Northern Ireland Highlights
Graeme O. Churchard

There are many, much publicised sights, which disappoint when one actually sees them. The Giants Causeway is not one of these! Quite the reverse! Especially if you see it in the company of Paul Lyle.

Sitting on the causeway.

Paul is an expert on multi-tiered lava flows of which the Giants Causeway is a prime example. If you look at the flow you will see that it can be divided into three units. At the base there is the well-known columnar-jointed section, or tier, called the Colonnade. At the top of this there is a sharp transition into a zone of narrow, often curved columns. This is called the Entablature or Curvi-columnar zone. This passes upwards into the Upper Colonnade which is made up of wider, less obvious columns. Sometimes this is called the pseudo-columnar zone.

Another thing you will notice about the Giants Causeway is the thickness of the lava flow. It is much thicker than many others in the area and this is because it fills a large, deep river valley which had been carved in the land surface. It is the presence of the valley and the river which are essential to the development of such exceptional columns.

The lava erupts (but no one knows where) and flows into the valley where it comes to rest. It cools against the valley bottom and a crust forms on top. Columns start to grow upward from the bottom and downward from the top. These columns are formed by cracks forming at many points. These are 3-pronged and they intersect to form 3, 4, 5, 6 or 7 sided polygons. These cracks are relatively tight.

The collonade meets the curvi-columnar zone

The top surface also has polygonal contraction cracks at a spacing of 3 to 5 metres. These also propagate downwards as master joints. They are rather open cracks.

Now the top of the lava flow is flooded by the water which used to flow down the river valley. A rather interesting lake must have formed. Some of this water goes down the open joints and starts to cool the lava. Things must have been rather steamy! The presence of water at a spacing of 3 to 5 metre intervals tends to complicate the shape of the cooling front at the top of the lava. And as cracks propagate at right angles to the cooling front the curvi-columnar zone is formed. It is a feature of the Causeway that large polygonal fallen blocks, stuffed full of curved columns, and about 3 metres on a side are ubiquitous. These are bounded by the master joints.

The collonade

The Colonnade continues forming in a dry state, with regular shaped columns propagating upwards. This continues till it meets the downwards propagating Entablature which is cooling in a wet environment. Column formation then ceases although the lava is still very hot. Mechanical contraction of the columns continues giving the characteristic horizontal ball and socket joints which characterise the vertical columns.

And there are more lava flows around the corner.

Was there a meteorite impact at the base of the Jurassic?
 

Mike Simms took us to Waterloo Bay in Larne to look at the Triassic-Jurassic boundary. At first sight a fairly straight-forward succession where the enthusiast can pinpoint the start of the Jurassic by finding the first ammonite - Psiloceras, conveniently located near the sewerage pipe.

But a few metres below the Jurassic boundary there is a highly disturbed 4 or 5 metres of sediment. The beds look as if they have been shaken violently. The top of the shaken beds is eroded and above this is a metre or so of sediment with ripples indicating it was laid down under strong currents.

If the shaken beds were thinner one would ascribe them to an earthquake - and we could call the sediment a seismite. But there is about 4 metres of them! That would be quite an earthquake! The amount of energy required is more than we can get from a terrestrial event. But an extra-terrestrial event - such as a meteor strike - has no energy limits and a 4 metre seismite becomes a possibility.

And we know that earthquake and meteor impacts are followed by tsunamis. A tsunami could explain the erosion surface and the current deposited sediment - it is a tsunamiite!

There is a meteor crater at Manicouagan in Quebec which is at a suitable position to be a candidate but it is at the wrong time by a couple of million years. But the crater for the K-T event took a lot of finding, so one more than 3 times older may well be even more difficult to find.

All this raises the question of where is the base of the Jurassic. The Jurassic is separated from the Triassic because the fossils are different between the two.

There was a mass extinction - and a meteor impact would be a good means of achieving that. Given that; the place for the start of the Jurassic is the base of the tsunamiite. The conventional place marks where a new fauna has established itself - some time after the event which allowed it to flourish.


As Mike Simms said, its like dating the new millennium not by the fireworks at midnight, but by the first bus on the second of January.