Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Natural Hazards and Disaster

Natural Hazards and Disaster

Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Types of Geohazards Types of Geohazards

Geohazard denotes geological hazard. Types of Geohazards

Geohazard denotes geological hazard. Relevant sciences: • Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. • Geophysics: Applies physics to studying the planet - originally included meteorology, physical oceanography • Geochemistry: studies the chemistry of the planet Geohazards include: Spatial scales can range from: • earthquakes, • local events such as a rock slide or coastal erosion • volcanic activity, • events that threaten humankind such as a supervolcano or • landslides, meteorite impact. • ground motion, Temporal scales: • tsunamis, rapid rock falls and short earthquakes: seconds to a few • ﬂoods, droughts, minutes • meteorite impacts and prolonged volcanic eruptions: days to years; • health hazards of geologic materials. slow slope motion and subsidence: years and more. Types of Geohazards

Tectonics: • Stress and strain (tectonic movements) • Earthquakes • Tsunamis • Volcanic activity • Salt Tectonics Ground instabilities and movements: • Landslide • Soil Creep • Ground Dissolution • Collapsible Ground • Running Sand/ Liquefaction • Shrink-swell clays • Compressible Ground Anthropogenic ground instabilities: • Induced seismicity (reservoirs) • Ground water management • Oil and gas extraction • Mining • Underground construction • Engineered ground • Fracking Types of Geohazards

Tectonics: Other geohazards: • Stress and strain (tectonic movements) • • Earthquakes health hazards of geologic materials: • Tsunamis - radioactivity (non-human and human caused) • Volcanic activity - atmospheric aerosols • Salt Tectonics - chemical elements (e.g. mercury, heavy metals) Ground instabilities and movements: - water quality • Landslide - anthropogenic pollution • Soil Creep • ﬂoods, droughts • Ground Dissolution • • Collapsible Ground sediments • Running Sand/ Liquefaction • meteorite impacts • Shrink-swell clays • Compressible Ground Anthropogenic ground instabilities: • Induced seismicity (reservoirs) • Ground water management • Oil and gas extraction • Mining • Underground construction • Engineered ground • Fracking Natural Hazards and Disaster

Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Natural Hazards and Disaster

Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Plate tectonics Plate tectonics Plate tectonics Plate tectonics

Similarity of coast lines noted already in 1598 Plate tectonics

Alfred Wegener, 1 Nov. 1880 - Nov. 1930 Plate tectonics

Alfred Wegener, 1 Nov. 1880 - Nov. 1930 Published the idea of “continental drift” in 1912 Plate tectonics

Alfred Wegener, 1 Nov. 1880 - Nov. 1930 Published the idea of “continental drift” in 1912 Supportive evidence: • Similarity of coast lines; • Similarity of rocks on both sides of Atlantic Plate tectonics

Alfred Wegener, 1 Nov. 1880 - Nov. 1930 Published the idea of “continental drift” in 1912 Supportive evidence: • Similarity of coast lines; • Similarity of rocks on both sides of Atlantic

Theory not accepted because no explanation of the forcing processes Plate tectonics

Magnetic patterns on ocean ﬂoor Plate tectonics

gtalumni.org/Publications/magazine/win98/images/rev2.gif Plate tectonics

gtalumni.org/Publications/magazine/win98/images/rev2.gif Plate tectonics Earth’s Internal Structure Earth’s continental and ocean crust are the thinnest, outermost layers of the planet. Earth’s outermost crust, on which we live, is often described by analogy with an egg’s shell. Although high mountains and deep ocean trenches may seem enormous, on the scale of the planet as a whole they are The internal structure almost unnoticeable wrinkles. The continental and of Earth. Layer d’’ is the transition zone oceanic crustal rocks and their underlying lithosphere, between the liquid which together comprise the tectonic plates, are on outer core and the average 100 km thick and for the most part they are base of the lower rigid and brittle. mantle.

Earth’s lithosphere is chemically and mineralogically part of the upper mantle. Its rock is predominantly peridotite, which is a coarse-grained, dense, igneous rock that is high in iron and magnesium. Beneath the lithosphere is the asthenosphere, which although solid, is capable of Earth’s tectonic plates, sometimes referred to as ﬂowing slowly due to its high temperature (1300°C). On lithospheric plates, are the surface, the rock would melt at such high made of the lithosphere temperatures, but the high pressures at depth keep it in and the overlying a solid state. oceanic and/or continental crust. Plate tectonics

Using Earthquakes To Map Earth’s Internal Structure Seismic waves generated by earthquakes are useful for mapping Earth’s internal structure.

