Tag: igneous rock

geology historical / biographies

Mysteries of time: A quest for the age of the Earth

David Alexander Gardiner (UK) The question of the age of the Earth and its former inhabitants is one of great interest to us all. Most are aware that the Earth is understood today to be approximately 4.6 billion years in age, but what is the story of the momentous quest – to unravel the mystery of time? Many early speculations as to the age of the universe abounded in ancient and medieval times. We are all familiar with the literalist understanding of the Old Testament, from which Archbishop Ussher famously calculated a 4004 BC date for the beginnings of the Earth. Yet, this was one of the shortest chronologies in existence: the Babylonians spoke of many hundreds of thousands; the Egyptians of many tens of thousands; and the Hindus many billions of years in their cosmological speculations of the past. However, all these early traditions were not scientific in basis. Rather, they were religious or philosophical and not based upon experimentation and observation. It would not be until after the Renaissance that people started employing scientific methodologies to unravel the mystery. Various early scholars speculated upon the Earth’s geological history, including Leonardo da Vinci, the universal genius. Leonardo noted that fossils had once been actual living creatures and that the ocean must have once covered the land. As regards the age of the world, however, few people dared to challenge the conventional wisdom based upon the Genesis narrative – one wonders what da Vinci’s own view might have been. However, … Read More

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Stop the press: The Jurassic Coast starts in the Permian

Mervyn Jones (UK) This Geologists’ Association field meeting followed the publication of Professor John Cope’s Geologists’ Association (GA) Guide No 73, Geology of the South Devon Coast. It is also the companion to GA Guide No 22, Geology of the Dorset Coast. John retired in 2003 after lecturing at Swansea and Cardiff universities. Since then, he has been an Honorary Research Fellow at the National Museum Wales in Cardiff, and has a wide field experience in the UK and Europe, with publications covering many fossil groups over a wide stratigraphical range. Most recently he has been working on redrawing the geological map of South Wales, the subject of an upcoming GA lecture. And, each year, for the past six years, he has provided weekend geological trips to the West Country. Fig. 1. Prof Cope demonstrates bedding and cleavage. We met up at Meadfoot Strand to the east of Torquay Harbour. Our mission for the weekend was to examine the complex Devonian succession in the Torbay area and its unconformable relationship to the Permo-Triassic cover. Of great interest was the marine Devonian, first described by Adam Sedgwick, assisted by Roderick Impey Murchison, who finally realised that these facies were contemporaneous with the familiar Old Red Sandstone found north of the Bristol Channel. Since then, the Devonian Stages have been named after rocks in the Czech Republic, Germany and Belgium. The base of the Devonian was the first ‘Global Boundary Stratotype Section and Point’ (GSSP), defined by graptolite zones at Klonk, in … Read More

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Colourful bluffs in Long Island recall the most recent ice age

Deborah Painter (USA) Imagine a tremendous piece of land moving equipment that scraped up the soil and some of the surface bedrock from four states within the United States’ Eastern Seaboard, carrying and dragging it all the way, before dumping it on a ridge off the shoreline. That is what essentially occurred with the final advance of the Wisconsinian ice sheet, the only one which left glacial deposits visible in New York State today. Long Island is a ridge of Cretaceous bedrock with glacial deposition. The moraines there have not been ground into sandbars and spits along the western end of the north shore as much as elsewhere, because of the sheltered nature of the Long Island Sound. Therefore, shoreline bluffs expose rocks as well as glacial loess. Fig. 1. Fishermen’s Drive takes you to the loess deposits. To park at the beach requires a permit. (Photo by JB Steadman.) If you find that your journeys take you to New York City, one of the world’s largest metropolitan areas, try to make time to visit Caumsett State Park at Long Island Sound. My own visit began when planning a visit to New York State’s Long Island to see my friend, Joyce Raber. She suggested various things that we might do: go to a Broadway play, go shopping and so forth. However, my list of things to do was typically “eco-tourist”. I wanted to visit the famed American Museum of Natural History in Manhattan, then see nearby Central Park, where the … Read More

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La Gomera: A short geological guide

Ken Madrell (UK) The island of La Gomera has an area of 370km2, it is 25km in diameter, has a maximum altitude of 1,487m (Alto Garajonay) and is situated approximately 40km west of Tenerife. Unlike the other Canary Islands, La Gomera has experienced a long and continuing eruptive break and is in a ‘postshield erosional stage’. Carracedo and Troll (2016) describe this as the stage when active volcanism has ceased, and erosive and denudational landforms are predominant (p. 39). The submarine base of the island shows that it rests on a shallower ocean bed than the surrounding islands. The emerged land mass is semi-circular in shape, with a radial drainage pattern from its centre near Alto de Garajonay. The dating of the island has proved problematic, as some of the earlier measurements placing its age between 15 Ma and 19 Ma have since proved to be inaccurate. More reliable estimates now put its age at between 10 and 11 Ma. Fig. 1. Roque Argando viewed from Lomo de la Mulata. La Gomera’s general stratigraphy comprises of three main rock sequences: A Miocene basaltic shield, including a basal plutonic complex (that is igneous rock formed by solidification at considerable depth beneath the earth’s surface).A nested felsic (that is, igneous rocks that are relatively rich in elements that form feldspar and quartz) stratovolcano (which is built up of alternating layers of lava and ash).The youngest Pliocene volcanism.Fig. 2. Sketch map of La Gomera, showing the main towns and geology of the island. … Read More

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What’s so special about South Devon?

