GEOG 274 - Endogenic System Review PDF

GEOG 274 – Review - Part I: Endogenic system Earth internal structure, energy, dynamics, rock transformation and cycling, associated landforms. Overview: Earth is a dynamic planet. Two broad systems in play: endogenic and exogenic. The endogenic system encompasses internal...

GEOG 274 – Review - Part I: Endogenic system Earth internal structure, energy, dynamics, rock transformation and cycling, associated landforms. Overview: Earth is a dynamic planet. Two broad systems in play: endogenic and exogenic. The endogenic system encompasses internal processes that produce flows of heat and material from deep below Earth’s crust. Radioactive decay and the geothermal energy are the principal sources of power for these processes. The materials involved constitute the solid realm of Earth. Earth’s surface responds by moving, warping, and breaking, sometimes in dramatic episodes of earthquakes and volcanic eruptions, constructing the crust, crustal landforms, and causing tsunami. The exogenic system consists of external processes at Earth’s surface that set into motion air, water, and ice, all powered by solar energy. These media carve, shape, and wear down the landscape. Thus, Earth’s surface is the interface between two vast open or partially open systems: one driven by endogenic energy that builds the landscape and creates topographic relief, and one driven by exogenic energy that tears the landscape down into relatively low elevation plains of sedimentary deposits. As a planetary body, Earth is an open system in terms of energy, and a closed system in terms of physical matter and resources. Key learning concepts: After reviewing lectures notes and using this study guide (cf. the Chs.12 –13, and Ch.16 (Tsunami) of the Geosystems textbook - see course outline for details), you should be able to: Distinguish between the endogenic and exogenic systems that shape Earth, and name the driving force for each. Explain the principles of uniformitarianism, superposition, and discuss the time spans into which Earth’s geologic history is divided. Depict Earth’s interior in cross-section, and describe each distinct layer. Describe the three main groups of rocks and diagram the rock cycle. § Describe the concept of plate tectonics, and explain the physical evidence that crustal drifting is continuing today. Outline the pattern of Earth’s major plates on a world map, and relate this pattern to the occurrence of earthquakes, volcanic activity, and hot spots. Describe the three types of plate boundaries associated with orogenesis, and identify specific examples of each. Distinguish between an effusive and an explosive volcanic eruption, and describe the key tectonic settings and factors determining the type of volcanic eruption; name and discuss the associated products of volcanic eruptions. Explain the processes of folding and faulting, and describe the principal types of faults. Describe earthquake fault mechanics, and explain the tectonic settings associated to major earthquakes types; their major characteristics, and measurement Describe and explain the tsunami. Introduction 1|Page  The Earth–atmosphere interface is the meeting place of internal and external processes that build up and wear away landscapes. The endogenic system consists of processes operating in Earth’s interior, driven by heat from radioactive decay and geothermal heat, while the exogenic system consists of processes operating at Earth’s surface, driven by solar energy and the movement of air, water, and ice. The endogenic system encompasses internal processes that produce flows of heat and material from deep below the crust, powered by radioactive decay and gravitational potential (initial compression). The exogenic system includes external processes that set air, water, and ice into motion, powered by solar energy. These media are sculpting agents that carve, shape, and reduce the landscape—all under the pervasive influence of gravity. Geology is the science that studies all aspects of Earth—its history, composition and internal structure, surface features, and the processes acting on them. The study of Earth’s surface landforms, specifically their origin, evolution, form, and spatial distribution, is geomorphology—a subfield of both physical geography and geology. Earth’s interior is organized as a core surrounded by roughly concentric shells of material. It is relatively evenly heated bottom-up by the interior materials, compressed by gravity, and unevenly heated by the radioactive decay of unstable elements. A rock cycle produces three classes of rocks through igneous, sedimentary, and metamorphic processes. The surface expressions of these forces include earthquakes and volcanic events. Review Questions 1) Define the endogenic and the exogenic systems. Describe the driving forces that energize these systems. The endogenic system consists of processes operating in Earth’s