The Layered Earth, Continental Drift and Plate Tectonics

LAST MODIFIED: 3 October 2018: Material on Layered Earth moved from Lecture 3

POSTED: 26 September 2018

• Material Properties: Elastic, Ductile and Brittle

Elastic, ductile and brittle describes a material's reaction when a force (or stress) is applied to it.
elastic material
• can be bent without breaking
• returns to original state after bending force is removed
• example: a metal spring
• stores potential energy (e.g. a loaded spring)
ductile material
• also bends without breaking but deformation is plastic, not elastic
• does not return to original state after bending force is removed
• example: putty, play dough
• no storage of potential energy
brittle material
• may be elastic when force is applied slowly
• breaks if force is applied rapidly or if force is greater than the elasticity of the material allows
• may store potential energy (when force is acting slowly)
• potential energy released when material breaks

• The Layered Earth

• radius: 6371 km; diameter (without atmosphere): 12742 km
• circumference: ~40000 km
• dense, iron-rich core (radius: 3480 km; diameter: 6960 km); composition similar to metallic meteorites
• solid inner core (radius: 1222 km; diameter: 2444 km)
• liquid outer core (mostly iron), movement of electrically charged particles cause magnetic field
• core-mantle boundary: at 2891 km depth; transition from liquid iron to solid rock
• mantle around core; composition similar to chondritic, stony meteorites (peridotite)
• transition zone between upper and lower mantle: 410-660~km depth
• weak, ductile asthenosphere above 350~km
• strong, brittle lithosphere above about 125~km
• crust above 70~km; very brittle; rich in silicates (SiO2-rich rocks)
• the 3 layer model (layers by material): core (metal), mantle (less silicic rock), crust (more silicic rocks)
• the 5 layer model (mechanical properties): inner core (solid; mushy), outer core (liquid), mesosphere (solid; viscous mantle), asthenosphere (solid; soft mantle), lithosphere (solid; rigid mantle+crust)

• The Lithosphere and Asthenosphere; Earth's Layers relevant to Plate Tectonics

Lithosphere
• from Greek "lithos" for rock
• crust and uppermost mantle
• cool and strong but brittle
• zone of earthquakes (EQs)
• broken into 12 major plates and a few others
• oceanic: thin but dense
• continental: thick and less dense

Asthenosphere
• from Greek "asthenes" for weak
• uppermost mantle below lithosphere
• warm, soft and weak
• ductile

• Isostasy

principle of buoyancy
• 2 counteracting forces: gravity and buoyancy
• isostatic equilibrium: forces are balanced and body floats
Relevance to Earth
• lithoshere floats on the asthenosphere
• lithosphere less dense than asthenosphere
• rigid lithosphere floats on soft asthenosphere like icebergs in water
• floating continents have roots/keels beneath
• asthenosphere reacts to imbalance by flowing (e.g. like viscous honey)
Example 1:
• oceanic plate cools and thickens with age
• gets denser, eventually losing buoyancy
• sinks into Earth's mantle
• drives mantle convection and plate tectonics
• use of potential energy to drive plate tectonics
Example 2 (Load through Ice Sheets during Glaciation):
• ice sheet adds mass added on top of lithosphere
-> asthenosphere beneath flows away
-> lithosphere sinks
• elastic rebound after ice melts (postglacial rebound)

• Continental Drift - Seafloor Spreading - Plate Tectonics

Alfred Wegener first noticed that the continents fit like pieces from a jigsaw puzzle, but there are also other features across continents that suggest that they were once joined (see next section). In 1912, Wegener formulated the theory of continental drift. However, he failed to provide a mechanism for why continents drift. In 1960, Harry Hess first reported on volcanically active mid-ocean ridges that he first proposed in the 1950s (see "Sea Floor Spreading" below). The idea was born that lithosphere could be formed along these ridges, and the new material could push plates apart. The ultimate proof of this was the discovery of "magnetic stripes" on the seafloor later in the 1960s: the magnetic domains in oceanic rocks recorded reversal of Earth's magnetic field over time. The pattern was symmetric to the ridge, supporting the idea of symmetric seafloor spreading. We know from other geophysical observations (e.g. length of day) that Earth is not expanding so lithosphere has to be consumed somewhere else. The idea of subduction zones was born when deep sea drilling and dredging (picking up rock in a basket lowered to the seafloor) did not find any lithosphere that is older than 200 Mio yrs. With plate tectonics we have a theory that explains Wegener's observations and how lithosphere can be produced and consumed so that Earth does not change its size (expands or shrinks).


