University of Iceland, Department of Geology and Geography Introduction to the Geology and Geography of Iceland
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| University of Iceland, Department of Geology and Geography
Introduction to the Geology and Geography of Iceland Autumn semester 2004 Lectures of Páll Einarsson Office: Askja, room 364 Tel. 525 4816, email palli@hi.is
Reading material Björnsson, Helgi, and Páll Einarsson. Volcanoes beneath Vatnajökull, Iceland: Evidence from radio-echo sounding, earthquakes and jökulhlaups, Jökull 40, 147-168, 1990. Einarsson, P., 1986. Seismicity along the eastern margin of the North American Plate. In: Vogt, P. R., and Tucholke, B. E., eds., The Geology of North America, Volume M, The Western North Atlantic Region: Geological Society of America: pp.99-116. Einarsson, P. Earthquakes and present-day tectonism in Iceland. Tectonophysics, 189, 261-279, 1991 Einarsson, P. Crustal deformation at mid-oceanic plate boundaries: Constraints from seismicity. Faulting and Magmatism at Mid-Ocean Ridges. 1995 RIDGE Theoretical Institute. Lecture Notes, 16 pp. Einarsson, P., B. Brandsdóttir, M. T. Guðmundsson, H. Björnsson, K. Grönvold, F. Sigmundsson. Center of the Iceland Hotspot experiences volcanic unrest. Eos, 78, Sept. 2, 369-375, 1997. Pálmason, G., Model of crustal formation in Iceland, and application to submarine mid-ocean ridges. In Vogt, P. R., and B. E. Tucholke, eds., The Geology of North America, Vol. M, The Western North Atlantic Region: Geol. Soc. America, 87-97, 1986. Saemundsson, K., 1986. Subaerial volcanism in the western North Atlantic. In Vogt, P. R., and B. E. Tucholke, eds., The Geology of North America, Vol. M, The Western North Atlantic Region: Geol. Soc. America, 69-86. Sigmundsson, F., 1991. Post-glacial rebound and asthenosphere viscosity in Iceland. Geophys. Res. Lett., 18, 1131-1134. Sigmundsson, F., P. Einarsson, S. Rögnvaldsson, G. R. Foulger, K. M. Hodgkinson, G. Thorbergsson. The 1994-1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress. J. Geophys. Res., 102, 15,151-15,161, 1997. Staples, R., R. S. White, B. Brandsdóttir, W. Menke, P. K. H. Maguire, J. H. McBride. Faeroe-Iceland Ridge Experiment. 1. Crustal structure of northeastern Iceland. J. Geophys. Res., 102, 7849-7866, 1997. Tucholke, B., F. McCoy. Paleogeopraphic and paleobathymetric evolution of the North Atlantic Ocean. In: Vogt, P. R., and B. E. Tucholke, eds., The Geology of North America, Vol. M, The Western North Atlantic Region: Geol. Soc. America, 589-602, Plates 9 and 10. 1. Hot spot, plate tectonics. Key words: Hot spot, mantle plume, lithosphere plate, plate boundary, North-America plate, Eurasia plate, Mid-Atlantic Ridge, transform fault, fracture zone, spreading center, spreading segment, sea floor spreading, magnetic anomaly, magnetic time scale, oceanic crust, plate movements, ridge jump, Reykjanes Ridge, Kolbeinsey Ridge, Mohns Ridge, Charlie-Gibbs Fracture Zone, Jan Mayen Fracture Zone, Greenland-Faroes Ridge, Aegir Ridge, Jan Mayen Ridge. Some definitions: Crust Jarðskorpa The outermost layer of the solid Earth, with P-wave velocities generally below 8 km/s. It varies in thickness from 5-10 km beneath the oceans to 30-60 km beneath continents. At the bottom the crust is limited by the Moho discontinuity. Mantle Möttull The approximately 2300 km thick layer of the Earth below the Moho discontinuity where the velocity of P-waves jumps to 8 km/s or more from lower values. Lithosphere Stinnhvolf