Geol 40310 Lecture A2: Earth Structure (2023)

Geol 40310 Fossil Fuels and Carbon Capture & Storage (CCS) Lecture A2: Structure of the earth and plate tectonics Autumn 2023-24 T Manzocchi, School of Earth Sciences, UCD 1 1 Lecture 2: Structure of the earth and plate tectonics Seismic waves and the earth’s structure: P-waves, S-waves Crust, Ma...

Geol 40310 Fossil Fuels and Carbon Capture & Storage (CCS) Lecture A2: Structure of the earth and plate tectonics Autumn 2023-24 T Manzocchi, School of Earth Sciences, UCD 1 1 Lecture 2: Structure of the earth and plate tectonics Seismic waves and the earth’s structure: P-waves, S-waves Crust, Mantle Lithosphere, Asthenosphere The formulation of Plate Tectonics. Divergent Plate Boundaries - Oceanic / continental - Failed rifts. - Passive Margins. Convergent Plate Boundaries - Subduction zones - Mountain chains Transform Plate boundaries The three basic types of rock: igneous, sedimentary, metamorphic 2 2 Geol 40310 Lecture A2 1 Seismic waves Body waves P waves S waves Surface waves Love waves Rayleigh waves Particle motion Travel direction 3 3 The Earth’s Structure • P-waves travel through solid and liquid layers. Their velocity is higher in solid media and increases with depth. • S-waves are not transmitted through liquid media. • There is no S-wave propagation in the outer core and therefore it must be a liquid. • These data come from travel times following an earthquake recorded at seismometers located around the world. 4 4 Geol 40310 Lecture A2 2 The Earth’s Structure Crust Lithosphere Asthenosphere 1330° C Mantle 5 5 Crust and Mantle or lithosphere and asthenosphere? Moho (Mohorovičić discontinuity): Chemical Boundary between the Crust and the Mantle. Mineralogical phase change from basalt (plagioclase feldspar and pyroxene) to peridotite (olivine + pyroxene). Associated with an increase in density and seismic velocity. LAB (Lithosphere Asthenosphere Boundary): Mechanical boundary between the Lithosphere and Asthenosphere. Lithosphere comprises the crust and portion of the uppermost mantle that deforms by brittle deformation between rigid plates. The asthenosphere is the weaker region of the upper mantle that deforms by viscous flow. Low seismic velocity zone at top of asthenosphere. Lithosphere Asthenosphere 6 6 Geol 40310 Lecture A2 3 Continental and Oceanic crust Oceanic crust is young (generally < ca. 100 myr), thin (< ca. 10km), and consists of basaltic rocks (2.9 g/cm3). Continental crust is old (to ca. 3.5 billion yr), thick (ca 30-50km) and consists of granitic rocks (about 2.7 g/cm3) The mantle has a density of 3.3 g/cm3 and is dominated by the mineral olivene Thickness of the crust (km) 7 7 Plate tectonics: Brittle faulting in the Lithosphere Ductile flow in the Asthenosphere 8 8 Geol 40310 Lecture A2 4 Continental Drift • • • Alfred Wegener, a German meteorologist, observed similarity in shapes of Africa and South America and noted similarities in rock types, ages, fossils, structures, palaoeclimates. In 1912 he published his book ‘The Origin of Continents and Oceans’ proposing that continents formed and drifted apart. This contentious theory suggested that Earth not only moved vertically but also continents moved laterally. Crucially Wegener didn’t have a mechanism for how plates move, and his theory was widely ridiculed. Alfred Wegener (1880-1930) 9 9 Continental Drift Alignment of continental shapes and sedimentary deposits formed 255 million years ago. 10 10 Geol 40310 Lecture A2 5 Continental Drift Geographical distribution of land-based fossils 11 11 Reversals of the Earth’s magnetic field • In 1929 Motonori Matuyama realised that the magnetic polarity of basalts changed systematically with age. • The implied reversals of the earth magnetic field were to provide key evidence for plate tectonics. Modelled Magnetic field lines: Motonori Matuyama (1884 – 1958) 12 12 Geol 40310 Lecture A2 6 Seafloor Spreading - Evidence • In 1963 Vine and Matthews found evidence for seafloor spreading in the ages of magnetic striping in the Pacific (youngest at the mid-ocean ridges). • This provided the conceptual