Programme: 2006 - 2007
© Craven & Pendle Geological Society
Friday: 27 October
Ordovician trilobites of Cincinnati, Ohio, USA. Dan Cooper, Cincinnati, Ohio, USA.
Friday: 24 November
Volcanic eruptions into Iceland's glaciers. Dave McGarvie Ph.D., Open University
Friday: 15 December
Members Presentations & Christmas 'Jacob's Join'.
Friday: 26 January
Causes and consequences of orogenesis: the Himalaya as a natural laboratory. Yani Najman Ph.D., Lancaster University
Friday: 23 February
The geology of asteroids. Professor Lionel Wilson, Lancaster University
Friday: 23 March
Virtual reality geology of the Suez rift. Paul Wilson Ph.D., Manchester University
Field Meetings for 2007
Sunday, 27th May
Crummack Dale, Anticlines, Unconformities & the Norber Erratics.
Sunday, 24th June
The Sedgwick Trail at Sedbergh and the Taythes Inlier. Jean Chicken
Saturday, 28th July
The position of the Whin Sill within the geological framework of Upper Teesdale.
Saturday, 25th August
The South Craven Fault at Settle. Paul Kabrna
Sunday, 23rd September
Carboniferous geology around Dunsop Bridge in the Craven Basin. Paul Kabrna. This is a joint meeting with Huddersfield Geology Group.
www.isotelus.com Email: Dan Cooper
The Ordovician System of Ohio is probably the most famous of the state’s Palaeozoic rock systems. The alternating shales and limestones of the upper part of this system crop out in south-western Ohio in the Cincinnati region and yield an incredible abundance and diversity of well-preserved fossils.
The most desirable fossils from Cincinnatian rocks seem to be trilobites. The remains of these arthropods are found in considerable abundance in some beds. The most common Cincinnatian trilobite, Flexicalymene, is best known from the Corryville Member of the Grant Lake Formation and from the middle part of the Waynesville Formation. Much less common are well-preserved specimens of Isotelus, Ohio’s official State Fossil
The first serious study of Ohio’s Ordovician rocks was undertaken by the first Geological Survey of Ohio in 1837-1838. At this time John Locke mapped portions of the southwestern corner of the state. Among Locke’s many discoveries were partial remains of a large specimen of Isotelus. Because of its size, Locke named the trilobite Isotelus maximus. He later changed the name to Isotelus megistos, but today Isotelus maximus is the accepted species name. Locke collected only the pygidium (tail) of the trilobite but, by proportional comparison, he estimated that the complete trilobite would have been about 21 inches in length.
Trilobites are an extinct class of the Phylum Arthropoda, which includes among its living members the horseshoe crab, crabs, lobsters, shrimp, scorpions, spiders, and insects. Trilobites first appeared in the fossil record about 542 million years ago and became extinct about 251 million years ago. They lived in marine environments, where they burrowed in sediment, crawled along the sea floor, or were free swimming. Most trilobites ate mud from the sea floor, whereas others filtered food directly from the water, scavenged, or were predators. They grew by periodically moulting their exoskeleton, a hard, outer shell similar in composition to fingernails. Thus, one trilobite could leave behind numerous fossil fragments representing shed exoskeletons. For defence against predators, some trilobites had sharp spines on their exoskeletons, and all had the ability to enroll, much like our hedghog.
Ohio's State Fossil
On 20th June, 1985, Ohio House Bill 145 designated the trilobite genus Isotelus as the official state invertebrate fossil of Ohio. Isotelus is a most suitable selection for the state fossil. Not only are specimens of this trilobite, or at least fragments, moderately abundant in the rocks exposed in southwestern Ohio, but they are represented by the Huffman Dam specimen, which is one of the largest complete trilobites ever collected. This giant specimen of Isotelus measures I4½ inches long by I0¼ inches wide. The Huffman Dam trilobite still occupies a prominent position in the palaeontological exhibits at the Smithsonian and is still one of the largest, complete trilobites of any kind ever collected.