Professor John CW Cope (National Museum of Wales, Cardiff UK) Take a trip to the South Devon coast around Easter time and you are bound to come across student parties from universities engaged in fieldwork. What is it about this area that makes it so popular as a centre for this? The simple answer lies in a single word — variety. There is probably no other area in the UK where such a wide variety of rock types and ages is well-exposed in such a small geographical compass. Let’s have a look at some of the factors. The geological succession The oldest rocks exposed in South Devon are of Devonian age and, unlike many other areas of the UK, the Devonian rocks are in marine facies virtually throughout. Looking back over the history of geology, the age of these rocks had initially proved difficult to identify and it was only after Murchison had seen the marine successions in The Rhineland and Russia that he realised that these marine rocks were the equivalent of the Old Red Sandstone farther to the north. The Devonian rocks present a variety of marine facies, with the Middle Devonian limestones being of particular note. The limestones are a local development whose presence, in an otherwise deeper water succession, is due entirely to local shallowing of the water caused by thicknesses of volcanic rocks extruded along extensional fault lines as the local basins developed. This shallowing allowed reef-building organisms to flourish and the principal ones of … Read More

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Armboth Dyke, Lake District

Mark Wilkinson (UK) The Armboth Dyke makes a good half day geology excursion in a scenic but quiet part of the UK Lake District. Parking is on the west shore of Thirlmere, in a pay-and-display car park accessed by the narrow road that winds around that side of the lake (Grid reference NY 305 172). The car park is in an excellent setting, with direct access to the wooded lake shore, and would be a great place for the non-geologically minded to wait while you venture onto the adjacent hill. It is probably worth noting at this point that the dyke itself is mostly exposed on rather featureless rolling moorland at around 400m above sea level (Fig. 1), and might not be a good place to visit in thick mist, unless you are very confident with a map and compass. If you happen to be in the business of teaching students to make geological maps, this site makes a great practise day, without too many problems of recognising weathered rocks in the field. Fig 1. Moorland with the dyke just visible as a slightly lighter patch of rock below the red arrow, where the edge of the dyke is exposed. University of Edinburgh students for scale. So, assuming you have decent weather, leave the car park and take the path uphill from the west side of the minor road, a few metres to the north of the car park. The path is steep-ish, and can be slippery if wet, so … Read More

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Features in the field: Ignimbrites of the Yr Arddu syncline

Stephen K Donovan (The Netherlands) In July 1979, I was one of more than 20 undergraduate students at the Department of Geology, University of Manchester, to undertake their final year mapping project in the Snowdonia National Park in North Wales. My mapping area was the Yr Arddu Syncline, about 4km southeast of Beddgelert in Gwynedd. The rock succession is comprised of slates and sandstones, overlain by acid volcanic rocks, with a range of intrusions (mainly acidic), such as microgranite, but also including dolerite. A feature of this succession was the range of features beautifully exposed in the volcanics and intrusions (Figs. 1 to 4). Fig. 1. Features in acidic igneous rocks, Yr Arddu syncline, North Wales (Upper Ordovician).A: [NGR SH 6267 4554] Large acidic fragment (about 60cm maximum dimension) in Pitts Head Tuff Formation. The fragment shows lenticular lapilli. Such large fragments are the exception rather than the rule in the Pitts Head Tuff Formation.B: [NGR SH 6334 4594] Contact between the Pitts Head Tuff Formation (left) and the Composite Intrusion weathered out as a crack to the right of the hammer. Note that the cleavage of the ‘baked’ tuff has not been picked out by weathering, unlike the unbaked rock to the far left.C: [NGR SH 6267 4579] Bedding in Rhyolite Tuff, dipping steeply to the right. Finer grained tuff (left of centre) overlies tuff with small fragments. The finer grained tuff is overlain, in turn (right of centre), by rubbly tuff with numerous small rhyolitic fragments and then … Read More

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Dinosaurs footprints on the Isle of Skye, Scotland

Mark Wilkinson (UK) If you think of dinosaur hunting, you probably imagine trekking through a parched landscape, reaching the crest of a low hill and catching the first glimpse of a complete skeleton lying half exposed in the next depression. While this might just be true in some parts of the world, the reality of hunting for dinosaurs in Scotland could not be much more different. Hence, a cold and damp day in April 2015 found a small group of geologists from the University of Edinburgh on a slippery foreshore on the northwest extremity of the Isle of Skye. We were hoping not for complete skeletons but, if we were lucky, an occasional bone or tooth – well, perhaps we were hoping, but plenty of geologists have been here before, so the chances of a large find seemed pretty slim. Having said that, the total number of dinosaur bones that have been found in Scotland is still small, so that any bone is likely to be of interest – and could well be a new species, or evidence that a larger taxonomic group known from elsewhere was present on the island in the Jurassic. To add extra scientific interest, the exposures on Skye include a thick Middle Jurassic sequence, representing a time of a rapid dinosaur evolution, but with a poor fossil record worldwide. So any find might be of great importance. We visited several locations on the excursion. There are well-known dinosaur footprints at Staffin Bay on the east … Read More

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