interior, driven by heat from radioactive decay and geothermal heat, while the exogenic system consists of processes operating at Earth’s surface, driven by solar energy and the movement of air, water, and ice. The endogenic system encompasses internal processes that produce flows of heat and material from deep below the crust, powered by radioactive decay and gravitational potential (initial compression). This is the solid realm of Earth. The exogenic system includes external 2 processes that set air, water, and ice into motion, powered by solar energy. This is the fluid realm of Earth’s environment. These media are sculpting agents that carve, shape, and reduce the landscape—all under the pervasive influence of gravity. Broadly speaking, exogenic processes tend to wear down the landscape, while endogenic processes create new land. Within the rock cycle, exogenic processes wear away rock, creating sedimentary rocks. Endogenic processes are responsible for creating igneous and metamorphic rocks through the processes of volcanism and subduction. 2) Explain the principles of uniformitarianism and superposition in the Earth sciences. Uniformitarianism: The same physical processes active in the environment today have been operating throughout the geologic time (earthquakes, volcanic eruptions, landslides, orogenesis). 2|Page  Superposition: Rock and sediment are always arranged with the youngest bed “superposed” toward the top of a rock formation and the oldest at the base, if they have not been disturbed. Uniformitarianism assumes that the same physical processes active in the environment today have been operating throughout geologic time. The phrase “the present is the key to the past” is an expression coined to describe this principle. In contrast, punctuated equilibrium places catastrophic events into the larger, slower, picture of uniformitarianism. The catastrophic events such as earthquakes, tsunami, and massive floods occur as small, localized events within the larger gradual process of landscape evolution. Nicolaus Steno in 1669 described two basic geologic principles. The first stated that sedimentary rocks are laid down in a horizontal manner, and the second stated that younger rock units were deposited on top of older rock units. In 1815 Smith produced a geologic map of England in which he successfully demonstrated the validity of the principle of faunal succession, by (1) showing better the preservation of the fossils closer to the surface, and (2) - the increasing amount of fossils with advent of new life forms. Eventually, the basic principles of stratigraphy were formalized: A. Original Horizontality: all sedimentary rocks are originally deposited horizontally. Sedimentary rocks that are no longer horizontal have been tilted from their original position. B. Lateral Continuity: sedimentary rocks are (were) laterally continuous over large areas. "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." C. Superposition: "...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." D. Cross-Cutting Relations: "If a body or discontinuity cuts across a stratum, it must have formed after that stratum." E. Law of Inclusions: this law states that rock fragments (in another rock) must be older than the rock containing the fragments. F. Law of Faunal Succession: Fossil groups were succeeded by other fossil groups through time. This allowed geologists to develop a fossil stratigraphy and provided means to correlate rocks throughout the world. 3) How is the geologic time scale organized? What era, period, and epoch are we living in today? What is difference between the relative and absolute ages of rocks? The geologic time scale (also Figure 12.1 in the Geosystems 4ed.) reflects currently accepted names and the relative and absolute time intervals that encompass Earth’s history (eons, eras, periods, and epochs). The sequence in this scale is based on the relative positions of rock strata above or below one another. An important general principle is that of superposition, which states that rock and sediment always are arranged with the youngest beds “superposed” near the top of a rock formation and the oldest at the base—if they have not been disturbed. The absolute ages on the scale, determined by scientific methods such as dating by radioactive isotopes, are also used 3|Page to refine the time-scale sequence. The figure presents important events in Earth’s life history along with the geologic time scale. 3.1 Thoughts about an “Anthropocene Epoch” Many of the key divisions in the geologic time scale are based on observable events in the geologic record. The massive changes caused by humans to Earth will be clearly visible in the geologic record - massive deforestation, mass extinctions, and significant changes to the composition of the atmosphere. As a matter of fact, now humans (1) affect the Earth’s systems greater than natural forcing, (2) affect the Earth’s systems faster than natural forcing, and (3) generate number of (harmful) consequences for life as we know it. Based upon this and further evidence, we are clearly living in the Anthropocene, which may indeed be defined as a separate geologic period. 