1. Skellig Michael, Stratigraphic Report with Specialist Papers, 1986-2010.
This is one of the best sources of archeological information on the Skellig Michael monastery and South Peak area available. It is complete and well-illustrated including a LIDAR scan of the monastery site. Everything is labeled and explained with measurements and historical details. There is also a brief summary of the island’s geology.

2. Skellig Michael World Heritage Site Management Plan, 2008-2018
This report has a thorough section on the history and site description of Skellig Michael and the monastery with many photos. As befits a management statement there is also a section on the historic and cultural value of the site.

3. Bozeman Science – Mr. Anderson lectures on plate tectonics (2011) 9 mins
There are numerous videos online about continental drift and plate tectonics and I am including this one as just one example. It is well narrated with an excellent graphics package and easy to understand. If you want to learn more about this geological phenomenon, check out the other videos listed on You Tube.

4. Interactive Map of Tectonic Plates
This highly detailed map of the world shows all the continental plates and oceanic plates, the mid-oceanic ridges, fault lines, volcanoes and even the spreading rates of the plates. You can zoom in on specific areas for more detailed examination.

5. Geology of Ireland
The wiki entry has a lot of useful information and details about the timeline of the formation of Ireland including a color-coded map of the major rock types and formations and where they are found today.

The Geology of Skellig Michael Island Refuge of Early Christians and the Last Jedi

By MollyCrilly (Own work) via Wikimedia Commons

My first thought when I saw a photo of Skellig Michael was how in the world did this craggy pyramid of an island come to be there just eight miles from the Irish coast and how long ago was it formed. If you have seen the movie, The Last Jedi, you know how strikingly beautiful is the rocky island of Ahch-To with its jagged, layered grey rocks and emerald verdure. And you probably also know that the scenes were shot on the island of Skellig Michael, a scant eight miles off the southwest coast of Ireland. But, how did this craggy eruption from the sea and its sister island, Little Skellig, come to be here?

By Jerzy Strzelecki (Own work) via Wikimedia Commons

The rock of Skellig Michael is 370 million year-old sandstone
Skellig Michael is an up-thrust of one of the furthest west outcrops of a type of rock with the picturesque name of Old Red Sandstone. Sandstone is rock that begins, as you might guess by the name, as sand and other particles that settle out of water onto the bottom of an ancient sea or shore as sediment. Over millions of years, the mass of material pressing down and the cementing action of ocean salts creates a sedimentary rock, sandstone.

This Old Red Sandstone was first laid down between 360 and 374 million years ago during a geological period called the Devonian, named for Devon, England, where rocks of this age were first studied. The sediments that became the rocks of Skellig Michael were deposited by rivers flowing into the sea over perhaps 25 million years primarily as sand on the coastal plain of an ancient sea. But, the location of that sea was very different from where the island of Skellig Michael and Ireland are today. Over hundreds of millions of years of geological time, the land masses that form the continents have drifted around the globe forming new continents and seas.

Via Wikimedia Commons

The supercontinent of Pangaea and the idea of continental drift
During the Devonian geological period, there was an exceptionally large land mass in the southern hemisphere that geologists call the supercontinent of Pangaea. Most people have noticed at some time the curious fact that the coastline of Africa seems to fit neatly against the shape of South America while North America and Europe match up pretty well. Alfred Wegener was a German meteorologist and explorer who also observed this apparent continental alignment. In 1912, he proposed the hypothesis that the shorelines of Africa, South America, North America and Europe seem to align because they were once a single huge land mass that somehow was pulled apart. He called this hypothetical supercontinent Pangaea and the process of separation, continental drift.

Evidence for continental drift from the fossil record
To test his theory, Wegener traveled to the various continents, collected fossils from each and compared them. The striking similarities, especially among plant fossils, from the matching land masses convinced him that they were once contiguous but separated and moved apart to form the continents we see today. Also, he found fossils of tropical plants in the Arctic where they could not possibly have originated. In spite of his extensive evidence, Wegener's theory was rejected by most geologists. The crucial discovery that continents are parts of huge plates that float upon a sea of molten rock in the Earth's mantle would not be made until the 1950s. Thus, Wegener could not offer a plausible mechanism for how the gigantic and seemingly fixed continents could possibly have moved, even over a space of hundreds of millions of years.