• Initial Observations - Continental Drift

first postulated by Alfred Wegener in 1915 continents fit like pieces of jigsaw puzzle fossil records match at continental boundaries across oceans glacial deposits, coal and other geologic units match at continents boundaries across oceans apparent polar wander curves don't match between continents

A cool video explaining Continental Drift and its fight for acceptance in the scientific community can be found on YouTube: www.youtube.com/watch?v=MEh4B1Pv8YE.


• Earth's Magnetic Field

• here, helps to support continental drift
• seafloor-spreading magnetic stripes provide ultimate proof of plate tectonics

some basics on Earth's magnetic field:
• looks approximately like that of bar magnet, with N and S poles (dipole field)
• description of the field through three parameters:
  • intensity: local strength of magnetic field
  • declination: local angle of magnetic field line with respect to Geographic North
  • inclination: local angle of magnetic field line with respect to horizontal
• convection currents in metallic outer core (mostly Iron) sustains Earth's magnetic field (magnetic dynamo); movement of free metallic electrons cause an electric current that induces a magnetic field
• currents in core change over time, so magnetic field also changes
• in extreme cases, magnetic field flips over time (reverses)
• magnetic reversals generally do not cause mass extinctions
• when rock freezes from melt, it is able to record current orientation of magn. field
• a stack of rock layers of different age "records" Earth's magnetic field over time

some useful things to know about Earth's magnetic field
• true Magnetic North changes dramatically over time due to variations in Earth's magnetic field; apparent polar wander curves therefore show a combination/superposition of the movement of a continent and the wander of Magnetic North; scientists study apparent polar wander curves and try to separate the two effects
• In 2000, magnetic North was at 81.5 deg N, 111.4 deg W. Magnetic North is moving at about 15km per year. The current declination in Southern California is about 13 deg E, which means that true north is 13 deg counterclockwise from what your compass indicates as North. If this is not accounted for when you go hiking, your error in the east direction is 0.25 mi for every mile going north.
• GPS measures Geographic North. It receives a radio signal from at least 4 of currently 27 satellites in orbit. At least 4 satellites are needed for trilateration, the determination of your geographic position and altitude.

• Apparent polar wander curves

An apparent polar wander curve (also called apparent polar wander path) displays the apparent position of Magnetic North over time. Here is how these curves are compiled:
• measure inclination and declination in subsequent layers (strata) of a geologic profile at one location
• determine age of layers (either by comparing fossils or radioactive dating method); the youngest layer is most likely on the top
• for this one location, construct polar wander curve
• construct polar wander curves for different continents
• try to match different curves
• if curves match, then continents were together for this time period
• if curves of all continents match, then they were together in one supercontinent (e.g. Pangaea) and the curve shows either the drift of this supercontinent or the true polar wander, i.e. how much Magnetic North changed over time. Most likely, a mix of both.

• The Proposal for Sea Floor Spreading

• the mechanism that is needed to explain why continents move
• postulated by Harry Hess in the 1950s
he observed that
• sea floor deepens away from mid-ocean ridges (MORs) -> sea floor "settles" over time?
• sediments thicken away from MORs -> MORs younger?
• heatflow is greater at MORs than away from MORs -> MORs let heat out?
• dredging (gathering rock samples from ocean floor) revealed that oceanic rocks are different from continental rocks
• earthquakes along MORs -> young ocean cracks?
the idea then is that new oceanic rock is formed at MORs that then moves away from MORs as it cools.

