocean. Stretching continental crust eventually thinned enough for oceanic crust to develop.

Eventually that ocean began to close. The oceanic crust was subducted down below a continental plate rising from the south due to its higher density. However, a fragment was obducted - dragged and dumped on what is now south Cornwall.

#structuralgeology #coverack #cornwall #cornishgeology #geology

As it is St David’s Day today I thought I would post some Welsh geology - the aptly coloured red, green and white rocks at Freshwater East in Pembrokeshire.

These rocks are from the latest Silurian, around 420 million years ago. They represent a transition from marine to terrestrial conditions, and the start of the Old Red Sandstone.

The bands of green and red in the Moor Cliffs Formation here are all terrestrial, hosting beautiful sedimentary structures deposited in braided river systems and adjacent floodplains on tropical dryland alluvial/coastal plains.

The Welsh-themed colours are actually controlled by post-depositional changes in iron oxidation states.

This is between reducing Fe2+ (green) and oxidising Fe3+ (red). All of the beds here are sub-vertical, caused by folding during the Variscan orogeny in the Carboniferous.

Happy St David’s Day to our Celtic cousins across the sea!

#pembrokeshire #wales #geology #geologist #stdavidsday

No need for a thin section of this rock - the minerals are the size of your fist!

The minerals we see are supersized pyroxenes (dark green) and plagioclase feldspar (white).

This “pegmatite” gabbro is part of the Traboe Cumulate Complex towards the northeast of the Lizard Ophiolite Complex.

The Lizard peninsula in south Cornwall is largely made up of rocks that form a fragment of oceanic crust and upper mantle called an ophiolite.

Geology intro textbooks tell you that change in grain size in igneous rocks is down to cooling rates. Having spent most of my career working with granites, I tend to think it’s more to do with the presence of volatiles that control this.

It’s back to basics here though - these monsters grew over a long time in small pods of molten rock in a very hot migmatite.

A migmatite is a high-grade metamorphic rock where it gets so hot the rock begins to partially melt.

The word translates from Greek as “mixed rock” as it includes a mixture of the metamorphic rock with igneous rock from the crystallised partial melt.

These pods are surrounded by beautiful ductile shear zones where highly stretched rocks were being deformed during Early Devonian rifting.

Soon after gabbroic magmas intruded in a mid-ocean ridge setting and began to crystallise at around 1200°C, they were being stretched as a consequence of this rifting event.

Because the rock is already hot, the stresses in the Earth’s crust caused deformation, high-grade metamorphism, and remelting of the gabbro, resulting in these remarkable textures.

It’s a good opportunity to ponder over some geological philosophising here.

We often think of metamorphism through increased temperature and pressure of cold rocks buried from surface.

But here we have newly formed intrusive igneous rocks almost immediately experiencing the highest grade of metamorphism to the point where they melt again.

This is all down to tectonics and the environment these rocks form in. If a newly crystallised igneous rock immediately experiences high pressures, their residual heat provides the right conditions to undergo high-grade metamorphism first.

#mineral #cornwall #cornishgeology #geology #geologist

Variscan chevron folding at Millook Haven in north Cornwall.

The section visible in this photo is more than 20 m high, with a total cliff height exceeding 80 m.

These layers of sedimentary rock were originally deposited in the Culm Basin around 320 million years ago, in a shallowing ocean basin as more sediment was dumped in it as sediment gravity flows. It is stratigraphically known as the Crackington Formation.

These rocks were buried and folded as Variscan continental collision progressed northwards throughout the Carboniferous. Cornwall was effectively squeezed together like an accordion in a mountain chain for over 70 million years. This caused the inversion, or expulsion if you like, of the Culm Basin sedimentary rocks.

#millookhaven #structuralgeology #tectonics #cornwall #geology

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(+44) 07727 096492

Explore Cornwall's extraordinary geological story with the Cornwall Geologist. Learn all about the dynamic story of Earth's history locked in the rocks beneath our feet and discover how it has shaped the Cornish landscape.

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Explore Cornwall's extraordinary geological story with the Cornwall Geologist.
Learn all about the dynamic story of Earth's history locked in the rocks beneath our feet and discover how it has shaped the Cornish landscape.