Coverack Beach, Place To Visit

Location & Access:

Coverack is a small fishing village located on peninsula in Cornwall. It can be accessed via the A3083 and B3294 roads from . Public bus is available from Helston—service 36A. 2 car parks can be found at North Corner (TR12 6TF) and a small car park can be found at Dolar Point (TR12 6TQ). Coverack can also be accessed via the . Red Serpentine boulder. Photo: P Berry

Key Geography: Sand and pebble beach, fossil ‘Moho’ - boundary between the Earth’s mantle and the crust, geology - serpentine, peridotite, troctolite, basalt, and gabbro rocks.

Description: . The sand and pebble beach at Coverack is one of the few places in the world where it is possible to see a fossil ‘Moho’ – the boundary between the Earth’s upper mantle of serpentine and its lower crust of gabbro. Both were once molten rock, and as they cooled they fossilised into the large lumps of rock you can see embedded in the beach.

The Moho boundary is less deep beneath the oceans, where it typically lies around 7 kilometres beneath the sea floor. Below the continents it is found at depths between 25 and 60 kilometres. It is possible to see this boundary at Coverack, which was once about 5 kilometres below the ocean floor, but is now exposed at Coverack Beach. Photo: P Berry the surface and laid flat.

‘Moho’ is short for ‘Mohorovicic Discontinuity’, and is named after Croatian seismologist, Andrija Mohorovicic, who discovered in 1909 that seismic waves changed velocity as they passed through the Earth’s crust and into the mantle.

At the beginning of the 20th century, scientists were almost certain that the Earth contained many layers– but they didn’t know exactly where these layers started and ended. Mohorovicic studied the records of an earthquake in Croatia in October, 1909, and found that two different sets of shock waves existed. Because the second set of waves exactly mirrored the first tse but was offset in time, he realised that the velocity of seismic waves is affected by the material it is moving through. Mohorovicic suggested that there must be a change in the composition of rocks at depth, with higher density material being present the deeper one goes. By gauging the time difference he was able to accurately calculate the exact depth of the denser material. Scientists are now able to calculate the depth of the boundary between layers, and have discovered that the Moho varies not only in depth but also in its thickness. (continued overleaf)

Curiosity Questions:

# Which rock type found on Coverack Beach is the official state rock of California? # If peridotite contains over 90% olivine, it is known by what different name? # For the last 50 years, Coverack residents have held a fund raising event for Cancer Research on Christmas Day. What does it involve?

Further information: Reviewer: Paul Berry B Ed (hons) M Sc FRGS www.devongeography.wordpress.com Former Assistant Vice Principal and Head of Geography at South Molton Community College with 35 years of classroom experience. www.southwestcoastpath.org.uk/walksdb/19/ Now an Iceland Field Studies Tutor with Rayburn Tours. Blog: www.devongeography.wordpress.com Twitter: @unicorn4275 Coverack Beach, Cornwall Place To Visit

To understand the geology of this part of south west England, it is necessary to turn the clock back some 375 million years, where at the bottom of an ancient sea known as the Rheic Ocean, about 30 degrees south of the equator, the molten rock which became the Lizard was forced through the Earth’s crust from about 10 kilometres below. The enormous pressure of these eruptions brought up a complete slice of rocks from the mantle to the crust. Over the next 80 million years, earth movements caused the two supercontinents separated by the Rheic Ocean to close the gap between them. The ocean disappeared and the Lizard was bulldozed onto Cornwall. The new North America/Asia continent formed by the joining of these two continents began to move north, crossing the equator 250 million years ago and arriving in its present location just before the last Ice Age flooded the English Channel and separated Britain from the rest of Europe. One of the rocks thrust up from beneath the crust was peridotite, which can be found on the beach at Coverack. As it passed through the crust and onto the Lizard it was changed into serpentine, the Wave worn Peridotite. Photo: P Berry beautiful rock of many colours for which the Lizard is famous. It can appear in many different colour forms, ranging from bright green to the deep red serpentine which is found at Coverack.

To explore the special geology of Coverack, a start could be made at the south end of the beach – near the harbour. Here, you will find serpentine rock from the mantle.

Serpentine is a rock composed of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Minerals in this group are rich in magnesium and water and have a slippery feel. They are Serpentine pebbles. Photo: P Berry formed by serpentization– a sea water hydration and metamorphic transformation of ultra-mafic rock from the Earth’s mantle. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

If you continue northwards to around halfway along the beach, you will be in the transition zone from mantle to crust – the Mohorovicic Discontinuity. This area is characterised by the intermingling of serpentine and gabbro, along with the intrusion of basalt in the form of narrow dyke ridges.

Basalt is an extrusive igneous rock formed by rapid cooling of molten rock near the surface. It is fine grained, and usually black in colour. It is typically rich in magnesium and calcium oxides with around 45% silica content.

Peridotite is also easy to spot in the transition zone, sometimes highly polished by wave action. Peridotite is a dense, coarse-grained ultramafic (< 45% silica) igneous rock consisting mostly of olivine and pyroxene. It is derived from the Earth’s mantle. It has many colour and texture variations – and consequently, many different names! On closer inspection, red Red veins in peridotite. Photo: P Berry patches and veins can be identified in the peridotite, representing light serpentization. (continued overleaf) Coverack Beach, Cornwall Place To Visit

Towards the harbour end of the beach, lightly serpentized peridotite appears as ‘lizard skin’ rock boulders.

It is also possible to spot a rare red and white rock in this transition zone known as troctolite. Coverack is a classic location for this attractive rock which is sometimes known as ‘troutstone’, and is a variety of gabbro with less than 55% silica. It is rich in olivine and pyroxene depleted. Troctolite can be identified in wide veins cutting through serpentized peridotite, or as deep red boulders rounded by wave action.

Troctolite boulder. Photo: P Berry Still proceeding northwards, you will finally enter the area of gabbro which marks the start of the oceanic crust. Gabbro is part of a large group of dark, coarse-grained intrusive mafic igneous rocks. It is chemically equivalent to basalt. Gabbro is a plutonic rock formed when molten magma is trapped beneath the Earth’s surface and then cools into a crystalline mass. This part of the beach resembles a boulder field, and it is worth searching for unusual manifestations of gabbro in some individual rocks. For instance, you might come across boulders of darker gabbro within a matrix of lighter coloured gabbro. The lighter gabbro has been partially

changed by post-magnetic alteration, while the darker Gabbro boulders. Photo: P Berry form is the unaltered equivalent. This rock is sometimes known as brecciform gabbro.

By strolling along the beach at Coverack from south to north, you have ‘walked the Moho’ – from mantle to oceanic crust. Well done! A very well produced information board in the lower car park gives a good account of the geology of the beach, and is well-illustrated to help you search out different rock types. However, please note that Brecciform gabbro. Photo: Jo Harris Coverack beach is a SSSI (Site of Special Scientific Interest), so please do not remove any rocks from the area.

Car park interpretation board. Photo: P Berry

Answers to Curiosity Questions:

# Which rock type found on Coverack Beach is the official state rock of California?(Serpentine) # If peridotite contains over 90% olivine, it is known by what different name?(Dunite) # For the last 50 years, Coverack residents have held a fund raising event for Cancer Research on Christmas Day. What does it involve? (Swimming in the harbour) Coverack Beach, Cornwall Place To Visit

Coverack Beach To Falmouth Helston To A394

Helford River

B3293

B3294 Mullion Coverack A3083

Lizard To Goonhilly

B3294

Coverack Coverack Cove

Harbour

Dolor Point