base for development of the theory of plate tectonics. Magnetic striping mapped by oceanographic surveys offshore of the Pacific Northwest 13 13 Plate tectonics • In 1962 Harry Hess proposed a mechanism whereby continents move – a series of plates, some carrying continents, forming and subducting. • This theory proposed that convection of the mantle was driving force. Harry Hess (1906-1969) Lithosphere Asthenosphere 14 14 Geol 40310 Lecture A2 7 Age of Ocean-floor basalts 15 15 Plate tectonics driven by Mantle Convection • Plate movement is driven by mantle convection. Hot material rises towards the surface at midocean ridges forming new oceanic crust. Cold material sinks carrying subducting places at collision zones. • The details of the convection cells is still poorly constrained. Convection could involve small cells convection within parts of the mantle, or large cells involving the entire mantle. 16 16 Geol 40310 Lecture A2 8 Tectonic Plates The surface of the Earth is subdivided into a number of plates, some large and others small. New material is created at the spreading ridges (mid-oceans) and old oceanic crust is destroyed at subduction zones (e.g. Aleutian Islands in Alaska, New Zealand). Where continents collide mountains are produced (e.g. Alps, Himalayas). 17 17 Active volcanos in the past 10,000 years 18 18 Geol 40310 Lecture A2 9 Earthquake locations by hypocentre depth M > 5.5, 1975-1999 • Shallow earthquakes in narrow belts at mid-ocean spreading centres. • Narrow belts of deep earthquakes at continental margins (e.g. South America, Western Pacific). • Diffuse zones of deep and shallow earthquakes in mountain chains (Alps, Himalaya). • Narrow belts of shallow earthquakes at continental margins (e.g. West coast of North America). NB: Not all continental margins are plate margins (e.g. Africa, NW Europe) 19 19 Earthquake locations by hypocentre depth M > 5.5, 1975-1999 • Shallow earthquakes in narrow belts at mid-ocean spreading centres. Divergent plate boundary • Narrow belts of deep earthquakes at continental margins (e.g. South America, Western Pacific). Convergent plate boundaries (subduction zones) • Diffuse zones of deep and shallow earthquakes in mountain chains (Alps, Himalaya). Convergent continental plate boundaries • Narrow belts of shallow earthquakes at continental margins (e.g. West coast of North America). Transform Plate Boundaries NB: Not all continental margins are plate margins (e.g. Africa, NW Europe) 20 Passive Margins. 20 Geol 40310 Lecture A2 10 Three basic types of plate boundary Divergent Convergent Transform 21 21 Divergent oceanic plate boundary • • • Divergent (spreading) boundaries occur where plates pull apart, faults and earthquakes occur and the crust thins, allowing the production of volcanic (igneous) rocks. These are typically basalts (fine-grained) low in silica, and normally containing feldspar and ferromagnesian minerals (e.g. olivine, hornblende, augite). Such boundaries are typically located at mid-ocean ridges. North American plate Eurasian plate Bridge across the Álfagjá rift valley in southwest Iceland 22 22 Geol 40310 Lecture A2 11 Mid Ocean Ridge Grain size in igneous rocks reflects rate of cooling Rapid cooling, fine grain size - Basalt Slow cooling, coarse grain size - Gabbro 23 Imperial college Rock library 23 Classification of Igneous Rocks: Grain size and composition Marshak 2015 24 24 Geol 40310 Lecture A2 12 Basalt Pahoehoe lava flow, Hawaii Giant’s Causeway, Northern Ireland Fine grained, mafic rock Rapid cooling. Low silica, high iron content 25 25 Divergent continental plate boundaries An artificial rendering of the Albertine Rift, which forms the western branch of the East African Rift. 26 26 Geol 40310 Lecture A2 13 Cross-section through a rift basin km e.g. Lake Tanganyka rift, GeoExPro 2014. 