4) Make a simple sketch of Earth’s interior, label each layer, and list the physical characteristics, temperature, composition, and range of size of each on your drawing. See details in Figures 12.2 and 12.3 (in the Geosystems 4ed.) and/or lecture notes, as the basis for this sketch. 5) How does Earth generate its magnetic field? Is the magnetic field constant, or does it change? Explain the implications of your answer. The fluid outer core generates at least 90% of Earth’s magnetic field and the magnetosphere that surrounds and protects Earth from the solar wind. A present hypothesis by scientists from Cambridge University details spiralling circulation patterns in the outer core region that are influenced by Earth’s rotation; this circulation generates electric currents, which in turn induce the magnetic field. An intriguing feature of Earth’s magnetic field is that it sometimes fades to zero and then returns to full strength with north and south magnetic poles reversed! In the process, the field does not blink on and off but instead oscillates slowly to nothing and then slowly regains its strength. (New evidence suggests the field fades slowly to zero, then when it returns it tends to do so abruptly.) This magnetic reversal has taken place nine times during the past 4 million years and hundreds of times over Earth’s history. The average period of a magnetic reversal is 500 000 years, with occurrences as short as several thousand years possible, though this statistics is only good at a time scale of millions years or more. Earth’s magnetic field presently is losing strength at the rate of approximately 7% per 100 years. The field was about 40% stronger 2000 years ago, according to the latest published research. This means, a new reversal is expected in some future. 6) Describe the asthenosphere. Why is it also known as the plastic layer? What are the consequences of its convection currents? The extreme upper mantle, just below the crust, is known as the asthenosphere, or plastic layer. It contains pockets of increased heat from radioactive decay and is susceptible to convective currents in these hotter (and therefore less dense) materials. The depths affected by these convection currents are the subject of much scientific speculation. Because of this dynamic condition, the asthenosphere is the least rigid region of the mantle, with densities averaging 3.3 4|Page g cm-3. This section of the mantle is known as the plastic layer due to its dynamic activity. About 10% of the asthenosphere is molten in asymmetrical patterns and hot spots. Think of Earth’s outer crust (densities of 2.7 g cm-3 for continental crust and 3.0 g cm-3 for oceanic crust) as floating on the denser layers beneath, much as a boat floats on water. With a greater load (e.g., ice, sediment, mountains), the crust tends to ride lower in the asthenosphere (see isostatic adjustment in the lecture notes). Convection currents in the asthenosphere disturb the overlying crust and create tectonic activity. In return, the movement of the crust - collision, divergence - may influence currents in the mantle. 7) What is a discontinuity? Describe the principal discontinuities within Earth. A discontinuity is a place where a change in physical properties occurs between two regions deep in Earth’s interior. A transition zone of several hundred kilometres marks the top of the outer core and the beginning of the mantle. An analysis by scientists at the California Institute of Technology determined that this transition area is bumpy and uneven, with ragged peak-and-valley-like formations. Some of the motions in the mantle may be created by this rough texture at what is called the Gutenberg discontinuity. The boundary between the crust and the rest of the lithospheric upper mantle is another discontinuity called the Mohorovičić (Moho) discontinuity, named for the Yugoslavian seismologist who determined that seismic waves change at this depth, owing to sharp contrasts of materials and densities. 8) Define isostasy and isostatic rebound, and explain the crustal equilibrium concept. The principle of buoyancy (that something less dense, like wood, floats in denser things like water) and the principle of balance were further developed in the 1800s into the important principle of isostasy to explain certain movements of Earth’s crust. The entire crust is in a constant state of compensating adjustment, or isostasy, slowly rising and sinking in response to its own weight, and pushed and dragged about by currents in the asthenosphere (cf. Figure 12.4 in the Geosystems 4 up. ed., and lecture notes). 