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Iceland is the largest landmass exposed along the Mid-Ocean Ridge System. It has been constructed over the past 16 million years by basaltic to silicic volcanic activity occurring at the Mid-Atlantic Ridge. Large quantities of igneous material are produced in association with the Iceland hot spot, the centre of which is thought to be located beneath Vatnajökull ice cap. Although it is paradoxical that Iceland’s hottest region boasts its biggest ice cap, it is no coincidence: the ice sheet is huge and permanent, precisely because lava flowing from the mantle plume has built the mountains so high. Iceland is made of such paradoxes. Its mountains and valleys are sculpted primarily through the interplay of molten rock which builds volcanoes, and the glaciers’ solid water which ferociously erodes the landscape. The Icelandic people are continually confronted with a three-pronged pestilence of fire, flood, and ice.
Askja (1516 m) is a large basaltic central volcano that forms the Dyngjufjöll massif which last erupted in 1961. It is truncated by three overlapping calderas, the largest of which is 8 km. A major explosive eruption on the SE caldera margin in 1875 was one of Iceland’s largest during historical time. It resulted in the formation of a smaller 4.5 km wide caldera, now filled by Öskjuvatn lake, that truncates the rim of the larger central caldera. The latest volcanic event in Dyngjufjöll occurred in 1961.
This volcano was visited by William Lord Watts in 1875 “… beneath us lay a pandemonium of steam and hideous sounds…the most hideous shrieks, groans, booming and screaming sounds rose from all parts of this terrible depression, the bottom of which was now utterly obscured. The sides of the crater were evidently falling in, and huge wide cracks, even where we stood, showed us that our position was not altogether a safe one. So we lit our pipes and waited until we could obtain a better view.”
Herðubreið is a classic example of a “tuya” or flat-topped, steep-sided subglacial volcano. Icelandic tuyas are usually composed of pillow lavas at the base, followed by hyaloclastic breccias, capped by horizontal lava flows. For a tuya to form basalt lava is erupted into ~1.5 km of ice. The subglacial confinement prevents the lava from free flow away from the vent. In the case of Herðubreið, the subaerial basalt lavas quenched and fragmented when they reached a meltwater lake and formed breccias. Successive subaerial lava flows accumulated to make the characteristic lava cap. In Iceland these tuyas are the subglacial equivalents of basaltic shield volcanoes.
The Gjálp eruption in Vatnajökull (1996)
An eruption started beneath the Vatnajökull glacier in Central Iceland in the late evening of 30th September and lasted for 13 days. The eruption site which was discovered by aircraft was a subglacial basalt fissure eruption. Cauldron formation indicated that the glacier was being melted along a 4 km long fissure beneath the glacier. In fact the eruption took only 31 hours to melt through 600 metres of ice and 0.45 km3 of basalt melted 3 km3 of ice.
Meltwater from the eruption site drained into the Grímsvötn caldera, raising the ice shelf on the Grímsvötn caldera lake. The cauldrons widened and deepened and it is estimated that 0.3 km3 of water were added to the Grímsvötn lake in less than 24 hours. Meltwater escaped as a flood (jökulhlaup) 3 weeks after end of the eruption.
The subglacial Grimsvötn volcano, which is Iceland’s most frequently-erupting volcano, spectacularly erupted beneath the Vatnajökull ice cap between the 1st - 6th November, 2004. In the Grimsvötn caldera ice 150-250 m thick floats on a subglacial lake. The lake drained in November 2004, triggering a small subglacial eruption that eventually became ermergent. Clouds of ash and tephra were ejected beyond the vent (but not far). Preservation of eruptive material is generally poor because as soon as the eruption ceases, snow and ice rapidly cover the evidence.
The volcano Kerlingarfjöll is the main feature in the chain of mountains and glaciers of the Kjolur area which covers about 150 km² area southeast of Hofsjökull. Subglacial rhyolite eruptions into thick ice at Kerlingarfjöll have produced steep-sided piles of fragmental material up to 300 m tall. Several edifices are capped with lava flows and these structures are therefore broadly equivalent to the ‘tuyas’ formed by subglacial basaltic eruptions. The pyroclastic material typically consists of massive, unconsolidated breccias containing blocks and lapilli of pumice within an ash matrix. Unlike the basaltic tuyas of Herðubreið, ponded water is present only during the initial phases of the eruption, and that the transition from explosive to effusive, lava cap producing, activity takes place in essentially ‘dry’ conditions.