27 27 2km Clastic sedimentary rocks – deposition of eroded grains 20km Alluvial fan: grains are poorly sorted and angular – they have not travelled far. Submarine fans – intebedded sandstones and shales 28 reflecting the energy of the individual turbidite flows 28 Geol 40310 Lecture A2 14 The formation of new oceans and passive margins e.g. East African Rift. e.g. North sea e.g. Atlantic Ocean Allen and Allen (2013) 29 29 Passive margins Allen and Allen30(2013) 30 Geol 40310 Lecture A2 15 Global Sediment Thickness Allen and Allen31(2013) 31 Three basic types of plate boundary Convergent Divergent Transform 32 32 Geol 40310 Lecture A2 16 Convergent plate boundaries (subduction zones) Granite (coarse grained) • • • Rhyolite (fine grained) Convergent (subducting) boundaries occur where plates collide, faults and earthquakes occur, the downgoing oceanic plate is subducted into the mantle and melts. Some of the melt material rise through the crust and erupts as a volcanic arc adjacent to the trench. The volcanic material is different to that of divergent boundaries and tends to be silicarich andesites or rhyolites. They are typically gaseous and form violent eruptions. The subducting plate steepens with depth to and angle of about 45, and earthquakes occur on it to a depth of 660km (base of Upper mantle). 33 33 The great East Japan (Tohoku) 2011 earthquake Earthquake magnitude is proportional to the amount of rock moved. Pacific Plate Magnitude 9 earthquake: 650 km*300 km rupture, maximum slip ca. 10 m 34 34 Geol 40310 Lecture A2 17 The great East Japan (Tohoku) 2011 earthquake • • • Earthquakes defining the subducting plate can be deep and are associated with tsunami. The leading edge of the plate sticks and the continental plate bends. When it eventually ruptures, the plate ‘springs up’, displacing the water. The Tusnami wave generated by this earthquake was up to 20m high. Rizuzen-Takata City 35 35 Convergent continental plate boundaries Time 1: Before • Time 2: After Where subduction results in the collision of two continents, the density of the continental material is such that it cannot subduct. Such continental collision results in the formation of a mountain belt (e.g. the Alps, Himalayas, Zagros), with occasional slivers of oceanic material preserved as ophiolites along the continent-continent suture. A large crustal root is preserved beneath the fold belt and the subducting oceanic slab drops off into the asthenosphere. 36 36 Geol 40310 Lecture A2 18 Convergent continental plate boundaries The Alps 37 37 Regional Metamorphism Regional metamorphism occurs during continental collision. With every 1 km burial the temperature increases by 30-40º C and the pressure by 300-350 bars. Rock A is buried and at depths of several km to 10’s km and is metamorphosed (e.g. recrystallised). New minerals grow at the expense of minerals that are unstable at the increased confining pressures and temperatures. Rocks change (e.g. mudstones become slates and eventually schists) and their minerals and porosity/permeability characteristics also change. 38 38 Geol 40310 Lecture A2 19 Metamorphic rocks Textural and mineralogical changes as rock is deformed, pressurised and heated. Sedimentary rock Metasedimentary rock 39 39 Three basic types of plate boundary Divergent Convergent Transform 40 40 Geol 40310 Lecture A2 20 Transform Plate Boundaries • • • • Transform plate boundaries occur where one plate slides past another with a strike-slip motion. No new material is created, nor is material destroyed through subduction. Where the boundary is curved there is the potential for sticking to occur, followed by a buildup of stress and a sudden release resulting in an earthquake. The San Andreas Fault lies along a transform plate boundary. 41 41 Plate tectonics – Main components USGS • • • • • New lithosphere is created at mid-ocean ridges where seafloor spreading occurs. Old lithosphere is returned to the asthenosphere at subduction zones. Earthquakes occur both in regions of sea-floor spreading (shallow earthquakes) and at subduction zones (shallow, intermediate and deep earthquakes). New igneous material (basalt) is produced at seafloor spreading centres and also (andesites and rhyolites) at arc systems above regions of collision. 42 Metamorphic rocks are produced in subduction regions and in areas of continent-continent collision. 42 Geol 40310 Lecture A2 21