Ch 12 Tectonics Earthquakes and Volcanism Earth s Surface Relief Features Relief vertical elevation differences in the land scape Topography The lay of the land Crustal Orders of Relief and Hypsometry Orders of Relief o 1st order is coarsest consisting of oceans and continents Continental landmasses the portions of the crust that reside above or near sea level including the undersea continental shelves on the coastlines Ocean basins are entirely below sea level o 2nd order is the intermediate level of landforms for both continental and oceans Include mountain plains and lowlands i e the Rockies Alps and Tibetan Plateau o 3rd order is most detailed including individual mountains cliffs hills The average elevation of Earth s surface is actually below sea level exhibiting valleys etc the depth of the oceans Earth s Topographic Regions Plains high tablelands hills and low tablelands mountains widely spaced Plains areas with local relief of less than 100 m and slope angles of 5 mountains and depressions degrees of less High tablelands have elevations exceeding 1520 m Hills and low tablelands dominate Africa Mountain ranges local relief exceeding 500 m Occur on each continent Crustal Formation Processes Tectonic activity is very slow Endogenic processes result in gradual uplift and new landforms Continental Shields Continental shield a region where a craton is exposed at the surface Craton the nucleus around which a continent grows Building Continental Crust and Terranes Explanation of crust formation on page 337 Each major plate is actually composed of fragments from other plates Terranes slow moving crustal pieces that attach to the plates at least 25 of growth of western North America can be attributed to the accretion of at least 50 terranes since the Jurassic Crustal Deformation processes Three types of stress o Tension stretching o Compression shortening o Shear twisting or tearing Strain how rocks respond to stress o Expressed by folding bending or faulting breaking If a rock is brittle it will break more easily but if it is ductile it will bend Folding and Broad Warping Folding types o Along the ridge of a fold layers slope downward away from the axis forming an anticline In the trough of a fold layers slope downward toward the axis resulting in a syncline Synclinal ridge forms within a syncline because different rock strata offer greater resistance to weathering processes Faulting Faulting when rocks on either side of a fracture displace relative to the other side Fractures cause earthquakes Fault plane the fracture surface along which the two sides of a fault move Normal fault forms when rocks are pulled apart by tensional stress Rocks on one side move vertically along an inclined fault The downward shifting side is a hanging fault which drops relative to the footwall block thrust or reverse fault results when rocks are forced together by compressional stress Strike slip fault forms when rocks are torn by lateral shearing stress Has right lateral top plate slides right or left lateral movement top plate slides left Orogenesis Mountain building The birth of mountains Orogeny mountain building episode Laramide orogeny Formed Rocky Mountains about 40 80 Ma Nevadan Orogeny formed Siierra Nevada and Klamath Mountains about 29 35 MAa Alleghany orogeny Appalachian mountains and Canadian Maritime Provences 250 300 Ma Associated with collision of North America and Africa Alpine orogeny Alps of Europe 2 to 66 Ma Himalayan orogeny 45 54 Ma beginning with the collision of the Indiana plate and Eurasia plate and continuing to the present Types of Orogenies Oceanic plate continental plate collision orogenesis now occurring along the Pacific coast of the Americas and formed the Andes and Rockies Folded sedimentary formations Volcanic activity inland from the subduction zone Oceanic plate oceanic plate collision orogenesis an produce either simple volcanic island arcs or more complex arcs like Indonesia and Japan Continental plate continental plate collision orogenesis mechanical Large masses of continental crust are subject to intense folding Earthquakes crustal plates don t glide smoothly past one another Focus Epicenter Foreshock and Aftershock focus also called hypocenter Subsurface area along the fault plane Epicenter area above surface directly above the focus Aftershock may occur after the main shock Foreshock precedes the main shock Earthquake Intensity and Magnitude Tectonic earthquakes associated with faulting Intensity scale useful in classifying and describing damage to terrain and structures Rated on the Mercalli scale Ricter scale gives a more specific numerical measurement of an earthquake s magnitude by recording seismic waves Moment magnitude scale considers the amount of fault slopeage produced by the earthquake and the size of the surface and the nature of the materials that faulting The Nature of Faulting Elastic rebound theory describes the basic process of how a fault breaks Two sides along a fault appear to be locked by friction resisting any movement despite the powerful forces acting on the adjoining pieces of crust Stress continues to build long the fault plane storing elastic energy like a wound up spring When the strain buildup finally exceeds the frictional lock both sides of the fault abruptly move to a condition of less strain releasing a burst of mechanical energy Volcanism Volcanic Features A volcano forms at the end of a central vent or pipe that rises from the asthenosphere and upper mantle through the crust into a volcanic mountain Crater forms at or near the summit Pyroclastics solid debris from a volcanic eruption Aa rough and jagged basaltic flow filled with pyroclastics Pahoehoe smooth fluid basaltic flow Cinder cone small cone shaped hill usually less that 450 m high Make of pyroclastic material Caldera large basin shaped depression Forms when summit material on a volcanic mountain collapse inward after an eruption i e Crater lake Effusion eruption produces low viscosity magma that is fluid Has small eruptions and few pyroclastics Shield volcano a volcano built from effusive eruptions that is gently sloped gradually rising from the surrounding landscape to a summit crater Plateau basalts when lava comes out through hot spots or elongated fissures Explosive eruptions Caused by gas being trapped underneath the surface When the pressure becomes too great it pushes out the trapping it in causing a large eruption Results in a lot of pyroclastic material