Earthquakes and very large explosions release energy in Compressional (P) and shear (S) the form of seismic waves that travel in all directions seismic body waves. S-waves cannot be transmitted through a through the Earth, as well as along the crust’s surface. liquid. Seismic surface waves cause ground shaking and are responsible for most of the damage caused by earthquakes, but the different physical properties of compressional (P) and shear (S) seismic body waves are useful for mapping Earth’s internal structure. P-waves have a smaller amplitude, shorter wavelength,

and travel faster than S-waves through Earth’s crust, which An S-wave ‘shadow zone,’ is why they are the first to arrive at seismograph stations shown in pink, occurs on the opposite side of Earth to an after an earthquake. However, unlike P-waves, the S-waves earthquake at position ‘0’. No cannot travel through fluids and on the opposite side of S-waves are received by seismograph stations in the Earth to an earthquake there is always a ‘shadow zone,’ shadow zone; only P-waves within which no S-waves are received by seismographs. can travel through Earth’s fluid outer core, although they are The recognition of this shadow zone allowed scientists to refracted at the core-mantle infer the existence of Earth’s fluid outer core. boundary. Plate tectonics Tectonic Plates Lithospheric plate boundaries are divergent, convergent, or transform, or a combination. Earthquakes and volcanic eruptions occur mainly because Earth’s lithospheric plates are constantly in motion. Relative to an Earth-fixed reference frame, they move with few exceptions with velocities of less than 10 cm/year and relative velocities between neighboring plates can reach up Earth’s major tectonic plates and some of the smaller plates (boundaries to 25 cm/year. When summed over thousands and millions outlined in red). of years, the plates can accomplish a great deal of motion. The three main types of plate boundary are: divergent, where plates move apart along mid-ocean spreading ridges; convergent, where one plate moves over the top of another; and, transform, where plates slide past one another. There are also more complex boundaries with a combination of different motion types. The boundaries and relative movement vectors of the larger plates are well-defined, but Earth’s divergent (D), convergent (C), and transform (T) plate boundaries there are also dozens of micro-plates whose boundaries and have a distinct appearance on an ocean floor elevation map. Not all plate boundaries are simple; a few have more complex D+T or C+T movement relative motions are still the subject of research activity. and some plate boundary regions, such as in the western Pacific, contain numerous micro-plates. Plate tectonics Plate tectonics Plate tectonics Plate tectonics Plate tectonics

Divergent boundary Transform boundary

Convergent boundary Plate tectonics Earth’s Magnetic Field Note: rotation axis and magnetic ﬁeld axis are not parallel

http://www.scifun.ed.ac.uk/card/images/left/earth-magﬁeld.jpg Plate tectonics

3. Evidence from ‘magnetic stripes’ in oceanic crust Plate tectonics Divergent Plate Boundaries Divergent plate boundaries are where new lithosphere is created, along mid-ocean ridges. At mid-ocean ridges, hot, buoyant basaltic magma reaches the surface where it solidifies into basalt rock in undersea mountain ridges. As the magma cools, it reaches a temperature at which iron atoms in magnetic Earth’s magnetic field polarity in normal (left) and reverse (right) orientation. Earth’s field is more complex than is illustrated and has minerals such as magnetite lock their magnetic field into reversed polarity without any observable ill-effects in the fossil record. the same orientation as that of the Earth’s magnetic field. Magnetic ‘stripes’ of normal This temperature, called the Curie Point, is about 570°C (red) and reversed (black) for magnetite. polarity in ocean floor basalts have a symmetrical pattern Earth’s magnetic field reverses direction about every consistent with the rock ages 200,000 to 300,000 years. When a ship-towed on either side of the Mid Atlantic Ridge. Similar magnetometer crosses a mid-ocean ridge it receives a patterns are seen across most signal of alternating normal and reverse polarity in the mid ocean ridges. ocean floor rocks. These alternating patterns are consistent with a symmetrically increasing age of the rocks and their overlying sediments on either side of the midocean ridges, and point to ocean lithosphere Age distribution of Earth’s divergence along the ridges. Hence the term ‘sea floor oceanic lithosphere, created at divergent plate spreading’. boundaries. Plate tectonics

