structure of the earth tectonics

Chapter 22ISCI 2001

Structure of the EarthTectonics

Structure of the Earth

Crust(1). Thin, brittle

Oceanic Basalt rock (dark); greater

densityContinental

Granitic rock (lighter); less dense

Less dense than mantle Floats

Mantle• (1). Most of the volume (82%)

and mass (65%)– Thickest layer

• (2). Rich in Si and O– Contains Fe, Mg, Ca (causes increased

density)• Density increased by weight of crust also

• (3). Hotter than crust– High pressure

• Caused by radio active decay of elements • Flow of heat from core

• (4). Regions – Upper Mantle – Lithosphere

• Stiff, cool – similar to crust (forms one layer together)

– Asthenosphere• Solid, plastic (solid flow)

• (5). Upper Mantle– Solid and rigid – Plastic but not as much as upper mantle

Core Consist of:

Metallic Fe2x dense as mantle

(2). Inner CoreSolid 4000 to 7000 C

Radioactive elements Earth development; matter hitting the surface Core sinking to the center

(3). Outer CoreFlows due to less pressure Rotation of Earth stirs up core material

Produces magnetic field (flowing electrical charge)

Plate Tectonics(1). Continents sit atop of tectonic ‘plates’

What are the plates made of?What is a continent?

(2). PlatesConsist of mantle and the crust (lithosphere) Plates move atop the asthenosphere (plastic)

Continents move because they are embedded into the plates

(3). Plate boundaries and continents Continents and plates do not have the same boundaries

(overlapping) Figure 22.18A lot of action occurs at the plate boundaries!

Plates

How were the Plates Discovered?

(1). Alfred Wegener Proposed the theory of “Continental Drift”

Continents are in motion; drifting over geologic time

Pangea (figure 22.8)Same fossils found in several different

continentsMatching rocks on both sides of Atlantic

Ocean

(2). Evidence for CDSeafloor SpreadingMagnetic Stripes

Pangea

Evidence for Continental Drift –Seafloor Spreading

(1). Magma flows out of breaks in lithosphereNew lithosphere is formed and old lithosphere

will be recycled back in trenches (22.16)Mid-ocean ridge

Pushes the continents in specific directions

Evidence – Magnetic Stripes (1). Lava contains Fe and magnetite (Fe and

O)Crystals are magnetic – line up with the

magnetic field of the EarthCrystals point North and south / cool and freeze and

become locked inContains a record of the history of the magnetic field of

the Earth Gives a striped, bar pattern (22.17)Alternates normal (today’s field) and reversed field from

years ago. Runs along the spreading seafloor

How do the Plates Move?(1). As magma is heated deep in the Earth it

moves upward via convection currents (22.21)Hot rocks from lower mantle move upward cool in the

upper mantle and returnHit the lithosphere and can crack it – seafloor

spreading Plates move atop ‘convection cells’

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)

Seafloor Spreading Rift Valley in Kenya

Mid-Atlantic Ridge

Convergent Boundaries

(1). When two plates collideOne plate usually subducts Most dense or oldest plates

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

Marianas Trench

Oceanic – Continental

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)

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

Continental – Continental Massive plate collisions (both granitic)

No subduction, why?Both have same density

Massive mountains are formedHimalayas

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

Faults Parts of a Fault Normal Faults

Dip-slip Reverse Faults Strike-Slip

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

(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)

(1). EarthquakesTransform faults

Compression 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

Tsunami

Sumatra EQ and Tsunami

184,000 People died

Surface and Ground Water

Watershed

Ocean Floor

Surface Water – Surface Processes

Glaciers