Open University Geological Society Journal Spring Edition 2006 Contents The Geoff Brown Memorial Lecture: Volcanic eruptions into Iceland’s glaciers 1 Dr Dave McGarvie, Staff Tutor, Faculty of Science, OU Yorkshire Region An introduction to the pyroclastic rocks of the Bandas del Sur, Tenerife and 7 field guide to the area around Tajao Duncan Woodcock Field Observations in the Silesian rocks on the west coast of Carmarthen Bay 12 John Downes The Palaeolithic in Britain 17 Gladys Dinnacombe Mineralisation and Mining at Minera, North Wales 23 Peter Appleton Within this hill 27 Rob Heslop Scratching the surface of New Zealand: North Island 31 Robin Lewando Geology of El Cusuco National Park, Honduras 37 Olwen Williams Assessment of earthquake-triggered tsunami hazards in the Straits of Messina 39 and eastern Sicily, Italy Laurence Ghier BSc (Hons) Geosciences (Open), University of Caen, France Teaching rocks to KS 1 & 2 49 Alan Bates Some observations on tectonic plate movement 52 Martin Beaney Petrology of Millstones 55 Tony Browne ISDR-UNESCO workshop on urban policies and decentralisation 58 Annette Kimmich Book reviews 22, 30, 48, 51, 54, 62 Winning photographs of the Moyra Eldridge Photographic Competition for 2005. Plates 2 & 3a&b Centre colour spread. It is the responsibility of authors to obtain the necessary permission to reproduce any copyright material they wish to use in their article. The views expressed in this Journal are those of the individual author and do not represent those of the Open University Geological Society. In the opinion of the author the description of ven- ues are accurate at the time of going to press; the Open University Geological Society does not accept respon- sibility for access, safety considerations or adverse conditions encountered by those visiting the sites. Committee of the Open University Geological Society 2006 Executive Committee Members President: Dr Angela Coe, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA Chairman: Joe Jennings Secretary: Linda Fowler Treasurer: Bob Morley Membership Secretary: Penny Widdison Newsletter Editor: David Jones Information: Linda McArdell Events Officer: Glynis Sanderson Sales Manager: Lesley Laws Non-voting postholders Gift Aid: Ann Goundry Journal Editor: Jane Clarke Archivist/Review Officer: Jane Michael Minutes secretary: Sam Aderson OUSA Representative: Alasdair Farquharson OUSA Deputy Representative: Karen Scott Branch Organisers East Anglia: Andrew Fleming East Midlands: Don Cameron East Scotland: Anne Burgess Gogledd Cymru: Rachel Atherton Ireland: Phyllis Turkington London: Sue Vernon Mainland Europe: Annette Kimmich Northumbria: Annie Hedley North West: Phil Horridge Oxford: Sally Munnings Severnside: Janet Hiscott South East: Roger Baker South West: Angela Scarrott Walton Hall: Michael Friday Wessex: Sheila Alderman West Midlands: Chris Gleeson West Scotland: Stuart Fairley Yorkshire: Dave Williams Past Presidents of the OUGS 1973-4 Prof Ian Gass 1985-6 Dr Peter Skelton 1997-8 Dr Dee Edwards 1975-6 Dr Chris Wilson 1987-8 Mr Eric Skipsey 1999-0 Dr Peter Sheldon 1977-8 Mr John Wright 1989-90 Dr Sandy Smith 2001-2 Prof Bob Spicer 1979-80 Dr Richard Thorpe 1991-2 Dr David Williams 2003- 4 Prof Chris Wilson 1981-2 Dr Dennis Jackson 1993-4 Dr Dave Rothery 2005 - Dr Angela Coe 1983-4 Prof Geoff Brown 1995-6 Dr Nigel Harris Vice Presidents of the OUGS Dr Evelyn Brown Dr Michael Gagan Norma Rothwell The Geoff Brown Memorial Lecture: Volcanic eruptions into Iceland’s glaciers Dr Dave McGarvie, (Staff Tutor, Faculty of Science, OU Yorkshire Region) Author’s Note. As you can see, the diet was dominated by meat and fat, with very Although this is an abridged account of the memorial lecture, little carbohydrate. Yes – it looks like the forerunner of the Atkins some material has also been added to help those who were not at diet. It is the cold of course. In a cold climate a lot of fat is exact- the lecture (and who did not see the many illuminating images) to ly what you need to keep you going. The diet obviously worked gain better insight into these unusual eruptions. because the Watts expedition was very successful. My talk is about volcanic eruptions into Iceland’s glaciers. (Like Icelanders I use the term ‘glacier’ very loosely, to include all icy coverings – ice sheets, ice caps, valley glaciers, and so on.) Of relevance to this particular talk is that I went with Geoff Brown to Iceland on four trips between 1985 and 1988, and in those were sown the seeds of the research that I am doing at the moment. The talk will cover the following: • An introduction to Iceland. • Working with Geoff at the Askja volcano. • What a basalt ‘tuya’ is and how it is formed (i.e. a large basalt eruption into thick