Convergent Plate Boundaries Cold, dense lithosphere is subducted into the mantle at convergent plate boundaries, while the mantle above partially melts to form volcanic arcs. Cold, old, oceanic lithosphere is much more dense than continental crust or than hot, young oceanic lithosphere. As Lithospheric slabs sink into the mantle along subduction zones, creating a counter-flow of asthenosphere (red arrows) above the down-going the cold lithosphere thickens with distance from the slab. Stars represent earthquakes where the slab bends and slides under the more buoyant lithospheric plate. spreading ridge, gravitational forces eventually cause it to Topography of the Puerto sink back into the mantle along subduction zones. Flexure Rico Trench (purple) and volcanic arc (brown) of the sinking slab creates deep ocean trenches at surface, above the subduction but neither the bending nor the descent of lithosphere into zone where cold lithosphere of the North the mantle is smooth. Some of the deepest and largest American Plate sinks earthquakes occur in, and just above, the down-going slab. beneath the Caribbean Plate. Ocean trenches Above the sinking slab, the lithosphere and asthenosphere often exceed depths of are heated and partially melted by an upward counter-flow 6,000 m. of mantle. The resulting magma rises, heating and melting part of the lithosphere and crust as it goes. These magmas incorporate more crustal rock as they ascend, and the resulting silica- Volcanic arcs develop above subduction enriched magma erupts in volcanoes to form a volcanic arc. zones, where lithosphere descends back into the mantle. Graphic not to scale. Plate tectonics Plate tectonics Plate tectonics Continent-Continent Convergence When all of the oceanic lithosphere has been consumed in a subduction zone, the result is a convergence of continental lithosphere. The region of Tibet, north of the Himalayan Mountains, has an average elevation of 4,500 m above sea level. One reason for such a high elevation is that the continental crust there is twice its normal 40 km thickness. At about 70 million years ago, oceanic lithosphere beneath a long-gone ocean called Tethys, on the The northward migration of the Indian plate began at about 70 northern margin of the Indian Plate, began to be subducted million years and ended in collision beneath the Eurasian Plate. India moved quite rapidly northward as and suture with Eurasia about 40 the Tethyian oceanic lithosphere was subducted. By 40 million million years ago. years ago, the convergence of the Indian and Eurasian plates had consumed all the Tethyian lithosphere that had been between them, along with some micro-plates, and the continental crust of India collided with that of Eurasia. The descending lithosphere continued to pull the Indian plate northward, under Eurasia, resulting in a doubling of the crust beneath Tibet. The many large Profile across the suture between Indian (light purple) and Eurasian (gray) continental crust. Light orange = Tethys Ocean earthquakes and landslides that occur in the Himalayas, Tibet, and floor sediments; red = rocks squeezed from deep in the crust. even far into China, are due to the continuing adjustments along this continent-continent plate boundary. Plate tectonics Continent-Continent Convergence When all of the oceanic lithosphere has been consumed in a subduction zone, the result is a convergence of continental lithosphere. The region of Tibet, north of the Himalayan Mountains, has an average elevation of 4,500 m above sea level. One reason for such a high elevation is that the continental crust there is twice its normal 40 km thickness. At about 70 million years ago, oceanic lithosphere beneath a long-gone ocean called Tethys, on the The northward migration of the Indian plate began at about 70 northern margin of the Indian Plate, began to be subducted million years and ended in collision beneath the Eurasian Plate. India moved quite rapidly northward as and suture with Eurasia about 40 the Tethyian oceanic lithosphere was subducted. By 40 million million years ago. years ago, the convergence of the Indian and Eurasian plates had consumed all the Tethyian lithosphere that had been between them, along with some micro-plates, and the continental crust of India collided with that of Eurasia. The descending lithosphere continued to pull the Indian plate northward, under Eurasia, resulting in a doubling of the crust beneath Tibet. The many large Profile across the suture between Indian (light purple) and Eurasian (gray) continental crust. Light orange = Tethys Ocean earthquakes and landslides that occur in the Himalayas, Tibet, and floor sediments; red = rocks squeezed from deep in the crust. even far into China, are due to the continuing adjustments along this continent-continent plate boundary. Plate tectonics

Transform Plate Boundaries Transform boundaries are where the lithospheric plates slide, jarringly, past one another. Earth is not a perfect sphere, but it is spheroidal and therefore as its tectonic plates spread apart in some places and converge in others, some segments of their boundaries must slide past one another. This slip occurs on transform fault zones, such as along the San Andreas Fault zone in coastal California. Transform fault zone between the North Transform fault zones have a very steep to vertical orientation. American and Pacific The majority of large earthquakes on these faults occur in the tectonic plates. most brittle, upper section of crust, at depths of 3 to 4 km. Exactly how deep the brittle behavior extends beneath the base of a typical transform fault system is still a topic of Profile and map view of the San Andreas and related transform intensive research. What is known is that the many faults that faults system. View to the north. comprise a transform fault zone are not perfectly planar and Earthquakes occur in the upper, brittle crust. Beneath the any one of them can get ‘stuck’ and be unable to slide – until transform fault the rocks are stresses build to the point that the rocks rupture. When the more ductile and able to flow to accommodate the relative fault finally slips, it releases its potential energy as an plates’ movement. earthquake. Plate tectonics

