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GLG110 Textbook Notes Exploring Geology 2nd Ed Chapter 4

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University of Toronto St. George
Earth Sciences

Chapter 4 – Introduction to Natural Hazards  Most large tsunami’s occur at subduction zones (narrow strip of earth’s lithosphere where one tectonic plate moves beneath another).  Volcanic eruptions and earthquakes are the result of internal forces explained by plate tectonics. o Most earthquakes and active volcanoes occur at boundaries b/w plates.  The “processes” we consider to be hazards are derived from the internal heating of the earth and heat from the sun. Hazard – Natural process that threatens human life Risk – Probable severity that destructive event will occur multiplied by the likelihood the event will impact people. Disaster/Catastrophe – Events that cause serious injury or property damage. Mitigation – Used by scientists and planners to describe efforts to prepare for disasters and minimize their effects. Floods, hurricanes, earthquakes, volcanic eruptions, and large wildfires are most likely to create catastrophes. Geologic Cycle - Processes that produce the mineral resources, fuels, land, water, and atmosphere we require for survival. The cycles include:  the Tectonic Cycle  the Rock Cycle  the Hydrologic Cycle  Biogeochemical Cycles The Tectonic Cycle – The large-scale geologic processes that deform earth’s crust. Driven by forces deep within earth. Involves the creation, movement, and destruction of tectonic plates. Earth’s Lithosphere and Crust  Outer most outer layer of earth is the lithosphere. It is stronger and more rigid than the deeper material. Below the lithosphere is the asthenosphere, a hot layer of low-strength rock that extends to an average depth of 250km.  The upper part of the lithosphere is the crust. o Two types: Oceanic and Continental o Oceanic crust is denser and thinner than continental. Types of Plate Boundaries  Lithosphere is broken into fragments called tectonic plates that move.  Plate boundaries can be divergent, convergent, or transform. o Divergent – Two plates move away from each other and new lithosphere is produced. Underwater mountain ridges called mid-ocean ridges are formed by a process called seafloor spreading. Cracks in the underwater rift zone fill with molten rock/magma and new lithosphere forms. o Convergent – Two plates collide head on. A higher density oceanic plate is drawn beneath a lower density continental plate (usually). This is called subduction, and convergent boundaries like this are called subduction zones. The oceanic plate heats as it moves beneath the continental plate. The high temperatures cause lower crustal rocks to melt and magma moves up through the crust along fractures. Some of the magma reaches the
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