ice). • The 1996 Gjálp eruption (an example of a small basalt erup- tion into thick ice). • The 2004 Grimsvötn eruption (an example of a small basalt eruption into thin ice). Figure 1. Geoff Brown at Askja in 1985. • Thick-ice rhyolite eruptions at the Kerlingarfjöll mountains. • Thin-ice rhyolite eruptions at the Öraefajökull stratovolcano. At Askja with Geoff One of the other places visited by William Lord Watts was the Some basic facts about Iceland remote volcano of Askja in the central highlands of Iceland. It is Iceland’s highest point is just over 2km above sea-level, the old- usually only accessible for fieldwork for about eight weeks each est rocks are c.16 Ma old and there is an eruption every five years year, and I went there with Geoff on four trips from 1985-1988 or so. About 90% of Iceland is basalt and about 10% is rhyolite. (Figure 1). Geoff wanted to do geophysics at Askja, in particular Iceland is also notable because it is building new land, and dur- to use gravity-meters to measure density variations in the crust to ing the 1960s the island of Surtsey was created off Iceland’s south try and understand the subsurface structure of the volcano (look- coast. Of particular relevance to this talk is that once it emerged ing for magma chambers etc). I was there as his Icelandic geolo- above the sea surface and seawater was denied access to the vent, gy specialist, and he also asked me to challenge/correct the more subaerial basalt lava flows were able to pour out over the new wayward ideas of the over-imaginative geophysicists. Much of island till they reached its coast and flowed into the sea. Here they Askja is constructed of fragmental basalt formed during eruptions quenched and fragmented, and the fragments tumbled down the into glaciers, and this was the first time I had had a serious look underwater slopes to form breccias. This process also occurs dur- at these rocks (Figure 2). The observations I made then remained ing large basaltic eruptions into thick glaciers, and its significance with me until I rejoined the OU in 1996 and started thinking about will become apparent later. Iceland is home to the massive glacier of Vatnajökull, into which all the other ice in Iceland and Northern Europe could fit with room to spare. One of my heroes, a chap called William Lord Watts, trekked across Vatnajökull in 1875 with five Icelandic guides. It is fascinating to compare and contrast his explorations and adventures with those of modern-day fieldwork in Iceland. A reconstruction of his route across Vatnajökull shows that he passed near to what would one day become the 1996 and 2004 subglacial eruption sites at Gjálp and Grimsvötn, which we will cover later. His food supplies were ‘interesting’ and somewhat different to those of modern-day expeditions: 100lbs of pemmican in skin bags; 15lbs of dried mutton; 15lbs of dried fish; 3 tins of Peek and Frean’s meat biscuits; 50lbs of butter; 12lbs of gravy soup; 2lbs of cocoa; 6 tins of chocolate and milk; 10lbs of Danish ship-bis- Figure 2. Examining fragmental basalt produced during a cuits; and 4lbs of sugar. subglacial eruption at Askja. OUGS Journal 27(1) 1 Spring Edition 2006 Figure 3. Askja’s two calderas – the water-filled 1875 caldera and the wall of the older and larger caldera in the back- Figure 4. On the outer slopes of the Askja volcano. Dark rib- ground. In the right foreground is the small Viti crater. bon is the 1961 basaltic lava flow from the most recent doing some research again after a decade-long hiatus. So my time eruption. The prominent mountain is Herdubreid – a with Geoff at Askja played a key role in inspiring me to start work- tuya formed during a large basaltic eruption into thick ing on the unsolved problems of volcanic eruptions into glaciers. ice. The ice level was at least as high as the prominent break of slope over half way up the left-hand side. Askja has two calderas (Figure 3); the largest (and also the older of the two) is about 8km in diameter. An eruption in 1875 (rhyo- Herdubreid requires a large eruption into very thick (c.1.5km) ice. lite and basalt) created the smaller and younger (c.4km diameter) The pronounced vertical construction of Herdubreid is due to caldera, which now contains Iceland’s deepest lake (220m deep). thick ice confining the escape of erupting basalt. Everywhere the Only one of the 1875 vents escaped the caldera collapse, and this basalt tries to escape to it meets either ice or meltwater, and so it forms a small crater called Viti (which is Icelandic for ‘Hell’ or quenches and has to build upwards. (Without confinement by ice, ‘mouth of Hell’). Viti is a common name for eruptive vents in such a large basalt eruption would simply produce a broad low- Iceland.