Geology Of The Giants Causeway

View of the Amphitheatre from thetop of the cliffs

Geology Of The Giants Causeway

“When the world was moulded and fashioned out of formless chaos, this must have been the bit over—a remnant of chaos!”

Thackeray’s quote describing the Giants Causeway is not far from the truth. Although the original chaos was on a much larger scale and a very very long time ago.

The causeways 40000 plus columns are so regular that they even look man-made. However, this is far from the truth. The individual columns – the remains of a deep lava flow – are predominantly 5 sided (pentagonal) or 6 sided (Hexagonal); they are so tightly packed that they form a pavement hence causeway) like structure.

Of the three causeways that protrude out into the North Atlantic, none of them actually, despite the legends, continue underwater to Scotland; the causeways stop quite abruptly a short distance offshore, where the sea bed is mostly covered in sand, shell and gravel.

So how did this landscape of the Causeway come to be? The Late (100-66Ma) Between 66 and 100 million years ago, the Cretaceous period, was a time of significant global tectonic change, seeing the breakup of the supercontinents Gondwana and Laurasia, and the opening of the Atlantic Ocean.

It was in this time, during the Upper Santonian age, roughly 85 million years ago, that the maximum period of transgression occurred; sea levels were at an all-time high due to elevated levels of atmospheric CO2 and therefore higher global temperatures and warmer oceans. 

Deposits from these warm oceans include the Upper Cretaceous Chalks that can be seen all along the coast and in particular at Whiterocks Beach near Portrush.

Chalk is considered to be a very fine-grained, pure limestone composed of billions of microscopic nannoplankton called coccolithophores. 

These marine algae bloomed in the warm oceans, and subsequently, their remains rained down onto the ocean floor between 100 and 500m depth, accumulating as a white ooze and solidifying as chalk. The deposits can reach hundreds of metres in thickness, forming the spectacular white cliffs we see can see in the image above.

Chalk is a soft, highly porous type of pure limestone. The chalks of the north coast are incredibly refined, with less than 0.5% insoluble residues. However, they are also notoriously hard and dense compared to other Cretaceous chalks. 

Close examination of the cliff faces reveals thin, laterally continuous crinkled lines that connect to flint nodules. These lines are caused by pressure dissolution of the limestone. 

The carbonate material is dissolved into the solution due to increasing overburden, but insoluble material such as silica is left behind, accumulating in thin bands and migrating to form flint nodules that are capable of engulfing and preserving the body and trace fossils Uplift throughout the Jurassic to early Cretaceous period exposed the chalks at the surface and formed the white cliffs we see today.

What caused this chalk to become so condensed? Walking down Whiterocks beach, the homogenous white cliffs are interrupted briefly by a much darker igneous rock. This intrusion, known as a volcanic plug, gives us the first indication of the genesis of the Antrim basalts.

As the North Atlantic began to open at the end of the Cretaceous, magma began to erupt through the chalk firstly in the form of isolated cinder cone volcanoes. 

The explosive volcanism brecciated the chalk in many places, and forcibly injected magma blocks into the surrounding rocks, which can be seen as dark coloured boulders within the white cliffs. Over time these vents solidified to produce the volcanic plugs, upon one of which sits the spectacular Dunluce Castle.

As rifting continued, extensive fissures opened up in the earth’s crust resembling those seen in Iceland or Hawaii today, allowing basaltic lava to pour out on top of the chalk. 

Three successive pulses of rifting resulted in three distinct phases of volcanic activity; the lower, middle and upper basalts, separated by periods of calm.

The Giant’s Causeway is comprised of the middle basalts. During each phase, successive lava flows erupted onto the surface and pooled in natural hollows in the landscape. Flows range from 7 to 18m in thickness.

The renowned hexagonal pillars of the Giant’s Causeway are formed from the cooling of these immense pools of lava. As the lava cools, it loses heat to the atmosphere at the top, and to the colder country rock through the base of the pool. These cooling fronts move towards each other to the centre of the pool as the lava cools and solidifies.

As it does, the resulting basalt uniformly contracts laterally and cracks into mostly five- and six-sided columns. 

These cracks extend upwards and downwards, perpendicular to the cooling fronts, at roughly equal speeds. In an ideal situation, these cracks would eventually join each other at the centre of the flow, creating continuous columns separated by slightly offset cracks at the centre.

However, the main causeway lavas are divided into an upper colonnade, a central entablature and a thick basal colonnade. This is thought to be caused by water seeping into cracks as they were forming, accelerating cooling and disrupting large colonnade formation in the upper and middle sections. 

The most spectacular example of this junction is at the aptly named “Organ”.

Following the outpouring and cooling of each of these lava flows, a period of inactivity allowed the topmost section of the basalt to be exposed to intense, persistent tropical weathering, forming a soil rich in iron and aluminium, called laterite. 

Laterites form by the leaching of the parent rock during the wet season, the resulting solution is brought to the surface during the dry season and removed, progressively depleting the soil of easily dissolved ions such as sodium, potassium, calcium and magnesium, leaving behind the more insoluble elements such as aluminium and iron oxides. 

It is the iron oxides that give this soil its characteristic brick-red colour.

Laterite formation occurs on the surfaces of the basalt that are in contact with water; on the surface and within cracks in the rock. 

As a result, weathering propagates downwards and inwards from cracks, creating “cores” of unweathered basalt that resemble pillow basalt.

These cycles are spectacularly displayed in the amphitheatre shaped cliffs in the image above, from the iconic stepping stones of the middle basalts, through the distinct red layer of the laterite and into the columns of the upper basalts. 

The story of the Giant’s Causeway has evolved over many centuries, from myths of giants and man-made pillars to a tremendous primaeval ocean, but one thing that has never changed is the impact that this captivating landscape has had on mankind since we first set foot on the emerald isle, and will continue to evoke awe and wonder for centuries to come.

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