• Proof for Sea Floor Spreading

The final pieces of evidence for sea floor spreading came from magnetic stripes on the seafloor, together with ocean deep drilling:
• mapping magnetic anomalies (stripes) on the sea floor
• Earth's magnetic field reversed throughout its history (see above)
• ferromagnetic domains within partially molten rock can align with the ambient magnetic field; when rock cools below Curie temperature (about 500 deg C), the ferromagnetic domains retain direction of magnetization (i.e. they can no longer change)
• a rock magnetized in the direction of current field enhances the local magnetic field; a rock magnetized in the opposite direction (e.g. magnetized during a reversal) weakens the local magnetic field; the magnetization in the rock is called remnant magnetization
• magnetometer towed by a ship over the ocean floor records changes in magnetization from which remnant magnetization can be deduced
• alternating remnant magnetization over time period of several reversal of Earth's magnetic field causes "magnetic stripes" in record
• stripes are symmetric with respect to mid-ocean ridge axis
• get age of seafloor from dredging or ocean drilling (see below)
conclusion: new crust is formed at MOR that then moves away from MOR as it cools and ages
• ocean deep drilling
• drill into oceanic crust and determine age of basement rock
• oceanic crust away from MOR are older than near MOR
• no oceanic rock older than 200 Mio yrs (vs. oldest continental rocks are almost 4 Billion years old)
• determine ages of rocks in drill core and relate ages to magnetic reversals
conclusion: oceanic rocks must be consumed somewhere -> subduction zones


• Subduction Zones

• zone of deep trenches along some coasts (deepest places on Earth, up to 11 km below sea level)
• zone of earthquakes and volcanoes
• zone of accretionary prisms: triangular features consisting of sediments and pieces of Earth's crust; accretionary prisms can be likened to piles that are pushed together by a bulldozer

There are many good animations on YouTube. Here is a link to a BBC clip on mid-ocean ridges, subduction zones and mantle convection in general: http://www.youtube.com/watch?v=ryrXAGY1dmE
A few careful words on the movie narration though:
• plate tectonics is driven primarily by slab pull (see below), and only secondarily by the heated rising magma coming out at mid-ocean ridges
• in the subduction zones, water is subducted, together with other volatiles such as CO₂ that is mentioned in the clip
• it is the subducted water that makes the rock from the subducting slab melt (by lowering the rock's melting temperature), and only secondarily the heat (by increasing the temperature of the subjecting material); this topic is covered in the volcano lectures
• it is true that CO₂ comes up through a volcano and is recycled back into the atmosphere

• The Life Cycle of Oceanic Lithosphere

• New lithosphere is formed along mid-ocean ridges
• the new material pushes the two plates on either side apart -> seafloor spreading
• near the ridge, the new lithosphere is warm, has low density and is very buoyant
• then lithosphere cools and density increases; also material "grows" to bottom, so plate is thickening
• as oceanic plate becomes older, it eventually becomes so dense that it can't float on the asthenosphere any longer
• the plate starts to sink/subduct
• no oceanic lithosphere is older than 200 Mio yrs


• a plate usually consists of both continental and oceanic lithosphere
• only oceanic lithosphere subducts
• continental lithosphere too buyonant for subduction
• when two plates with oceanic lithosphere come together, the denser one is forced to sink
• the subducting part of a plate is also called a slab
• >when all old oceanic lithosphere of a plate is consumed, subduction stops and plate tectonics rearranges
• example 1: Himalayas: mountain building along convergent plate boundary after subduction stopped. This is now a continent-continent collision and along this type of convergent plate boundary, not subduction occurs. Instead, we find massive mountain building. Maps in some textbooks show a subduction zone at the front of the Himalayas. This suggests that the Indian continent still subducts. This is not the case. The "subduction zone" is a remnant of when the leading piece of the Indian plate was oceanic. This oceanic lithosphere is now gone on the surface but can be traced inside earth by seismic imaging. These remnants still sink beneath the Himalayas.
• example 2: the ancient Farallon plate and the San Andreas Fault 30 Mio years ago, the plate boundary system along the west coast of North America looked very different from today's; an oceanic plate (the now gone Farallon plate) subducted beneath North America where the overall movement was such that the mid-ocean ridge slowly approached the coast; consequently, there was volcanism along a stretch marked by today's Sierra Nevada. By the time the MOR reached the continent (about 15 Mio years ago), subduction stopped because MORs are too buoyant to subduct. Continued plate movement to the north and south forced the plate boundary along California's coast to go over into a transform boundary, with subduction still continuing in the north in the Cascadia subduction zone (the subducting Juan de Fuca plate is a remnant of the Farallon plate), and sea floor spreading in the opening Gulf of California in the south.