Eruptions of Öræfajökull have produced mafic and silicic magmas, and have taken place in both glacial and interglacial periods. Historically eruptions have been recorded in 1362 (rhyolite) and 1727 (basalt). The geology of the volcano records the differing response of magmas of contrasting composition to interaction with ice of variable thickness and gives insight into the development of a long-lived ice-covered stratovolcano. The southeast flank of Öræfajökull, is the first area of the volcano to have been mapped in detail. The oldest units comprise pillow lavas, hyaloclastite and jointed lava flows that were formed during subglacial basaltic eruptions involving abundant meltwater. Later a subglacial rhyolite eruption took place that was confined by ice to form a tephra pile over 200 m thick and that was subsequently intruded by dense rhyolite magma towards the end of the eruption.
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Among the most dramatic and visible creations of plate tectonic forces are the lofty Himalayas which stretch over 2900 km along the border between India and Tibet. This immense mountain range was formed by huge tectonic forces and sculpted by powerful denudation processes. Topographically, the belt has many superlatives: the highest rate of uplift (nearly 1 cm/year at Nanga Parbat), the highest relief (8848 m at Mt. Everest), and the highest concentration of glaciers outside of the polar regions. This last feature earned the Himalaya its name meaning in Sanskrit: «the abode of the snow».
Tethyan limestones in Tibet with Mt. Everest to the south
© Yani Najman Ph.D.
The Himalayas span six nations: Bhutan, China, India, Nepal, Pakistan and Afghanistan. It is the source of three of the world’s major river systems, the Indus Basin, the Ganga-Brahmaputra Basin and the Yangtze Basin supplying freshwater for more than one-fifth of the world population, and it also accounts for a quarter of the global sedimentation budget. An estimated 750 million people live in the watershed area of the Himalayan rivers which also includes Bangladesh.
The Himalayas formed when the Tethys Ocean closed and India and Eurasia collided. This continent collision is continuing today and many aspects of the formation and evolution of the Tibetan Plateau remain unresolved. The mountain belt consists of six lithotectonic zones, separated by thrust and faults. From north to south, these belts and their representative rock types are as follows:
1. The Trans-Himalayan Zone batholiths which are interpreted as the Andean-type northern margin of the Tethys.
2. The Indus Suture Zone (the northern limit of the Himalaya) represents the line of collision between India and Eurasia. Here the rocks are predominantly marine sediments, ophiolites (pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates), arc volcanics and mélange (a jumble of large blocks of varied lithologies of altered oceanic crustal material and blocks of continental slope sediments).
3. The Tibetan or Tethys Himalayan Zone is formed by strongly folded and weakly metamorphosed sedimentary rocks. In terms of age, they are Cambrian to Palaeocene sediments (the Tibetan Sedimentary Series) deposited on the Indian continental terrace.
4. The greater Himalaya (the backbone of the himalayan orogen which encompasses the areas with the highest topographical relief) is regionally metamorphosed Indian continental crust of mainly Proterozoic age.
5. The Lesser Himalaya is represented by non or weakly metamorphosed Indian continental crust ranging in age from Proterozoic to Palaeozoic and Palaeogene foreland basin sediments.
6. The Sub-Himalaya, which consists of foreland basin (foothills of the Himalayan Range), an area where the sedimentary record of material eroded from the mountain belt is preserved. The Foreland Basin provides a history of erosion, tectonism, and palaeodrainage in the orogen. A further sub-division of the early foreland basin into foredeep, forebulge and back-bulge depozones has been proposed.
Himalayan orogenic belt showing potential Himalayan source rocks for the sediments of the foredeep. MCT: Main Central Thrust; STDZ: South Tibetan Detachment Zone; MBT. Main Boundary Thrust; HFT = MFT: Main Frontal Thrust.
The collision process between India and Eurasia began at approximately 55 million years ago. The line of collision is the Indus-Tsangpo suture zone, which contains molasses, mélange, and ophiolitic material. The Indus River flows west along the line of suture zone and then cuts south over the Himalayas.
Timing of the surface uplift of the Tibetan Plateau has received much attention because of proposed links between high topography and change in regional and global climate such as monsoon intensification. A suggested late Miocene age of uplift assumes that east-west extensional faults and basins of late Miocene age in the southern part of the Tibetan Plateau developed in response to gravitational collapse of a thickened, high plateau.
The link between deep-seated tectonic processes and regional, if not global, environmental change is too tantalizing to ignore. In conclusion a variety of studies to test the interrelationship between Tibet’s growth and regional climate change and its impact on Tibet is the way forward.
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