Burren and Cliffs of Moher UNESCO Global Geopark - Geology

More documents

Info

The rocks of the Cliffs of Moher As the tourist season starts to accelerate, one of the most popular visitor locations will be the Cliffs of Moher. All visitors will experience a variety of feelings and emotions as they gaze out over the world-renowned cliffs. It is worth reminding ourselves about the geology they are looking at. All the rocks in the Cliffs of Moher started out as mud, sand, and silt around 320 million years ago. These sediments were eroded by rivers from a huge mountain range that extended along the equator, south of where we were located at that time. Like the modern Ganges or Brahmaputra rivers, they were fed by monsoonal rains and carried a lot of sediment to the sea where it was deposited as a delta. When rivers carry sediment to the sea, the coarser sand is deposited first, right at the shore, but the finer mud can be transported huge distances offshore before it finally settles to the seafloor. As the sediment keeps being deposited, the sheer volume of it can form new land as it builds outwards into the sea. So, if you were standing on the seafloor (for a very long time) far out to sea, you would initially see a lot of mud accumulating around you. However, over time you would be buried by increasingly coarser sediment as the shoreline got closer to you. Eventually it would fill the sea, land would appear, and plants would then colonize that land. Then imagine that sea-level rose and you were inundated again. As long as the rivers keep flowing the process would repeat itself and the sequence from mud to sand would be repeated in successive layers as the sediment filled the sea again. If sea-level then dropped, the coastline would move offshore and there would be erosion of the coastal area until a new equilibrium was reached. This interaction between fluctuating sea-level, monsoonal rains and large rivers is what happened 6,000 kilometres away and 320 million years ago to form the layers of sand and mud that became the sandstone and shale of the Cliffs of Moher. The changes in sea-level were caused by an ancient ice age that was happening at that time. The layers of sandstone are slightly harder than the rest of the cliff face, which is made of softer shale and siltstone. The softer layers get eroded by wind and waves, leaving the sandstone ledges slightly overhanging. These ledges are ideal nesting sites for the incredible bird population that returns here every year, an amazing sight for the visitors who also return every year. The rocks of Cliffs of Moher are a record of the changing patterns of sedimentation in an ancient delta that formed 320 million years ago, over 100 million years before the first birds had evolved and 320 million years before the first tourist. Figure caption: The layers of sandstone and shale of the Cliffs of Moher.
The corals of the Burren National Park The Burren National Park is located on the ancient Burren limestone bedrock. As you approach Mullaghmore you will notice that the layers in the limestone are gently buckled, the result of plate tectonic collisions almost 300 million years ago, more than 30 million years after the rocks were formed in a shallow tropical sea. The limestone in the Burren is rich in fossils, although they can be difficult to see sometimes. Along the main walking trail that ascends to the top of Mullaghmore there are some wonderful examples of colonial corals. Some of these corals are more than one metre in diameter. The corals stand out because they are darker than the surrounding limestone. They also literally ‘stand out’ as they protrude slightly above the limestone surface. The corals are called colonial corals because they are colonies of individual animals not because of any geopolitical history! These colonies are formed of clustered branches of the coral structure. This structure is the rigid framework that physically supported the soft creatures known as ‘polyps’ that lived there. An individual polyp would have lived at the end of each branch. Each polyp in a colony was an individual creature, but they were actually clones of each other, so would have been genetically identical. Corals make their hard structures from the white mineral calcite (CaCO3), which they extract from sea water. The fossil corals along the walking trail in the Burren National Park were originally calcite too, however the minerals have since been changed. The corals are now predominantly silica (SiO2), and silica doesn’t dissolve as easily as calcite, which is why they stand proud of the limestone surface. It is common for minerals to be replaced by other minerals over geological time. This process of change is known as ‘diagenesis’, and it is controlled by temperature, pressure and chemistry changes that take place as rocks get buried deeper below the surface. The question is, where did the silica come from? There were many possible sources of silica. Some creatures which may have been living with the corals, such as sponges and radiolaria, extracted silica from seawater to make their rigid structures, so their skeletons could have supplied the silica. The silica could also have come from wind-blown dust blown into the sea, similar to modern Saharan dust storms. Another source could have been mineral veins which were injected into the limestone during the tectonic collision that formed the buckling of Mullaghmore. It is also possible that freshwater flowing into the sea could have brought in silica, in much the same way that offshore springs around the Burren transport minerals from land to sea. Whatever the source, once buried, that silica dissolved and migrated and found a new home, in the corals of the Burren National Park. The corals are wonderful examples of colonial organisms but also highlight the complex paths that lead to the preservation of fossils as we see them today. Figure caption: Colonial coral replaced by silica in the Burren National Park.
Page 1 and 2: Managed By Supported By
Page 3 and 4: The patterns on Moher Flags are anc
Page 5 and 6: History in water flowing through th
Page 7 and 8: Our Fossil Roots In sandstone rocks
Page 9 and 10: Biokarst coastline, near Doolin. In
Page 11 and 12: Lough Garrig on top of Sleive Elva
Page 13 and 14: A clay layer between two layers of
Page 15 and 16: Big Brachiopods Large brachiopods a
Page 17: Our Geological Connection with Ukra
Page 21 and 22: Conodonts: bizarre fossils in the B
Page 23 and 24: Sedimentary layers: reading the pag
Page 25 and 26: Oh poop... We have quite a wide var

Burren and Cliffs of Moher UNESCO Global Geopark - Geology

Create successful ePaper yourself

Delete template?

Save as template?