GEOG 140
Intro Physical Geography
Lecture Notes
Tectonics, Earthquakes, and Volcanism
Continental Shields
-nucleus of ancient crystalline rock (craton) on which the continent grows by
addition of crustal fragments and sediments
Building Continental Crust and Terranes
-continental crust
-lighter than oceanic crust
-takes hundreds of millions of years to form
-sea floor spreading creates new oceanic crust
-crust collides with plate edge at some distant location
-subducted into mantle where it remelts
-magma rises and cools, creating new continental crust (intrusive
igneous rock-granite)
-erosion exposes intrusive igneous landforms
-terranes
-slowly migrating crustal pieces that are “scrapped off” of subducting
plates and attached to the land mass at the edge of plate boundaries
-microplate, foreign, or suspect terranes
-often different composition than continents to which they attach
-considerable accreted terranes along west coast of North America
-Alaska
-British Columbia
Plate Movements
-eight major and at least ten smaller plates
-plate boundaries take on three kinds of character
-divergence or spreading
-convergence or collision
-transform or lateral displacement
-divergence
-midoceanic ridges-seafloor spreading
-magma wells up from the athenosphere
-new lithosphere is created
-lithosphere on opposing sides of the ridges is pushed apart
-crust at these sites is thin-rifting occurs
-also happens on land-crustal spreading
-rift valley
-eastern Africa
-Red Sea (advanced stage of rift valley)
-convergence
-plates spreading outward must converge and collide at other points
-continental crust has lower density than oceanic crust
-it is “lighter”
-when oceanic and continental plates meet the continental plate overrides
the oceanic plate and pushes it downward
-subduction
-subducted plate is melted by the athenosphere
-molten rock forces its way up through vents and fissures to the
surface
-volcanism
-plate movement at these sites also results in earthquakes
-three types of convergent plate boundaries (causes of orogenesis or
mountain building)
-oceanic-continental plate convergence
-example - Nazca plate subducting beneath South American Plate
-oceanic-oceanic plate convergence
-example- Pacific, North American, Eurasian, and Australian
plates create island arcs in northern and western pacific
-Aleutian Islands and Japan
-continental-continental plate convergence
-example – Eurasian plate and Indian plate
-no melting-thickening of land instead
-earthquakes but no volcanism
-lateral plate contact
-plates slide past each other-transform faults
-example- San Andreas fault
Earthquakes and Faults
-Elastic rebound theory
-stresses build along fault plane surfaces storing elastic energy
-strain exceeds frictional forces and both sides move to release strain
-release mechanical energy
-Folding and Warping
-three types of stress
-tension (stretching)
-compression (shortening)
-shear (twisting or tearing)
-convergent plate boundaries compress rocks in process known as folding
-anticlines-layers slope downward away from axis
-compressional forces push folds far enough that they overturn on
their own strata (overturned anticline)
-synclines-layers slope downward toward the axis
-faulting
-fault zones-areas where fractures in the rock indicate crustal movement
-fault plane-fracture surface along which two sides of a fault move
-normal fault
-tensional stress
-reverse (thrust) fault
-compressive stress
-strike slip fault (no scarps but can produce linear rift valleys)
-shear stress
-right lateral
-left lateral
Volcanism
-Most volcanism
-seafloor spreading at midoceanic ridges
-subduction at convergent plate boundaries
-Volcano status
-active
-volcano has erupted in recorded history
-dormant
-no observed eruptions but evidence of recent activity
-no significant signs of erosion
-extinct
-no signs of life
-evidence of long term erosion
-still chances of resumed activity even though considered extinct
-Lava and Landforms
-viscosity varies with composition
-basaltic lavas
-low in silica
-high in iron and magnesium
-low viscosity –very fluid-move fast
-acidic
-higher in silica
-lower in magnesium and iron
-higher viscosity-move slower
-higher gas content-explosive eruptions
-lava thrown into the air (pyroclastics)
-bombs
-cinders
-ash
-landforms a function of rate of cooling
-composition and viscosity
-thickness
-nature of surface
-vents
-pipes-often create mountains
-fissure eruptions
-create extensive basaltic plateaus
-Columbia Plateau –Pacific Northwest
-Volcanic Mountains
-four types of landforms
-composite volcanoes
-successive eruptions create successive layers
-lava remains relatively close to vent
-creates steep sided, symmetrical mountains
-very large versions-stratovolcanoes
-acidic gas filled lavas
-highly explosive
-lahars
-floods of ash, mud, and water
-melting of summit snows
-nuee ardente (glowing cloud)
-release of pent up gas
-heavier-than-air cloud
-extremely high temperature
-lava domes
-acidic lava oozes to surface without pyroclastic activity and produces
small volcanic mound
-form as distinct landform
-form inside crater after explosive eruption
-cinder cones
-formed almost entirely of pyroclastics
-cinder size fragments
-also larger and smaller fragments
-shield volcanoes
-fluid basaltic lavas
-sufficient gases to create fountains
-hot basaltic lava flows in sheets
-successive sheets build rounded landform
-Hot Spots, calderas
-Lava types
-pahoehoe-fluid, ropey pattern
-aa-angular, blocky
-Igneous Rocks
-form from the cooling of molten magma (lava)
-at the surface (extrusive)
-faster cooling-finer mineral crystals
-inside the crust (intrusive)
-slower cooling-larger mineral crystals
-plutonic igneous rocks
-magma composition
-rich in silica (acidic/silicic)
-light colored rocks rich in feldspar and quartz
-poorer in silica (basic/mafic)
-darker rocks
-intrusive-gabbro
-extrusive – basalt
-intrusive forms
concordant-does not disrupt existing strata
-laccolith-domelike structure that pushes overlying strata into a
bulge without destroying them
-sill-thin horizontal layer formed between two sedimentary strata
discordant-cuts across existing strata
-batholith-pluton that has melted and assimilated surrounding rock
-dike-vertical barrier wall cutting across sedimentary strata