Plate Tectonics

Seismic tomography teleseismic imaging – using earthquake waves to image the Earth’s interior

Seismic Tomography

Difference in arrival time at point along a surface showing various wave types. In reality there are many different wave sets some of which pass through the core. These all lead to very accurate location of the quake.

Tomography : An MRI of the Earth

Seismic waves are sent deep into the Earth and geophysicists study the patterns when they come back to the sensors. A network of seismographs also has monitored world-wide earth quakes since 1960. The earthquakes provide a kind of mega-sonar that has provided geophysicists with compelling images of plates as they dive into the mantle at subduction zones.

What the mantle really looks like…. (Composite images outline cold, downgoing ocean plates as areas where seismic waves speed up).

– Physical properties framework: how does it behave mechanically (Lithosphere / Asthenosphere / Mesosphere / Outer Core / Inner Core

Compositional framework – what is it made of. Crust / Mantle / Inner Core / Outer Core

Earth’s compositional layers. Crust = enriched in Si Mantle = enriched in Fe, Mg

Subduction zones form crust through dehydration melting (water entrained by the down-going plate). Spreading ridges form crust by decompression melting.

Both types of crust are created by convection. Mantle plumes push the ocean floor apart at spreading ridges. Subduction zones form magma that rises to form volcanoes.

Crust consists of ‘continental’ and ‘oceanic’ crust.

Melting Processes:

The Earth’s interior gets hotter as you go down (this is known as the geothermal gradient).

Decompression melting – melting due to a lowering of lithostatic pressure. This creates ocean crust from mantle material leading to Fe, Mg enrichment Dehydration melting – melting due to the addition of water. This creates continental crust from partial melting leading to Si enrichment.

decompression

dehydration

Development of the Crust The crust is created at spreading ridges through decompression melting of the mantle (which is enriched in Fe and Mg) and at subduction zones through dehydration melting (which causes enrichment in Si, Na, Al, K)

The long term effects of exchange has made continental crust enriched in Si, Na, Al, K relative to ocean crust, which is relatively enriched in Fe and Mg.

mantle peridotite (high Fe, Mg; dense, heavy; primordial starting material)

Olivine – (Mg,Fe)2SiO4 Pyroxene – (Mg,Fe,Ca)xSi2O6

subduction

sea floor spreading

Ocean vs. Continental Crust Oceanic crust thinner (5 km), younger (< 200 Ma) and denser (more Fe,Mg). This is because it comes directly from the mantle which has more Fe and Mg

than the silica-rich crust.

Continental crust is thicker (15 km), older (< 3.8 Ga) and less dense (more Si, Al). This is because subduction magmas preferentially melt silica.

Ocean crust is typically darker because of its higher proportions of Fe and Mg. It is referred to as ‘mafic’ after magnesium (ma) and iron (Fe).

Continental crust has higher Si, Al, K) and is referred to as ‘felsic’ (after the mineral felspar.)

In the Beginning The issue of the age of Earth and evidence of significant forces acting on the crust contributed to the thinking that the surface of the Earth had unknown forces acting upon it.

"In the mountains of Parma and Piacenza multitudes of rotten shells and corals are to be seen, still attached to the rocks…” Leonardo Da Vinci

Clues Early cartographers noticed how continents naturally fit together.

Fossils and rock types were similar among distant continents.

1596 – Ortelius hypothesizes continental drift in his atlas, Thesaurus Geographicus, Benjamin Franklin was also a supporter.

Basalt – Nova Scotia Basalt – Morocco

Certain types of rocks can only be formed by glaciers.

Glossopteris

Pangea, The Supercontinent

Volcano in Italy Coal beds in Germany

Wavy layers in Switzerland Layers at different angles in England

Different ideas depended on what naturalists could see around them.

Earthquakes and Volcanoes Geologists and geographers saw that earthquakes and volcanoes are clustered together. They are also tend to be associated with prominent geographic features like mountain ranges or submarine trenches.

Magnetic Anomalies

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Basalt erupts from submarine volcanoes and records the orientation of the Earth’s magnetic field at the time of submarine eruption.

Olivine – (Mg,Fe)2SiO4 Pyroxene – (Mg,Fe,Ca)xSi2O6

Wegener’s Hypothesis PhD: Astronomy Title: Meteorologist Hobby: Record holding balloonist Controversial Theories: Continental drift (1915), the existence of the Jet Stream Death: got lost in a blizzard in Greenland the day after his 50th birthday.