The uppermost layer of the solid Earth comprising the crust and the rigid part of the upper mantle, the part that overlies the asthenosphere. The thickness of the lithosphere is 60-140 km beneath the oceans but may be as much as 300 km beneath continents. Asthenosphere Linhvolf A layer in the upper mantle where the seismic velocities get lower with increasing depth. The material of the asthenosphere is considered to be softer and closer to the melting point than the layers above and below. Plate boundary Flekaskil The boundary separating two lithospheric plates. The movement across the boundary may be convergent, divergent or conservative (transcurrent motion). Rifting Gliðnun The process of separation between two crustal blocks that are moving apart. Rifting is often intimately linked with magmatism. Mid-oceanic ridge Úthafshryggur Ridge formed at divergent plate boundaries in the ocean. The oceanic crust is formed by magmatism and spreading along mid-ocean ridges. Hotspot Heitur reitur A descriptive term referring to an area on the surface of the Earth with high volcanic activity, anomalously high elevation, and high heat flow. Some hotspots are close to plate boundaries, s.a. Iceland, Galapagos and the Azores. Others, s.a. Hawaii, are far from plate boundaries. Mantle plume Möttulstrókur An interpretative term referring to a plume of upwelling mantle material, transporting anomalously hot and light material from the lower mantle towards the surface. Melting in the upper part of the plume produces magma that feeds the excess volcanism of an overlying hotspot. Assignment: Define lithosphere plate, transform fault, fracture zone. Events in the history of the North Atlantic: 170 Ma Beginning of spreading between North America and Africa 118 Ma Spreading in the Bay of Biscay and Rockall Trough 67-50 Ma Spreading west of Greenland, first signs of the Iceland hotspot 50 Ma Spreading along the Reykjanes Ridge - Ægir Ridge 36-26 Ma Ridge jump to Kolbeinsey Ridge, formation of Jan Mayen Ridge 15 Ma Ridge jump, Vestfirðir rift to Snæfellsnes rift 6-7 Ma Ridge jump, Snæfellsnes rift - West rift 0-3Ma Ridge jump, West rift to East rift 2. Plate boundaries and earthquakes. Key words: Seismicity of ridges and fracture zones, earthquake source, fault plane solutions, magnitude, b-value, earthquake sequences, swarms, mainshock sequences, aftershock, foreshock, strike-slip faulting, normal faulting, bookshelf-faulting, Tjörnes Fracture Zone, Reykjanes Peninsula, South Iceland Seismic Zone, earthquakes and volcanoes, intraplate earthquakes, earthquake history, earthquake prediction. Some definitions Earthquake Jarðskjálfti Sudden ground motion caused by the passage of seismic waves. Most earthquakes are caused by the failure of crustal rock under stress. Magnitude Stærð A number that is a measure of the size of an earthquake. The magnitude is frequently based on the logarithm of the maximum amplitude of the earthquake waves as they are recorded at a specific seismograph station. An increase of one unit in magnitude thus means a tenfold increase in the maximum amplitude. The corresponding increase in the energy released in the earthquake is about 30-fold. b-value b-gildi The slope of the distribution curve for the logarithm of the number of earthquakes, N, as a function of magnitude M: log N = a - bM The b-value is a measure of the ratio of small to large events in a set of earthquakes. A high b-value indicates there are many small events for each large event.