• Plate Tectonics

• lithosphere is broken into several plates (12 major plates) that move about (a few cm per year)
• new crust is formed at MORs
• old crust is consumed at subduction zones
• both MORs and subduction zones are locations of EQs and volcanism
• in general: EQs and volcanoes mark plate boundaries; i.e. plates interact at boundaries
• globally, new crust is formed and old crust is consumed at same rate
• i.e. Earth does not expand


"the grand unifying theory of geosciences that explains"
• movement of continents
• earthquakes
• volcanism
• most major features on Earth's surface, incl. mountain building
• formation of new lithosphere
• consumption of old lithosphere

• The 2 Forces that Drive Plate Tectonics

mantle convection drives movement in the interior but is not the major driving mechanism to move plates about slab pull: the more dominant force; dense subducting slab pull downward ridge push: the less dominant force; buoyant magma at ridges push plates apart

• Plate Boundaries

• divergent
• rifting
• mid-ocean ridges
• continental rifts
• usually smaller, shallow earthquakes
• usually benign volcanism, especially when oceanic plates are involved
• convergent
• ocean-ocean subduction (e.g. Aleutians, Japan, Tonga-Fiji, parts of Indonesia)
• ocean-continent subduction (e.g. Nazca/South America, Juan de Fuca/North America, Central America, New Zealand)
• continent-continent collision/no subduction! (e.g. Himalayas, Alps, Zagros)
• large earthquakes, some deep
• EQs in oceans can cause tsunami
• violent volcanism
• transform
• plates slide past each other (e.g. San Andreas Fault/SAF, Dead Sea Fault, North Anatolian Fault)
• moderate-to-large earthquakes
• no volcanism
NB: A transform boundary is called transform because is connects one type of boundary with another type, e.g. a divergent along a mid-ocean ridge with a convergent in a subduction zone. The SAF connects the mid-ocean ridge system in the Gulf of California (divergent) in the south with the Cascadia subduction zone in the north (convergent).

• Plate Boundaries in a Map

  	A subduction zone is marked by a barbed line. The arrows, which show the direction of subduction, are attached to the subducting plate.
  	A spreading center or rift is often marked by a double line and double-sided arrows that mark the direction of spreading.
  	A transform boundary is marked by a simple line. Sometimes, arrows on each plate mark in which direction they move.

• Hot Spots and Mantle Plumes

• basic idea: mantle conduits remain fixed while plates above move
• explain intra-plate volcanoes (e.g. Hawaii, Yellowstone)
• form new volcanoes when plate moves over plume
• form hotspot tracks parallel to direction of plate motion
• can be used to track plate motion

• Earthquakes and Volcanoes and Plate Boundaries

• most earthquakes (EQs) and many volcanoes along plate boundaries
• deep EQs only behind trenches along subduction zones
• along trenches, EQs occur in a down-dipping band (Wadati-Benioff zone) that marks a sinking slab; EQs occur mostly near the top of the slab
• deep earthquakes occur to a maximum depth of 670km
conclusion: crust is consumed in trenches/subduction zones; below a certain depth, plates are no longer brittle

1. weak and ductile
2. warm and soft
3. below the lithosphere
4. above the lithosphere

1. gravity and friction
2. friction and stress
3. gravity and buoyancy
4. buoyancy and potential

1. true
2. false

1. mantle convection
2. a bar magnet at the Earth's center
3. currents in the liquid outer core
4. convection in the solid inner core

1. 2000 years
2. 2 million years
3. 20 million years
4. 200 million years
5. 2 billion years

1. continent-continent convergent
2. ocean-continent convergent
3. ocean-ocean convergent
4. all of the above
5. b) and c)