ISCI 2001Chapters 22-24
Plate Tectonics
Plate Activities – Divergent Plate Boundaries
(1). Plates may ‘diverge’Plates move apart
Lava fills spaces in between
(2). What types of structures are produced?Volcanic mountainsRift valleys
(3). Examples Mid-Atlantic Ridge Great Rift Valley (Africa near Nairobi
Kenya)
Rift Valley in Kenya
Mid-Atlantic Ridge
Convergent Boundaries
(1). When two plates collide One plate usually subducts
Most dense or oldest plates
(2). Types of convergence Oceanic – Oceanic
Trench formation (Marianas Trench) 11,000 m or 7.0 miles deep
Pacific and Phillipine Plates collide Formation of volcanic islands or arcs
Subduction plate mantle rock melts comes to the surface and cools
Oceanic – Continental Oceanic basaltic plate (more dense) subducts under granitic
continental plate Mantle rock melts, magma rises and cools forms island chains Volcanic Arcs (Peru)
Continental – Continental Massive plate collisions (both granitic) No subduction, why?
Both have same density Massive mountains are formed
Himalayas
Marianas Trench
Oceanic – Continental
The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form the towering peaks of the Andes, as seen here in the Pachapaqui mining area in Peru.
Continental – Continental
Transform Plate Boundaries (1). ‘Sliding Plate’ Boundaries
Slipping of plates causes ‘faults’
(2). Slipping causes plate movementsBoundaries move in opposite directions
against each other
(3). Where are they normally found?Mostly ocean basinsContinental plate: San Andreas Fault
San Andreas Fault
Types of Faults
(1). Dip-Slip (See figure 24.5)Hanging wall and vertical wall move
vertically along the fault plane Movement is vertical
(2). Strike-SlipMovement is horizontal San Andreas Fault motion
(3). ObliqueMove horizontally and vertically
Slip-DipConjugate Normal faults, Canyonlands National Park, Utah
Fault Types
Folding(1). Bending in Rock layers
Caused by compression
(2). Results Anticlines or synclines
Mountain Formation
(1). Folded MountainsDuring formation continental crust thickens
and wrinkles into vertical folds from compressionAppalachians, Rockies and Himalayas
(2). Unwarped Domed shaped
Single anticline (crust is heaved upwards; no folds produced)
Adirondack mountains NY
(3). Fault-BlockLand is ‘uplifted’ , stretched and elongated Very steep profileTetons (Wyoming); Sierra Nevada (California)
Mountain FormationFolded Mountain – Antarctica
Mountain Formation
Adirondack Mountains – unwarped
Mountain Formation- (Fault-Block)
Consequences of Plate Movements (1). Earthquakes
Transform faultsCompression and tension caused by stress of
plate movements -- SlippingFocus location
Rock is snapped or broken releasing ‘elastic’ energy
(2). TypesIntraplate (10%)
Away from plate boundaries New Madrid, Missouri
Interplate (90%)Plate boundaries
Transform plates (mild Eqs)Subduction zones (strong)
Earthquakes
Subduction Zones – Ring of Fire!
80% of all interplate EQs occur here
Powerful Interplate EQs and Tsunamis
(1). Coast of Sumatra Indian and Burma Plate collision Megathrust quake
100 billion tons of TNT
(2). Production of a Tsunami Quake took place in the Indian oceanAs subduction occurred
The seafloor bent as the other plate sankStress caused rock to snap and thrust
upwardsForce caused water to creat large wave30m +/- above sea level
Earthquakes – San Francisco 1906
Magnitude of Earthquakes –Richter Scale (1). Logarithmic scale
Each point represents a 10-fold increase in quake shaking strengthMeasures shaking Also indicates 30 fold increase in energy
output 1 thru 10
Examples 1906 San Francisco (8.2)Sumatran 2004 (9.0)
Richter Scale
Sumatra EQ and Tsunami
184,000 People died