Mantle convection Plate tectonics

Mantle convection Plate tectonics Tectonic Plate Motion Ridge push and slab pull are the two main driving forces for plate tectonics. Oceanic crust originates as basaltic magma in the upper mantle that is erupted at mid-ocean ridges. The basalt cools and thickens as the two sides of the ridge move slowly apart. At the same time, the uppermost asthenosphere under the new crust also cools, gradually creating an oceanic lithosphere that eventually reaches a thickness of 50 to 100 km. The dense Formation of oceanic crust and lithosphere at a mid-ocean ridge. oceanic lithosphere sags slightly into the underlying Heavy black arrows indicate the ridge-push effect that gravity imposes asthenosphere under the influence of gravity. This down-and- as the lithosphere cools, becomes more dense, and thickens. outward motion of the lithosphere, called ‘ridge push,’ is a major driving mechanism for plate motion. An even more important mechanism for plate motion than ridge push is ‘slab pull,’ which is where cold, dense slabs of old oceanic lithosphere sink back into the mantle down subduction zones, again under the influence of gravity. As the slab descends into the mantle it becomes heated and less rigid, eventually becoming almost indistinguishable from lower mantle. The combination of ridge push and slab pull,together with other, Subduction of cold, dense lithosphere slabs (blue) under the influence lesser driving forces, creates a circulating flow of material within of gravity. The down-going slab pulls along the adjacent mantle the mantle that carries the overlying lithospheric plates along. material and eventually becomes indistinguishable in its physical properties from the lower mantle Plate tectonics Plate tectonics Plate tectonics

Hot spot Plate tectonics

Hot spot Plate tectonics Plate tectonics Plate tectonics

Foulger (2010) Plate tectonics

Foulger (2010) Plate tectonics Plate tectonics Plate tectonics Plate tectonics Plate tectonics Plate tectonics Plate tectonics Plate tectonics Natural Hazards and Disaster

Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Natural Hazards and Disaster

Class 6: Geohazards • Types of Geohazards • Plate Tectonics • Other Causes of Geohazards Other Causes of Geohazards Rockfall Other Causes of Geohazards Rockfall Other Causes of Geohazards Rockfall

Landslide Other Causes of Geohazards Rockfall

Landslide

Mudslide Other Causes of Geohazards soil creep Other Causes of Geohazards soil creep Other Causes of Geohazards soil creep

Mass movements Other Causes of Geohazards Sink holes Other Causes of Geohazards Sink holes Other Causes of Geohazards Sink holes Other Causes of Geohazards

Shrink and swell clays Other Causes of Geohazards

Shrink and swell clays

BGS GeoSure: shrink–swell Shrinking and swelling of the ground (often reported as subsidence) is one of the most damaging geohazards in Britain today, costing the economy an estimated £3 billion over the past decade.

https://www.bgs.ac.uk/products/geosure/shrink_swell.html Other Causes of Geohazards

Shrink and swell clays

https://www.bgs.ac.uk/products/geosure/shrink_swell.html Other Causes of Geohazards Ground liquefaction Other Causes of Geohazards Ground liquefaction Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.

https://en.wikipedia.org/wiki/Soil_liquefaction Other Causes of Geohazards Ground liquefaction Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.

https://en.wikipedia.org/wiki/Soil_liquefaction Other Causes of Geohazards Induced Seismicity Other Causes of Geohazards Induced Seismicity Seismicity can be induced by: • loading (reservoirs) • deloading (groundwater) • injection • extraction Other Causes of Geohazards Induced Seismicity Seismicity can be induced by: • loading (reservoirs) • deloading (groundwater) • injection • extraction Between the years 1973–2008, there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States. This rate has ballooned to over 600 M3+ earthquakes in 2014 and over 1000 in 2015. Through August 2016, over 500 M3+ earthquakes have occurred in 2016.

https://earthquake.usgs.gov/research/induced/ Other Causes of Geohazards Induced Seismicity Seismicity can be induced by: • loading (reservoirs) • deloading (groundwater) • injection • extraction Between the years 1973–2008, there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States. This rate has ballooned to over 600 M3+ earthquakes in 2014 and over 1000 in 2015. Through August 2016, over 500 M3+ earthquakes have occurred in 2016.

See https://earthquake.usgs.gov/ research/induced/edge.php for more detail on injection-induced seismicity https://earthquake.usgs.gov/research/induced/