LECTURE DATE: 30th September 2016

MAIN TOPIC: Cambrian and Precambrian Rocks

In this lecture I learned about:

• the processes that lead to the formation of Ireland's oldest rocks such as the build up of sediments and through the process of metamorphosis
• the break up of the super-continent Rodinia into two smaller continents, Laurentia and Gondwana
• the different types of rocks formed in Cambrian and Precambrian times in various parts of Ireland
• the Cambrian life that existed at the time through the evidence present in fossils

The readings I completed are:

Sleeman, A, McConnell, B.& Gatley, S. (2004) Understanding earth processes, rocks and the geological history of Ireland, Geological Survey of Ireland, Dublin, pp.23-25

• This chapter focuses on the formation of Precambrian Rocks. These rocks formed due to severe metamorphosis almost two billion years ago. Precambrian rocks make up 0.5-1 percent of the land area in Ireland. These rocks are present in the Northwest, Southeast and Southwest of the country. The main rock present in these areas is gneiss. This gneiss comes from both igneous and sedimentary origin. This chapter also refers to the break up of Rodinia into two continents, Laurentia and Gondwana, due to crustal thinning and the formation of the Iapetus ocean between the two new continents.

Sleeman, A, McConnell, B.& Gatley, S. (2004) Understanding earth processes, rocks and the geological history of Ireland, Geological Survey of Ireland, Dublin, pp.29-32

• This chapter focuses on the formation of Cambrian rocks. These rocks are sedimentary rocks that were deposited on a continental shelf at the northern margin of Gondwana between 545 and 495 million years ago. They can be found in the East and Southeast of the country in Bray, Howth and South Wexford. The main rock types are greywackes and quartzite. This reading also contains information on Cambrian life. Cambrian rocks contain fossils that are referred to as Oldhamia. Oldhamia were burrowing marine organisms.

This lecture helped me understand how Cambrian and Precambrian rocks have contributed to the Irish Landscape by creating features evident across the country today. One of the prominent features formed from Cambrian rocks is The Sugarloaf Mountain. I found this particularly interesting as I have hiked this mountain several times with my family. I learned that The Sugarloaf consists of quartzite that was formed at the edge of Gondwana. Quartzite is formed from the separation of mud from sand on beaches. This sand is then lithified to form the rock. Quartzite is also found in Howth where plate tectonics and metamorphosis caused the sediment layers to mix, slump and become deformed. Quartzite is highly resistant to erosion which has allowed the rock to remain prominent in these areas today.

This topic also helped me to understand how the break up of Rodinia due to crustal thinning allowed for the formation of the same rock type on two separate continents. This was caused by plate tectonics where convection currents in the magma caused the landmass to split apart forming Laurentia and Gondwana. These plate tectonics also allowed the rocks in these areas to move repeatedly to be situated in their modern day positions in Ireland.

I also found it interesting to learn how the formation of rocks millions of years ago can be used as a form of revenue and employment in modern Ireland. For example, how the green marble of Connemara is now sold as an ornamental souvenir to tourists who visit the area.