9) Diagram the upper mantle and crust. Label the density of the layers in g/cm3. What two types of crust were described in terms of rock composition? See lecture notes as the basis for this diagram (Figures 12.2c and 12.4 in the Geosystems 4 up. ed). The two types of crust discussed in the text were oceanic crust, composed of basalt, a rock high in silica and magnesium (earning its name as simatic crust), and continental crust, composed mostly of granite, a rock high in silica and aluminum (earning its name as sialtic crust). 10) What is a mineral and mineral family? Name the most common minerals on Earth. What is a rock? A mineral is an element or combination of elements that forms an inorganic natural compound. A mineral can be described with a specific symbol or formula and possesses specific qualities. Silicon (Si) readily combines with other elements to produce the silicate mineral family, which includes quartz, feldspar, amphibole, and clay minerals, among others. Another important mineral family is the carbonate group, which features carbon in combination with oxygen and other elements such as calcium, magnesium, and potassium. 5|Page  Of the nearly 3000 minerals, only 20 are common, with just 10 of those making up 90% of the minerals in the crust. A rock is an assemblage of minerals bound together (such as granite, containing silica, aluminum, potassium, calcium, and sodium) or sometimes a mass of a single mineral, such as rock salt. 11) Describe igneous processes. What is the difference between intrusive and extrusive types of igneous rocks? Rocks that solidify and crystallize from a molten state are called igneous rocks. Most rocks in the crust are igneous. They form from magma, which is molten rock beneath the surface (hence the name igneous, which loosely means fire-formed in Latin). Magma is fluid, highly gas-rich, and under tremendous pressure. It is either intruded into pre-existing crustal rocks (known as country rock), or extruded onto the surface as lava. The cooling history of the rock—how fast it cooled, and how steadily the temperature dropped—determines its texture and degree of crystallization (minerals have time to form crystals). These range from coarse- grained (slower cooling, with more time for larger crystals to form) to fine-grained or glassy (faster cooling) 12) Explain what coarse- and fine-grained textures say about the cooling history of a rock. The location and rate of cooling determine the crystalline texture of a rock, that is, whether it is made of coarser (larger) or finer (smaller) materials. Thus, the texture indicates the environment in which the rock formed. The slower cooling of magma beneath the surface allows more time for crystals to form, resulting in coarse-grained rocks such as granite. The faster cooling of lava at the surface forms finer-grained rocks, such as basalt, the most common extrusive igneous rock. 13) Briefly describe sedimentary processes and lithification. Describe the sources and particle sizes of sedimentary rocks. Most sedimentary rocks are derived from pre-existing rocks, or from organic materials, such as bone and shell that form limestone, mud that becomes compacted into shale, and ancient plant remains that become compacted into coal. The exogenic processes of weathering and erosion generate the material sediments needed to form these rocks. Bits and pieces of former rocks - principally quartz, feldspar, and clay minerals - are eroded and then mechanically transported (by water, ice, wind, and gravity) to other sites where they are deposited. In addition, some minerals are dissolved into solution and form sedimentary deposits by precipitating from those solutions; this is an important process in the oceanic environment. The cementation, compaction, and hardening of sediments into sedimentary 5 rocks is called lithification. Various cements fuse rock particles together; lime (CaCO3, or calcium carbonate) is the most common, followed by iron oxides (Fe2O3), and silica (SiO2). Particles also can unite by drying (dehydration), heating, or chemical reactions. The two primary sources of sedimentary rocks - the mechanically transported bits and pieces of former rock and the dissolved minerals in solution - are known as clastic sediments and chemical sediments, respectively. See Table 6|Page 12.3 in the Geosystems 4ed, for the range of clast sizes and the form these materials take as lithified rock. 14) What is metamorphism and how are metamorphic rocks produced? Name some original parent rocks and their metamorphic equivalents. Any rock, either igneous or sedimentary, may be transformed into a metamorphic rock by going through profound physical and/or chemical changes under increased pressure and temperature. (The name metamorphic comes from the Greek, meaning to change form.) Metamorphic rocks generally are more compact than the original rock and therefore are harder and more resistant to weathering and erosion. 