Sea-floor mapping and plate tectonics solve the riddle
The beginning of the breakthrough in understanding continental drift came in the 1950s when the scientific project to map the Atlantic sea floor confirmed the existence of a mid-ocean mountain range extending from northeast of Greenland through Iceland all the way to the south Atlantic. Continuing improvements in sonar devices and more powerful computers allowed scientists to make better and better maps of the sea floor. The sonar data eventually showed that all the Earth's oceans had interconnected mid-ocean ridges. But the reason for this undersea phenomenon was not discovered until it was noted that seismic activity, earthquakes and volcanoes tended to cluster along the mid-ocean ridges.

By USGS Via Wikimedia Commons

Eventually, manned exploratory submersibles were built that could withstand the tremendous pressure of the deep ocean and allowed scientists to descend to the mid-ocean ridge. They saw for the first time cracks in the Earth's crust through which magma from the mantle, the area of molten rock beneath the crust, was emerging. By examining the rocks on either side of these cracks it was seen that the further they were from the ridge, the older they were. This could only happen if the rocks were actually moving away from the cracks over millions of years. In addition to the mid-Atlantic magma cracks, others were found all over the globe and by connecting them it appeared that the crust was actually split up into huge masses of rock. These continent-sized pieces of real estate were called tectonic plates. And, the truly mind-boggling but inescapable fact was that the plates were actually floating like icebergs on the liquid magma of the mantle because they were less dense. And, it was proved that they move--not very fast--only two to three centimeters per year, but enough to cause the splitting up of Pangaea over 350 million years into the separate continents we see today.

So, how did the Old Red Sandstone become the Skellig Michael of today?
Ireland and the Skellig Islands are part of the Eurasian tectonic plate that split off from the supercontinent, Pangaea, about 200 million years ago. The movements of the continental plates relative to the oceanic plates are not known exactly, but the general direction of the Eurasian plate was north. The rocks of Skellig and Ireland and the rest of Europe were being transported from the warm tropical seas of the equator to the cold North Atlantic at the rate of only a couple of centimeters a year, but it was enough. The next geological event to shape the landscape was a compression and folding of the rocks about 300 million years ago that led to the uplift of a mountain range and the formation of the craggy pinnacles of the Skelligs. Softer rock between the twin peaks on Skellig Michael eroded away leaving the valley formation known as Christ's Saddle. Skellig Michael and Little Skellig, then, are actually part of the same mountain ridge as seen on the mainland, but rising sea level turned them into islands. When we visit these marvelous places, we are seeing the results of 400 million years of geological activity. We humans are definitely newcomers on the scene!

The early Christian monastery on Skellig Michael--a wondrous spiritual place
I have not yet visited Skellig Michael, but the trip is definitely high on my list. The entire island is a UNESCO World Heritage Site and thus protected. The early Christians in Ireland would, I'm sure, have felt the powerful spiritual call of this isolated island. It is thought that the monastery on Skellig Michael was established in the 6th century by St. Fionan but the exact date is not known. The monks would have had a strenuous climb up the steep rock steps from the landing to the plateau 600 feet above the ocean where the monastery was built. It includes a church, a large and a small oratory, six cells for the monks and one for guests, a cemetery and terraced gardens.

By Cathal Curran Photography (Own work) [CC BY-SA 4.0
(], via Wikimedia Commons

The cells are ingeniously constructed of the only abundant building material on the island--rocks. A thick-walled foundation base of flat stones was laid and then a circular dome-shaped roof was dry-fitted with each course placed a little further inward until the roof was completed. Some of these beehive huts, as they are known, that were built elsewhere in Ireland had an opening at the top of the dome, but the ones on Skellig do not, although some have small openings apparently as windows. The largest cell, probably used communally, had walls six feet thick at the base and a dome sixteen feet high.

By Jibi44 (Own work) [GFDL (
CC-BY-SA-3.0 ( via Wikimedia Commons

By Arian Zwegers (Skellig Michael) [CC BY 2.0
(], via Wikimedia Commons

We do not know how many people lived at the monastery on Skellig Michael but perhaps a dozen monks and an abbot at the most. They had cisterns for collecting rain water and made terraced gardens for growing vegetables. They also caught fish and birds and ate their eggs. They lived and died and were buried there for several centuries and today we marvel at their religious devotion and asceticism. The monastery is a place of spiritual power and connection with those who came before us and well worth the pilgrimage to Skellig Michael.

By Ecmc23 (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons


What else would you like to know?