An earthquake sequence where no one earthquake is significantly larger than the others. Mainshock Meginskjálfti An earthquake in a sequence that is distinctly larger than any of the others, often more than one magnitude unit. Aftershock Eftirskjálfti An earthquake following a mainshock in an earthquake sequence. Foreshock Forskjálfti An earthquake preceding a mainshock in an earthquake sequence. Strike-slip fault Sniðgengi Normal fault Siggengi Bookshelf faulting Bókahilluhöggun Earthquake prediction Jarðskjálftaspá A statement on a future earthquake containing an estimate of its location, magnitude, and time of occurrence. Seismicity of Iceland Most of the seismicity of Iceland is due to processes related to the mid-Atlantic plate boundary, transform faulting, rifting and magmatism. THE SOUTH ICELAND SEISMIC ZONE 1. Zone of high seismic activity, takes up transform motion. 2. The partition of rifting between the rift zones may be uneven and change with time. 3. Defined by historic destruction areas, surface ruptures and epicenters. 4. Oriented E-W and is 10-15 km wide. 5. Faulting on N-S striking planes, perpendicular to the zone, "bookshelf faulting". 6. Earthquakes occur in major sequences, 45-112 years apart. 7. Long-term forecast of a major earthquake sequence within the next decades, fulfilled in June 2000. THE REYKJANES PENINSULA OBLIQUE RIFT 1. Structural continuation of the Reykjanes Ridge 2. Tectonic structure characterized by en echelon volcanic systems. 3. Strike-slip faults cut across the plate boundary at a high angle. 4. Fault plane solutions show least compressive stress horizontal and NW-SE. 5. The style of seismic strain release changes systematically along the peninsula. 6. Seismic activity on the peninsula is episodic. 7. Magmatic activity is also episodic, but on a longer time scale. 8. No seismic evidence for intrusive magmatic activity in recent decades. 9. Crustal deformation occurs in two different modes, wet and dry. THE TJÖRNES FRACTURE ZONE 1. Broad zone of seismicity, transform faulting and crustal dilation. 2. The transform motion is taken up by several parallel seismic zones. 3. Intimate relationship between rifting activity in the rift zones and transform faulting. 4. Earthquakes often occur on transverse structures, see the Reykjanes Peninsula.
1. Inflation of a magma chamber. 2. Deflation of a magma chamber. 3. Intrusion of a dyke. 4. Solidification and cooling of a magma chamber.
1. W-Iceland lithosphere tongue 2. Insular margin
Liquid formed by partial melting of crustal or mantle material. After erupting to the surface and degassing the magma forms lava. Magma chamber Kvikuhólf A region in the crust or mantle containing appreciable quantity of magma. Magma chambers receive magma from its source region and feed volcanoes at the surface. Magma chamber inflation Kvikuhólfsþensla A magma chamber inflates in response to increasing pressure. The inflation may be detected on the surface above by geodetic methods and increased seismicity. Magma chamber deflation Kvikuhólfshjöðnun A magma chamber deflates in response to decreasing pressure. The deflation may be detected on the surface above by geodetic methods and changes in seismicity. Dyke Gangur, berggangur Discordant or cross-cutting, tabular intrusion. Dykes are commonly formed in a vertical or near vertical position. See cone sheets, however. Volcanic tremor Eldvirkniórói Continuous vibrations of the ground associated with magmatic activity. Fissure swarm Sprungusveimur A collection of many similar, parallel or subparallel fissures occurring in a limited area. Caldera Askja A nearly circular depression in the central part of a volcano, often 2 - 10 km in diameter, formed by the removal of material. Central volcano Megineldstöð An area of high volcanic productivity. Central volcanoes are often associated with the production of rocks with high silca content such as rhyolite and dacite. A central volcano may also have one or more calderas and geothermal systems. Volcanic system Eldstöðvakerfi A structural and petrological unit consisting of a central volcano and associated fissure swarms. Important recent events of Icelandic volcanoes: Hekla. A large eruption. 1 km3 of lava and tephra, ranging from dacite to andesite in composition, erupted from the main Hekla fissure in 1947. Smaller eruptions, both along the main fissure and on the flanks in 1970, 1980-81, 1991 and 2000. Askja. A large eruption with a caldera formation in 1875. Small eruptions in the 1920ies. A basaltic lava eruption, 0.15 km3, from a short fissure near the E caldera wall in 1961.
December 1975 at Leirhnjúkur in the Krafla caldera, accompanied by large deflation, rifting and lateral magma injection into the adjacent fissure swarm marked the beginning of a major rifting episode at the plate boundary in North Iceland. During the following 9 years at least 20 magmatic injection events were recorded, 8 of which resulted in eruptions, April 1977, September 1977, March 1980, July 1980, October 1980, January 1981, November 1981, and September 1984. A total of 0.35 km3 of basaltic lava were erupted but a substantially larger volume was emplaced in the crust.
eruption occurred in 1983 from a short fissure near the S caldera wall. Another similar eruption occurred in 1998. Grímsvötn is inflating at the present time.