15) Briefly review the history of the theory of plate tectonics, including the concepts of continental drift and sea-floor spreading. What was Alfred Wegener’s role? In 1912, German geophysicist and meteorologist Alfred Wegener publicly presented in a lecture his idea that Earth’s landmasses migrate. His book, Origin of the Continents and Oceans, appeared in 1915. Wegener today is regarded as the father of the concept called continental drift. Wegener postulated that all landmasses were united in one supercontinent approximately 225 million years ago, during the Triassic period. The fact that spreading ridges and subduction zones are areas of earthquake and volcanic activity provides further evidence for plate tectonics, which by 1968 had become the all-encompassing term for these crustal processes. 16) Describe the process of upwelling as it refers to magma under the ocean floor. Define subduction and explain that process. The worldwide submarine mountain ranges, called the mid-ocean ridges, were the direct result of upwelling flows of magma from hot areas in the upper mantle and asthenosphere. When mantle convection brings magma up to the crust, the crust is fractured and new seafloor is formed, building the ridges and spreading laterally. When continental crust and oceanic crust collide, the heavier ocean floor will dive beneath the lighter continent, thus forming a descending subduction zone (see lecture notes and appropriate animations). The world’s oceanic trenches coincide with these subduction zones and are the deepest features on Earth’s surface. 17) Characterise the three types of plate boundaries and the actions associated with each type. The boundaries where plates meet are clearly dynamic places. Divergent boundaries are characteristic of seafloor spreading, where upwelling material from the mantle forms new seafloor and crustal plates are spread apart. Convergent boundaries are characteristic of collision zones, where areas of continental and/or oceanic crust collide. These are zones of compression. Transform boundaries occur where plates slide laterally past one another at 6 right angles to a sea-floor spreading center, neither diverging nor converging, and usually with no volcanic eruptions. 18) What is the relation between plate boundaries and volcanic and earthquake activity? 7|Page  Plate boundaries are the primary location of Earth’s earthquake and volcanic activity, and the correlation of these phenomena is an important aspect of plate tectonics because they are produced by plate/asthenosphere interactions at these boundaries. Earthquakes and volcanic activity are discussed in more detail later, but their general relationship to the tectonic plates is important to point out here. Earthquake zones and volcanic sites are identified on the world plate map (in Figure 12.21 in the Geosystems 4ed. and in lectures notes). 19) Define orogenesis, list the three types of plate collisions associated with orogenesis, and identify specific examples of each. What is meant by the birth of mountain chains? Orogenesis literally means the birth of mountains (oros comes from the Greek for mountain). An orogeny is a mountain-building episode that thickens continental crust. It can occur through large-scale deformation and uplift of the crust in episodes of continental plate collision such as the formation of the Himalayan Mountains from the collision of India and Asia. It also may include the capture of migrating terranes and cementation of them to the continental margins, and the intrusion of granitic magmas to form plutons. These granite masses often are exposed following uplift and removal of overlying materials. Uplift is the final act of the orogenic cycle. Earth’s major chains of folded and faulted mountains, called orogens, bear a remarkable correlation to the plate tectonics model. 20) Identify on a map several of Earth’s mountain chains. What processes contributed to their development? The mountain chain that includes the Andes, the Sierra of Central America, the Rockies, and other western mountains was created from the collision between oceanic plates and continental plates. The collision of the Nazca plate with the South American plate creating the Andes is a good example of subduction causing folded sedimentary formations, with intrusions of magma forming granitic plutons characteristic of explosive volcanism. Another mountain chain is the Himalayas, created by the collision of the India plate with the Eurasian plate 45 million years ago. This is an example of continental plate-continental plate collision. 21) What is a volcano? In general terms, describe some related features. A volcano is a landform that develops at the end of a central vent or pipe that rises from the asthenosphere through the crust into the volcanic mountain, usually forming a crater, or circular surface depression, at the summit. Magma rises and collects in a magma chamber deep below the volcano until conditions are right for an eruption. Other features related to volcanic activity are: calderas, large basin-shaped depressions formed when summit material on a volcanic mountain collapses inward after eruption or loss of magma; cinder cones, small cone- shaped hills with a truncated top formed from cinders that accumulate during moderately explosive eruptions; and shield volcanoes, which are created by effusive volcanism. 