Interior Structure of Earth Compositional Layering (Chemical) Crust - variable thickness and...

Interior Structure of EarthCompositional Layering

(Chemical)• Crust - variable thickness

and compositionContinental 10 - 70 km thickOceanic 8 - 10 km thick• Mantle - 3488 km thick,

made up of a rock called peridotite.

• Core - 2883 km radius, made up of Iron (Fe) with some Nickel (Ni)

Layers of Differing Physical Properties

• Lithosphere - about 100 km thick (up to 200 km thick beneathcontinents), very brittle, easily fractures at low temperature.

• Asthenosphere - about 250 km thick - solid rock, but soft andflows easily (ductile).

• Mesosphere - about 2500 km thick, solid rock, but still capableof flowing.

• Outer Core - 2250 km thick, Fe and Ni, liquid

• Inner core - 1230 km radius, Fe and Ni, solid

Plate Tectonics

• rigid lithosphere (oceanic, continental and upper mantle) consists of numerous variable-sized pieces called plates.

• These plates move over the hotter and weaker semiplastic asthenosphere as a result of some type of heat transfer system within the asthenosphere.

Contributions of The Plate Tectonic Theory

The distribution of volcanoes along the edges of continents and around the rim of the Pacific Ocean (Ring of Fire)

The association of deep ocean trenches with volcanic mountain chains,

The presence of a huge undersea mountain range with a central valley, which encircles the globe (see image above),

The geographic distribution of mountain ranges (and their various ages and types of deformation),

The geographic distribution of earthquakes, which occur in lines, and deep earthquakes which occur along inclined planes,

The geographic distribution of certain types of fossils The distribution of certain types of sedimentary rocks which can

be used as paleoclimate (ancient climate) indicators, The age of the oceanic crust Sediment thickness distribution patterns in the ocean basins.

Geologists recognize three major types of plate boundaries/margins/zone:

• divergent, • convergent, and • transform

Divergent plate boundariesor spreading ridges

• plates are separating and new oceanic lithosphere is forming.

• boundaries are places where the crust is extended ,thinned, and fractured as magma, derived from the partial melting of the mantle, rises to the surface.

• As successive injections of magma cool and solidify, they form new oceanic crust

• Divergent boundaries most commonly occur along the crests of oceanic ridges—the Mid-Atlantic Ridge

Oceanic ridges are thus characterized

• by rugged topography with high relief resulting from the displacement of rocks along large fractures,

• shallow-depth earthquakes,

• high heat flow, and• basaltic flows or pillow

lavas.

Continental Rifting

Convergent Plate Boundaries• Two plates collide and the leading edge of one

plate is subducted beneath the margin of the other plate and eventually is incorporated into the asthenosphere.

• A dipping plane of earthquake foci, called a Benioff (or sometimes Benioff-Wadati) zone, defines a subduction zone

• Three types of convergent plate boundaries are recognized: oceanic–oceanic, oceanic–continental, and continental–continental.

Oceanic–Oceanic Plate Boundary

• Two oceanic plates converge• subducting plate bends downward to form the

outer wall of an oceanic trench.• A subduction complex, composed of wedge-

shaped slices of highly folded and faulted marine sediments and oceanic lithosphere scraped off the descending plate, forms along the inner wall of the oceanic trench.

• Volcanic Island Arc (commonly known as Island Arc) (arc is nearly parallel to the oceanic trench)

• Back-arc basin – tensional stress

• In those areas where the rate of subduction is faster than the forward movement of the overriding plate, the lithosphere on the landward side of the volcanic island arc may be subjected to tensional stress and stretched and thinned, resulting in the formation of a back-arc basin. This back-arc basin may grow by spreading if magma breaks through the thin crust and forms new oceanic crust

Oceanic–Continental Boundaries• oceanic and a continental plate converge• denser oceanic plate is subducted under the continental

plate• Volcanic Arc

• 45 deg. – average angle of oceanic lithosphere subducting

• Depend on buoyancy- a plate may descend at an angle as small as a few degree or at 90 deg. Into mantle

Spreading Center (divergent zone)

Angle of Descent Earthquake

near the subduction zone(eg. Peru – Chile)

Small (buoyant) Regions experience great earthquake

Far from the subduction zone

(eg. Marianas Trench)

Steeper angle of Descent Few strong earthquake

• magma generated by subduction rises beneath the continent and either crystallizes as large intrusive igneous bodies (called plutons) before reaching the surface or erupts at the surface to produce a chain of ANDESITIC volcanoes called a volcanic arc).

Continental–Continental Boundaries• Two continents approaching each other are

initially separated by an ocean floor that is being subducted under one continent.

INTERIOR MOUNTAIN BELT IS FORMED CONSISTING OF-deformed sediments and sedimentary rocks, -igneous intrusions, -metamorphic rocks,-fragments of oceanic crust.

EVIDENCE OF ANCIENT SUBDUCTION ZONES-igneous of andesitic composition-intensely deformed rocks, “mélange”-sediments and submarine rocks are folded, faulted, and metamorphosed into a chaotic mixture of rocks - -andesite lavas, -ophiolites (obducted)

Transform Plate Boundary• plates slide laterally past each other roughly parallel to the

direction of plate movement.• Conservative• movement between plates results in a zone of intensely

shattered rock and numerous shallow-depth earthquakes

• diagnostic features- displacement of rocks

• -large displacements in ancient rocks can sometimes be related to transformfault systems.

Hot Spots• Location of Columns of hot material rising through

mantle (plumes)• Source seems to be core-mantle boundary area (D"

layer) - low velocity zones in lower mantle• Hot spot fixed in position underneath moving

lithospheric plates• Islands associated with hot spots (island chains, mid-

ocean ridges, triple junctions)– Iceland (mid-ocean ridge)– Galopagos Islands (triple junction)– Island of Hawaii (mid-plate volcanic chain; hot spot

trace)

Driving Forces• Plate movement due to Earth's attempt to lose

internal heat– Conduction– Convection

• Possible driving mechanisms: convection, ridge push, slab pull, hot spot push– Thermal convection cells

• Hot mantle material rises, moves laterally, driving plates and cooling

• Cold mantle material sinks back into mantle• Seismic data shows that slabs penetrate into lower mantle

=> whole mantle convection• Difficult to make consistent convection model for

arrangement of plates (fit them on a globe)

• Ridge Push– Not really a "push," instead

it's gravity-driven sliding– Plate slide down sides of mid-

ocean ridge– Gravity gives the push, not

magmas at ridge• Slab Pull (most geologists now

believe this is the main mechanism)– Subduction driven by density– Denser (older) lithosphere

sinks back into mantle pulling plate behind it

– Convection of mantle driven by plate movements

• Hot Spots (Plumes)– Flow of material associated

with hot spot plumes drives plates

– Columns of rising material vs. convection cells

• The formation of many natural resources results from the interaction between plates, and economically valuable concentrations of such deposits are found associated with current and ancient plate boundaries.

• Many metallic mineral deposits such as Cu, Au, Pb Ag, Sb, and Zn are related to igneous and associated hydrothermal (hot water) activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits.

Economic Aspect

Economic Aspect