Bendy Rocks

Folded rocks in Glen Gairn, Scotland

Solid as a rock or rock solid are expressions that are all too frequently used, abused and misused. For example describing a premiership football teams back four as a rock solid defence may be stretching the bounds of metaphor to breaking point, but such metaphor is in frequent use on the evening footy shows. And besides, solid as a rock is a bit of a misconception anyway.

Contorted Man 'O' War Gneiss from the lizard Peninsula, Cornwall.  Sample 15cm

The idea of rock being able to bend or flow seems a little counterintuitive, but is it? Solid state flow is something that we have been aware of (even if we could not explain it) for centuries. It has long been noted that stained glass windows (Original ones that is) in ancient country churches are thicker at the bottom than they are at the top. The glass has flowed – admittedly at an extremely slow rate – under the influence of gravity. That example takes place over a period of hundreds of years – over thousands or millions of years, under the influence of gravity, pressure, heat, and other forces such as pulling, stretching and offset lateral pressure, rocks can do the seemingly impossible. The gigantic nappes, folds, and antiform/synform structures observed in the Grampian Highlands of Scotland are classic examples of just this.

An agonizingly contorted pebble of Migmatite from Nigg Bay, near Aberdeen.  Sample 6cm 

So why does rock bend? In short, it's a matter of Competence. The pressure and heat generated by the Grampian Phase (the mountain building episode that produced the original Grampian mountains) did several things including changing the chemical and mineralogical nature of the Dalradian sediments (the bedrock at that time). It also contorted those bedrocks in seemingly impossible ways. Rocks bend as a reaction to pressure because of competency contrast. Competency is a measure of how rocks behave under pressure. Competent rocks are more viscous, maintain their thickness when deforming and may fracture, incompetent rocks are more ductile and will flow more easily. The contrast between the competency of different rock layers dictates how a rock sequence will adjust to applied pressure. In the Grampian Highlands, evidence can be seen of folding on a scale of a few millimetres to metres to a scale of many kilometres.

Microfolds in Dalradian Metasediment from Glen Gairn, Scotland.  Sample 2cm across

At a far smaller scale though, why are even individual mineral grains able to be distorted by these pressures? This question is at the heart of solid state flow, which is in turn a vital part of what keeps the Earth a dynamic living planet. We have all seen pictures, film footage (or maybe even in real life) of molten rock – lava – pouring out of volcanic vents, but this is just the depressurized surface manifestation of a far more significant flow. The rocks of the Earths mantle flow, but they are not molten. The pressure in the mantle is far too great for the rocks to melt. The flow, which results from the intense heat with in the mantle creating convection currents, happens because at those temperatures and pressures, any imperfections in crystalline structures (known as dislocations) allow the material to deform in a plastic manner rather than a brittle manner (as they would under applied pressure at the surface at room temperature). The slow, microscopic, high pressure creep is what keeps the dynamic Earth in operation. 

A Warm Spot

A view of the rugged South Coast of Madeira

Madeira is located 600Km west of Morocco and is the massive shield volcano at the end of hot spot island and sea-mount trail that stretches NE back to the Azores-Gibraltar fracture zone (a probable future plate boundary).  At 5 million years, Madeira is the largest and youngest of the islands and sea-mounts in the trail.  The trail gets progressively older and colder back to the 67 million year old Ormonde Sea-mount SW of Portugal.  There have been no historically documented eruptions, but there is a history of land slippage due to internal movements - an indicator perhaps that this giant may only be sleeping.  Does this fact make Madeira a warm spot rather than a hot spot?

For the uninitiated, a hot spot is were an anomalously hot plume of mantle material (surprisingly termed a mantle plume) rises up through the mantle and pools beneath the crustal plate passing across the top of it.  This results in a linear sequence of islands and sea-mounts at the surface were the plate has passed over it.  Imagine passing a sheet of paper over a candle flame leaving a trail of burns.  In this case, the African plate was corkscrewing in a clockwise direction, passing over the plume and creating the hot spot trail.

South Coast 

Madeira rises to a high of 1861m above sea level, but don't let that fool you.  It continues down to the abyssal plain over 3000m below the waves - It is big.  On land it has been eroded into a spectacularly rugged edifice - the coastal scenery is dominated by imposing sea cliffs (one of which is the 4th highest in Europe) cut by impressive gorges, some of which penetrate all the way into the central massif.

The central Massif from the road to Sao Vincente

Geologically, it is the result of four phases of volcanic out-pouring followed by erosion and remod-elling of the land.  This can be seen quite strikingly at Sao Vincente on the North Coast were eroded and redeposited volcanic materials can be seen covered by more recent lava flows - the new covering the old, so to speak.

The new and the old

Organ Pipes - a section of columner lava formed by thermal contraction as the lava cooled, near Sao Vincente

A result of weathering of volcanic material can be seen all over the island.  The warm, relatively humid climate of Madeira results in the chemical weathering on the underlying volcanic rocks to form an iron and aluminium rich heavy clay soil known as Laterite.  Fields, road cuttings and red running rivers bare witness to this weathering, and it has been going on throughout the islands history, as witnessed by successive layers of laterite topped by lava flows, topped by more laterite, topped by more lava flows, etc.  In the Quinta Grande area, the laterite deposits can be seen with vertical cross cutting volcanic intrusions (Dykes) cutting through them - The dykes are of a more resistant volcanic rock (Hawianite).  Similarly weathering-resistant volcanic bombs (known locally as onion stones due to their concentric layering) can also be found in the laterite - usually exactly were they had landed after having been forcibly ejected from whatever volcanic vent spewed them out.

Laterite

A dyke cross-cutting laterite in a road cutting at Quinta Grande

To the North of the island are a series of caves created as lava tubes.  These were created when a solid crust developed over still flowing lava.  The flow of lava eventually dropped off leaving a long tubular space which was subsequently covered by later lava flows.  These are open to the wondering public and are presented as part of a Disney-esque tourist sideshow.  This is not geology in the raw - it's a well intentioned educational tourist trap that removes any sense of wonder or adventure  from the experience of going into a lava tube.  By hanging back from the crowds on the official tour, I was able to gain a little of the exploratory spirit, but is was immediately removed when the official photographer pounced on me - The image was printed and framed before I had even left the cave!!

Lava Tube at Sao Vincente

In relatively recent times small vents have spewed out copious amounts of volcanic material along the Southern coast.  In the Funchal area, you can see lines of volcanic vents getting progressively older further in land - their output is all around and can be seen on the pebble beaches in and around Funchal.    If you look carefully you may notice little flecks of green in the grey basalt groundmass - sometimes there are larger fragments - xenoliths - little fragments of the mantle, unfractionated, plucked from the Earths mantle and brought to the surface - little bits of the centre of the Earth.

Basalt cobbles on the beach at Lido, Funchal

Funchal from Casa Girau - volcanic cones in the middle and far distance

A small peridotite (Dunite) Xenolith in a  basalt pebble.  Xenolith approx 5mm diameter.