in 1996. 0.7 km3 of evolved basaltic material were erupted and emplaced mostly subglacially. The meltwater accumulated in the Grímsvötn caldera and was released in a large jökulhlaup a month later. Katla. Large subglacial eruptions have occurred about twice a century, the last one in 1918. Probable small, subglacial eruptions occurred in 1955 and 1999. No tephra erupted through the glacier, but collapse cauldrons formed in the glacier surface and relatively small jökulhlaups occurred. Katla is inflating at the present time. Eyjafjallajökull. Earthquake swarms beneath the NE and S flanks in 1994-99 associated with intrusive activity. 4. The Earth´s crust in Iceland. Key words: Crust, moho, seismic refraction measurements, seismic reflection measurements, seismic layers, upper crust, layer 3, the thin-crust model, the thick-crust model, the Pálmason model of crustal accretion, seaward dipping reflectors, mantle viscosity, isostatic adjustment, glacial rebound. Definition of oceanic crust: Seismological: The crust is separated from the mantle by the moho-discontinuity where the velocity of P-waves jumps from about 6.7 km/s to about 8 km/s Petrological: The crust is made of igneous material made by partial melting of the mantle. Thin, hot crust model of Iceland 8-16 km thick crust on top of "anomalous" mantle. Molten layer at the bottom of the crust, 10-15% melting in mantle. Arguments:
Thick, cool crust model 24-35 km thick crust on top of solid mantle. No molten layer. Melt in isolated magma chambers. Arguments:
Implications:
Principle of isostasy: A vertical column through the crust and upper mantle must be of equal weight to surrounding columns. If it isn´t, the asthenosphere flows until equilibrium is established. Isostatic adjustment.
Example of examination questions: Exam of 1998: 1. The lavas of the Tertiary in eastern and western Iceland generally dip towards the interior of Iceland. How is this dip produced? Describe the processes that contribute to this structural relationship. 2. The largest earthquakes in Iceland occur in two transform zones, one in South Iceland and the other off the north coast. Describe the configuration of these seismic zones within the plate tectonic framework. Why is the word "transform" used, and why not "transform fault"? 3. What is a "volcanic system". Name three volcanic systems in Iceland and describe features that classify them as such. Exam of 1999 Review the evidence for a hotspot and an underlying mantle plume in the Iceland area. Be sure to mention the definition of a hotspot, also how crustal thickness and seismicity are connected to the presence of the hotspot. A large earthquake is expected to occur in South Iceland within the next decades. What is the evidence for this “prediction” and what are the characteristics of this event? Exam of 2000 The last two years have been rather eventful in Icelandic geology with two large earthquakes in South Iceland in June 2000, an eruption of the Hekla volcano in February 2000, a possible subglacial eruption of Katla in July 1999, an intrusion of magma beneath the Eyjafjallajökull volcano in 1999, and an eruption of Grímsvötn in December 1998. Put these events into a plate-tectonic context, i.e. where are they with respect to the plate boundaries, hotspot center etc? Draw a sketch map. Define the following terms: a. Hotspot b. Earthquake swarm c. Magma chamber inflation Exam 2001 The terms "hotspot" and "mantle plume" play a central role in Icelandic geology. Explain their difference and how they are connected. Also explain their relationship with: a. The existence of Iceland above sea level b. Ridge jumps c. The existence of two zones of large earthquakes, in North and South Iceland. Exam 2002The Plate Tectonics hypothesis grew out of research stimulated by the cold war, especially the effort to monitor nuclear bomb tests and to find nuclear submarines in the World's oceans. Explain the type of data used and the results that demanded the new hypothesis. A large (magnitude 5.8) earthquake occurred off the North coast of Iceland on September 16, last week. The focal mechanism shows strike-slip faulting was responsible for the event. Explain the tectonic environment of this earthquake. Exam 2003At least two volcanoes in Iceland are inflating at the moment, Grímsvötn and Katla. Discuss their similarities and differences, and ralation to the general geological framework (hotspot, transform zones, rift zones etc.). Download 50.73 Kb. Share with your friends:
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