22) Where do you expect to find volcanic activity in the world? Why? Review the plate tectonic map and volcanic activity. The location of volcanic mountains on Earth is a function of plate tectonics and hot spot activity. Volcanic activity occurs in three areas: along subduction boundaries at continental plate-oceanic plate or oceanic plate-oceanic 8|Page plate convergence; along sea-floor spreading centers on the ocean floor and areas of rifting on continental plates; and at hot spots, where individual plumes of magma rise through the crust. 23) Distinguish three tectonic settings related to volcanic activity and landforms, e.g. in western Canada. In western Canada, three tectonic setting can be identified. First is along the convergent plate boundaries experiencing subduction at the boundary of the North American plate and the Pacific plate in northern British Columbia and where the Juan de Fuca plate is subducting beneath the North American plate. Earthquake activity and subduction volcanoes are associated with these convergent motions. The second tectonic setting is at hotspots where plumes of magma break through the crust producing volcanoes like the Nazko Cone near Quesnel, BC. The third tectonic setting is associated with crustal rifting, e.g., the Tuya Volcanic Field in northern BC. Volcanic activity in Eastern Canada mostly dates back to between 3,800 and 125 Ma BP. 24) Compare effusive and explosive eruptions. Why are they different? What distinct landforms are produced by each type? Give examples of each. Effusive eruptions are the relatively gentle eruptions that produce enormous volumes of lava on the seafloor and in places like Hawaii. Direct eruptions from the asthenosphere produce a low-viscosity magma that is very fluid and yields a dark, basaltic rock (less than 50% silica and rich in iron and magnesium). Gases readily escape from this magma because of its texture. A typical mountain landform built from effusive eruptions is gently sloped, gradually rising from the surrounding landscape to a summit crater, similar in outline to a shield of armor lying face up on the ground and is therefore called a shield volcano. Volcanic activity along subduction zones produces the well-known explosive volcanoes. Magma produced by the melting of subducted oceanic plate and other materials is thicker (more viscous) than magma from effusive volcanoes; it is 50–75% silica and high in aluminum. Consequently, it tends to block the magma conduit inside the volcano, allowing pressure to build and leading to an explosive eruption. The term composite volcano, or composite cone, or stratovolcano, is used to describe explosively formed mountains. Composite volcanoes tend to have steep sides and are more conical than shield volcanoes (therefore they are also known as composite cones); also, mainly due to the high viscosity of magma, their eruptions tend to reach extreme heights (hence, stratovolcano). 25) Define the four basic types of faults. How are faults related to earthquakes and seismic activity? When rock strata are strained beyond their ability to remain a solid unit, they fracture, and one side is displaced relative to the other side in a process known as faulting (see Figure 13.11 and related text section in the Geosystems 4ed. and/or lecture notes). Of the two types of faulting (with dip-slip and strike-slip types of motion – see lecture notes), three main cases 8 of faulting are discussed: normal, transform, and reverse. A type of reverse fault where the fault plane is at a low angle (often associated with initial folding) is a thrust fault (the fourth case). Thus, fault zones are areas of crustal movement. At the moment the fault line shifts, a release of energy occurs, potentially generating an earthquake (or quake). 26) What is the relationship between an epicentre and the focus of an earthquake? 9|Page  The subsurface area along a fault plane, where the motion of seismic waves is initiated, is called the focus, or hypocentre (see lecture notes). The area at the surface directly above this subsurface location is the epicentre. Shock waves produced by an earthquake radiate outward from both the focus and epicentre. An aftershock may occur after the main shock, sharing the same general area of the epicentre. A foreshock is also possible, preceding the main shock. Both aftershocks and foreshocks are most likely linked to the asperities. 