Science Spin September 2009

Threatening western waves

By Sean Duke

Galway Bay is known the world over for its tranquil sunsets and natural beauty. But, there is a darker side to its nature. This is also a place that has been shaped by repeated storms, some deadly, ice movements, and the occasional tsunami wave into what they are today. Such natural powers have carved the coastline, and occasionally, taken human life.

The forces that crash onto Ireland's western coastline are formidable, and sometimes frightening. For example, in 1755, the Great Lisbon Earthquake triggered a tsunami that rushed northward and slammed into Galway Bay. Reports indicate that the wave swept up the bay, entered the city, damaged the Spanish Arch, and drowned many citizens.

The west coast also was hit hard in 1839, when one of the biggest storms Europe has ever known came in off the sea and slammed into the coastline. This storm was referred to at the time as 'the night of the big wind' - a description that greatly understated the event.

The Aran Islands exists "in a marine environment, which is totally unprotected from the most extreme sea states encountered in the North Atlantic," according to Professor Mike Williams, Department of Earth and Ocean Sciences at NUI Galway. "Spectacular 'megaclast' accumulations are found along the tops of vertical cliffs on these islands."

The most amazing thing, according to Professor Williams, is that clasts (a clast is a rock composed of broken pieces of older rocks) as heavy as 2.9 tonnes have been found on the Aran Islands, at 50 metres, or 164 feet, above sea level. If we are to draw a rough analogy in terms of the weight and heights involved here it means that some force of nature was powerful enough to lift a herd of African Bull elephants up onto the roof of Croke Park.

The orientation of the clasts on Aran is consistent with the prevailing storm wind direction for the area. That means whatever did this lifting came in from the Atlantic. The clasts are constantly being eroded due to the cliff erosion on Aran. This means that the clasts visible today were probably deposited in the last few centuries. For if they were deposited, say 1,000 years ago, it is likely they would have been eroded away by now.

STORMS

There is no record of a tsunami hitting the west coast of Ireland in the past few centuries, with the exception of the wave that hit in 1755. This implies, Prof Williams has stated, that these deposits are due to extreme wave conditions, and act as a record of these conditions going back about a century and a half. These are not the same as tsunamis.

The megaclasts on Aran, said Prof Williams, act as a perfect natural laboratory in which to study the origins of deposits left way above sea-level, and whether these were left there as a result of repeated major storms, or tsunamis. The study of these deposits can help determine the frequency of storm and tsunami events that hit the west coast in the past, and could also help to determine the rates of coastal erosion that have been occurring.

The NUI Galway geologist has pointed out that wave heights in the North Atlantic can reach a phenomenal height of 20 metres (65 feet) or even more. This figure is remarkable given that scientists estimate that the 2004 tsunami reached heights of 15 to 30 metres.

Clearly, the storm waves in the North Atlantic can be very powerful and deadly indeed.

The large non-tsunami waves occur more frequently than the tsunamis and Prof Williams thus believes that they are more likely to have caused clast uplift along the west coast.

SAMPLES

The Irish research vessel, the RV Celtic Explorer, has, in recent years, been helping to shed light on some extreme weather conditions along the west coast in the past.

The vessel has taken 5-metre 'cores' through sediments that lie at the bottom of Galway Bay. These samples, when retrieved and analysed, were shown to contain the shells of creatures that were known to have evolved only in the last 10,000 years. That meant that the deposits were less than 10,000 years old, and were laid down after the last Ice Age.

The cores contained sediments that are very fine grained, but then there are also layers of coarse shell material and pebbles. Prof Williams believes that the coarse shells and pebbles were deposited during very strong storms, or even perhaps tsunamis, such as the one that hit Galway in 1755.

There may have been other tsunamis to hit Ireland since then. For example, in 1854 the historical record describes a big wave hitting into Kilmore Quay harbour, Co Wexford. An eyewitness to that event reported that before the wave hit, all the water around the harbour rushed out to sea. This is the classic 'fingerprint' of a tsunami.

RISK

Ireland could be hit again by a tsunami in the future, and there have been moves to put a tsunami early warning system in place following the 2004 Tsunami in Asia.

These large waves can travel great distances from where an earthquake occurred, and the tsunamis that emanated from the epicenter in Sumatra in 2004 even reached Australia. The greatest risk to Ireland stems from the collision of two pieces of the Earth's crust in the Atlantic waters off the west coast of Portugal.

Here geologists believe that the African plate is moving underneath the Iberian Peninsula landmass, and sinking into the Earth's mantle. This movement causes a buildup of pressure, which when occasionally released, causes earthquakes.

Scientists believe that a repeat of the Great Portugal Quake of 1755 is possible, and this has potentially serious implications for us here in Ireland. It is estimated that - with a proper early warning system in place - Ireland would have about 3 ½ to 4 hours warning, should a major earthquake occur again off Portugal.

This should give the authorities time to evacuate Galway, and ensure its inhabitants went to safe ground inland, providing an emergency plan is in place, and is carried out.

However, time would be even tighter should a volcanic eruption happen on the Canary Islands, which could also trigger a tsunami, and which is also a geological possibility.

It is estimated that we would have 2 ½ hours to react, should the Cumbre Vieja volcano on the island of La Palma spring suddenly into life and erupt, causing a tsunami.

FACTFILE

• Historical records indicate that major storm waves or tsunamis have hit Ireland in 1640, 1755, 1839, 1852, 1854, 1941 and 1953.
• The evidence suggests that the events of 1755 and 1854 were tsunamis, and the 1852 event might have been a tsunami.
• Eyewitnesses reported of the 1839 event that "cod and conger were left squirming on the tops of sand dunes".
• During the 1852 storm/tsunami, 15 fishermen were swept off a cliff on Inishmore.
• The 1953 event killed 1,800 people in Holland, and 120 people died in the Irish Sea when a ferry sank.
• The Atlantic is prone to 'freak waves' which can reach up to 30 metres. These are more frequent than previously thought and are due to merging of waves during storms.