by Paul Braterman
My previous post here described Siccar Point, where an 80,000,000 year time gap is present between near-vertical tilted strata, and their roughly horizontal overlay. This gap corresponds to the formation and subsequent erosion of fold mountains thrown up when Iapetus, precursor to the modern North Atlantic, closed. Today's post is (mainly) about the Giants' Causeway, part of the enormous lava field first produced when the modern North Atlantic began to open, and still growing at the Mid-Atlantic Ridge and, most spectacularly, in Iceland. Fragments of the initial outpouring were separated as the Eurasian and North American plates moved away from each other, and now can be found as far apart as Greenland and Denmark.
The Antrim Lava field shown within the British Tertialry Volcanic Province, itself part of the North Atlantic Lava Field. By Hazel Muzzy (Own work), via Wikimedia Commons.
The Antrim Lava Field, of which the Causeway is part, was formed in three separate phases each consisting of many individual episodes. The most spectacular feature of the Causeway is provided by the second of these. Here, the lava cooled slowly, to generate a solid layer which stressed as it cooled, finally fracturing to give a complex array of columns, up to 10 metres high, and showing in places an almost regular hexagonal pattern. The lava of this second episode shows subtle chemical differences from the first, evidence of changes in the hot lava plume feeding the outflow. But what most excited me at the site was the existence of a band around 5 metres thick, between these columns and the lava beneath them. This layer is not a sediment, but a palaeosol, an ancient soil formed by in situ weathering of the top of the lavas deposited in the first episode. Its nature is confirmed by the presence of occasional unweathered lumps, and there are occasional round scars (“Giants' eyes”) in the exposed surface where these lumps have come away. Humid conditions are confirmed by the presence of valleys carved by streams, and filled in by the later lava flows. The chemical composition is like that of tropical
soils, which have undergone extensive prolonged leeching under warm and wet conditions, with the
most insoluble materials, iron and aluminium oxides, predominating towards the top, and there are traces of charred plant roots in the topmost layer. So here we have direct evidence of an extended interval, variously estimated at between 100,000 years and 3 million years, between the first and second phase of eruptions. After my visit, I discovered that this interbasaltic layer is found across the whole area of the Antrim Lava Field, and that there is another such layer between the middle and upper lavas. There are also extensive dikes, penetrating all the lower levels, caused by the eruption of the lava layers above. The entire coastline has been extensively reshaped and eroded over the intervening millions of years, and most dramatically during the Ice Ages, and subsequent exposure to the storms of the Atlantic. For more extensive descriptions, see here, p. 30, or here, and references therein.