27) Differentiate between the Mercalli and moment magnitude (M) and amplitude (Richter) scales. How are these used to describe an earthquake? Earthquake magnitude is estimated according to a system originally designed by Charles Richter in 1935. In this method, the amplitude of a seismic wave is recorded on a seismograph located at least 100 km from the epicentre of the quake. That measure is then charted on the Richter scale, which is open-ended and logarithmic; that is, each whole number on the scale represents a 10-fold increase in the measured wave amplitude. Translated into energy, each whole number demonstrates a 31.5-fold increase in the amount of energy released. Today, the Richter scale has been improved and made more quantitative. The need for revision was because at higher magnitudes, the scale did not properly measure or differentiate between quakes of high intensity. The moment magnitude (M) scale, in use since 1993, is considered more accurate than Richter’s scale for large earthquakes. Moment magnitude considers the amount of fault slippage produced by the earthquake, the size of the surface or subsurface area that ruptured, and the nature of the materials that faulted, including how resistant they were to failure. The USGS and other sources of seismic reporting only report moment magnitude ratings for earthquakes. This usually appears as for a 4.5 earthquake on the moment magnitude scale as M4.5, or 4.5 magnitude in the reports. Earthquake intensity is rated on the arbitrary Mercalli scale, a Roman numeral scale from I to XII representing “barely felt” to “catastrophic total destruction.” It was designed in 1902 and modified in 1931 to be more applicable to conditions in North America. Intensity scales are useful in classifying and describing terrain, construction, and local damage conditions following an earthquake. 28) How do the elastic-rebound theory and asperities help explain the nature of faulting? In your explanation, relate the concepts of stress (force) and strain (deformation) along a fault. How does this lead to rupture and earthquake? The elastic-rebound process is described by the elastic-rebound theory. Generally, two sides along a fault appear to be locked by friction, resisting any movement despite the powerful motions of adjoining pieces of crust. This stress continues to build strain along the fault surfaces, storing elastic energy like a wound-up spring. When movement finally does occur as the strain build-up exceeds the frictional lock, energy is released abruptly, returning both sides of the fault to a condition of less strain. Think of the fault plane as a surface with 9 irregularities that act as sticking points, preventing movement, similar to two pieces of wood held together by drops of glue rather than an even coating of glue. Research scientists at the USGS and the University of California have identified these small areas of high strain as asperities. They are the points that break and release the sides of the fault. 10 | P a g e 29) Summarize what is known about the recurrence of high magnitude earthquakes off the coast of British Columbia and the Pacific Northwest. This is the most tectonically active region in Canada with divergent, convergent and transform boundaries in proximity. Earthquake activity here is unusual in that few low magnitude events occur. Two of Canada’s largest recorded earthquakes occurred in this region in 1946 (M7.3) and 1949 (M8.1). Currently, the Cascadia Subduction zone that runs along the British Columbia, Washington and Oregon coasts is locked and storing energy. It is anticipated that a large megathrust earthquake with a magnitude greater than 8 is likely to occur in this region. 30) Explain how a seismic sea wave attains such tremendous velocities. Why is it given the name tsunami? Tsunami is a wave formed [mostly] in the large bodies of water by sudden and sharp displacement of a sizeable water column; the displacement usually associated with sharp motions at or near the lake- or seafloor, such as those caused by earthquakes, submarine landslides, or eruptions of undersea volcanoes. Usually, a small group of two or three long waves is generated. These waves often exceed 100 km in wavelength but are at large only a metre or so in height. Because of their great wavelength, tsunami waves are affected by the topography of the deep-ocean floor and are refracted by rises and ridges. They travel at great speeds in deep-ocean water (velocities of 600 to 800 km h-1 are not uncommon), but often pass unnoticed on the open water because their great length makes the slow rise and fall of water hard to observe. Throughout history Japan has suffered greatly from these seismic waves. Tsunami is Japanese for “harbour wave,” named for its devastating effect in harbours. Please also review all the materials shared in Moodle. Supplementary learning materials Rocks and minerals: mindat.org/ Volcanoes: www.si.edu/gvp/ volcanoes.usgs.gov/ Earthquakes: earthquake.usgs.gov/earthquakes/ www.earthquakescanada.nrcan.gc.ca/index-eng.php https://www.seismescanada.rncan.gc.ca/hazard-alea/simphaz-en.php 11 | P a g e