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