Wegener knew that the continents had moved around based on his studies of plate reconstructions, but there was little evidence of whole continents actually moving. Wegener hypothesized the correct mechanism for how plates moved (convection) but scientists did not have the technology to ‘see’ evidence of this yet. Clearer images of the Earth’s surface would later show what was going on.

Shape of the Sea Floor

Bear Seamount

MidAtlantic Ridge

Shallow volcanoes with coral reefs that gradually subside.

H. Hess mapped the seafloor with sonar during WWII.

Volcanic chains

Linear valleys

Hess’ Transport Route

Philippines

Mariana Trench is the deepest part of the ocean (6.77 mi) and is part of a tectonically active region of the pacific ‘ring of fire’.

channels

volcanoes

The locations of earthquakes showed us that certain areas of the crust are dynamic interfaces. The Earth’s crust consists of large regional plates that are moving around on the surface.

The Earth’s plates are moved by Earth’s internal heat. As a mechanisms for dissipating heat and material, plate tectonics changes the Earth’s surface, making it a dynamic environment. Plate tectonics also creates a wide diversity of environments on Earth’s surface.

Plate tectonics is driven by mantle convection. Convection is density driven movement of a fluid or gas across a thermal gradient. The less dense (heated) mantle plumes rise and exert a lateral force on the plates. This moves the plates around. It also helps generate new crust in certain areas.

No other planet that we know of has plate tectonics.

Three Types of Plate Boundaries Divergent Margins – plates moving apart, new crust created Convergent Margins – plates moving together, crust is consumed Transform Margins – plates slipping past each other

Divergent Margins Plates can move away from each other due to divergent forces of mantle convection cells and/or the pull of other plate interactions. Divergent margins occur as mid-ocean ridges or continental rifts.

The fissures that form at divergent margins are called ‘rifts’. New crust is created at rifts in the form of magmas that erupt out of volcanoes, vents and other volcanic features.

mid-ocean ridge

continental rift

Rifts may start on land and then become ocean spreading ridges.

East Africa Afar Triple Junction and the Great Rift Valley: Lakes, rivers, unique volcanism and Earth’s next ocean

The nature of the rift geography and arid climate lead to unique erosion and sedimentation patterns in the Great Rift Valley. Due to these factors some of the oldest and best preserved hominid fossils were found here by Richard Leakey and others.

Iceland

Sea of Cortez is an active rift zone with a strong component of strike slip motion.

Active Rifts: Baja

Failed Rifts = Rivers The Mississippi River flows along the Reelfoot rift, a ‘failed’ rift, meaning it opened a little and then became dormant. Many of the world’s major rivers flow along failed or dormant rifts (e.g., Rio Grande, Amazon, Nile).

Mid-continent rift – ore, deposits, lake basins

Reelfoot rift – Mississippi River, New Madrid Fault zone.

Rio Grande

Passive Margins

Convergent The down-going plate goes under the over- riding plate.

3. continent – continent collision: Mountains

2. ocean-continent collision: volcanoes and earthquakes

1. ocean-ocean collision: volcanoes and earthquakes

Himalayas

Continent – continent collision creates large mountains. The Himalayas are so big relative to the Earth’s crust, they are collapsing under their own weight. The extreme elevation also leads to extreme erosion.

South America

Seismic tomography: what subduction really looks like, plate has 3D structure

Longest continuous exposed mountain range on Earth. Atacama Desert – highest, driest, desert on Earth

Japan (also Indonesia and the Philippines are island arcs where an ocean plate is subducting beneath another ocean plate.

South Sandwich Islands – underwater volcanic arc near Antarctica. Arcs are chains of volcanoes formed by subduction zones.

Mariana Trench: ocean – ocean subduction. Trenches are the surface expression of where ocean plates descend at subduction zones

New Zealand – Mixed Motion Alpine fault is bound on both ends by subduction zones between the

Pacific and Australian plates. 300 miles of lateral offset, 43 miles of shortening (or compression).

California Carrizo Plain

The transform margin is actually a fault zone.

15 – 20 m.y.o, 350 miles of lateral offset.

Hot Spot: the plate moves, and the mantle plume stays put.

Yellowstone is an example of a hot spot on land. The Yellowstone caldera has produced the largest known volcanic eruption on Earth.

Tectonic Cycle

Rifts form oceans with passive margins on both sides. The passive margins accumulate thick loads of sediment. The heavy sediment pushes down and initiates subduction anew.

Supercontinents Past and Future

Continents are constantly moving. Over Earth’s 4.5 billion year history, they come together into super continents and then move apart.