Plate Tectonics

Tectonics on a Sphere - Key Concepts

• Lecture summary includes earth structure (main units), earthquake distribution, plate tectonics (assumptions and plate boundary types), plate boundaries on flat and spherical Earth, determining rotation poles/vectors, plate boundary evolution, triple junctions, and absolute plate tectonics.

Earth Structure: The Main Units

• Earth's compositional layers include the crust, mantle, and core.
• Earth's rheological (physical) layers are the lithosphere, asthenosphere, and mesosphere.
• Plate tectonics explains the function of Earth's uppermost layer.
• The crust is a product of mantle melting with higher magnesium/iron ratio and less silicon/aluminum than mantle rocks.
• Lithosphere is rigid over geologic time scales while asthenosphere deforms in a ductile manner.
• Oceanic and continental are the two types of lithosphere which make up tectonic plates that move relative to each other
• The lithosphere and asthenosphere form the upper mantle.
• Extending from the 660 boundary to the outer core, the mesosphere corresponds to the lower mantle.

Asthenosphere Phase Changes

• At 410 km depth: Mg, Fe, Si, and O are primarily within olivine and pyroxene.
• Below 410 km depth, olivine becomes unstable and is replaced by higher density polymorph—spinel.
• The material below 410 km has a similar overall composition, but the minerals have a more compact structure.
• At 660 km depth: Spinel gives way to Mg-perovskite and Mg-wustite.
• Mg-perovskite is the most abundant solid on Earth, and it appears stable through much of the mantle.

Earthquake Distribution and Belts

• Earthquakes are organized along distinct belts.
• The circum-Pacific seismic belt (ring of fire) is the world's greatest earthquake belt located along the rim of the Pacific Ocean.
• The second most important seismic belt, the Alpide belt, spans from Sumatra through the Himalayas and the Mediterranean, and out into the Atlantic.
• A third prominent belt is the mid-Atlantic belt, that is splitting Africa apart.
• Most of the world's volcanic activity is concentrated along earthquake belts.
• Seismic belts divide the earth's surface into plates

Key Elements of Plate Tectonics

• Pangea is the super continent formed by plate tectonics
• The process is attributed to Alfrado Wegener
• Seven major lithospheric plates include the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates.
• New plate material is generated at mid-ocean ridges through seafloor spreading (divergent plates), forming igneous rocks from magma.
• The oceanic lithosphere that is formed becomes part of a rigid plate.
• To maintain a constant Earth surface area, seafloor spreading must be balanced by plate consumption elsewhere.
• Plate motion is accommodated along plate boundaries, which are vanishing, and are measured.

Divergent Boundaries

• Plates move apart, magma rises and cools to form lithosphere.
• Divergent boundaries are typically expressed as mid-oceanic ridges, also called accreting or constructive.
• At divergent boundaries, new plate material derived from the mantle is added to the lithosphere, new oceanic crust is formed at the crest of an oceanic ridge.
• Two or more plates pull apart and molten material rises through a rift zone, with the newest magma at the rift, newer crust travels equally from the center, and oceanic crust records reversed and normal polarity.
• Seafloor spreading leads to new crest formation, the seafloor grows older with distance from the rift zone, cools, becomes denser, and is buried under sediments

Continental Rifting

• Continental rifting occurs when tensional forces acting on the lithosphere thin it, promoting upwelling in the mantle, potentially leading to the formation of a long, narrow sea.

Convergent Boundaries

• Plates move toward each other, forming mountain belts and volcanoes.
• Oceanic plates may sink into the mantle along a subduction zone, typically marked by a deep ocean trench.
• Subduction zones and areas of intense volcanic and earthquake activity make up destructive boundaries, as plates move toward one another
• Oceanic plates move toward each other with the denser plate undergoing subduction.
• The plate that is subducted takes water with it which is the origin of the volcano formation since liquidus rock melts easier
• Continental collisions occur when lithosphere caps overlap
• The downgoing plate in a convergent boundary often penetrates the mantle to about 700 km.

Transform Boundaries

• Plates slide past one another, creating fault zones and earthquakes that mark the boundary like the San Andreas Fault in California.
• Transform Boundaries Create big earthquakes because friction is the source of the energy released
• They are also called conservative boundaries.

Relative Velocity

• Plate motion can be described using relative velocities
• Relative velocities are used to describe how fast and in what direction plates are moving in relation to each other near plate boundaries
• The barbed line is the symbol for a subduction zone with plates moving relative to each other.
• The barbs are on the side of the overriding plate, pointing away from the subducting or down-doing plate.
• The single line is the symbol for a transform (conservative) boundary
• All lines are subject to magnitude and the velocity of plate motion

Plate Tectonics on a Sphere: Rotation Axes and Poles

• Plate movement on Earth's surface can be described by viewing the surface of the planet as a sphere
• Euler's fixed-point theorem states every displacement from one position to another on the surface of the earth can be regarded as a rotation about a suitably chosen axis that passes through the center of the earth.
• The axis of rotation is chosen appropriately to pass through the center of the earth and reference displacement
• Poles of rotation/Euler's poles mark where the axis of rotation cuts through Earth's surface.
• Poles of rotation are mathematical points whose positions describe all point's direction of motion along a plate boundary.
• The sign convention is that a clockwise rotation is positive from the Earth's center while anticlockwise is positive from outside Earth, with one pole positive and the other negative.
• The maximum relative velocity for the same place
• Angular velocity and relative motion have been related
• Rotation vectors describe the present-day motion of plates on Earth's surface

Determining Rotation Poles and Vectors

• Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates can be found by:
  • The strike of active transform faults.
  • Changes in spreading rate along a constructive boundary.
  • Data analysis from an earthquake.
  • Surveys of displacement across cross land plate boundaries using satellites.

Surveys of Displacements

• Surveys of displacement can be used where plate boundaries cross land to determine the local relative motions
• Displacement surveys measure rates of change through space and time
• Stream channels, roads, field boundaries and buildings may be displaced (for example).
• Satellites make it possible to accurately measure instantaneous plate motions:
  • Satellite laser-ranging system (SLR) determines differences in distance between two Earth sites over time.
  • Very-long-baseline interferometry (VLBI) is dependent on the sources known and baseline location
  • VLBI uses quasars as signal sources and terrestrial radio telescopes are recievers.
  • GPS (Global Positioning System) navigation is a global network of signal receivers

Satellite Laser-Ranging System (SLR)

• Measurement of distance (=range) between a ground station and a satellite = Satellite Laser Ranging (SLR)
• Ground station transmits a very short laser pulse from telescope to a satellite
• The laser pulse is retro-reflected by corner cube reflectors on the satellite back to the ground telescope
• Very precise clock at the ground station measures round trip time and measures Time measurement accuracy < 50 picoseconds, or < 1
• Three stations, one satellite position satellites if station is known
• Three satellites, one station positions station if satellite's orbit is known

Plate Boundaries: Changes Over Time

• Plates and plate boundaries do not stay the same for all time.
• Formation/destruction of plates are obvious causes for changing plate boundaries/motions.
• A plate may be lost down a subduction zone.
• Two continental plates may coalesce into one with resultant mountain building
• The shape of the boundary will directly effect the boundaries orientation and velocity
• If pole position changes, relative motions change.
• A drastic change in pole position (90°) would completely alter the status quo.

Triple Junctions

• Multiple plate boundaries and triple junctions exist on Earth. Triple junctions are points at which three plates meet.
• Stable triple junctions maintain relative plate motion and boundary azimuth.
• Unstable junctions exist temporarily before evolving
• 4+ plates at one point = always unstable so the system becomes two or more triple junctions instead.
• A ridge is written as R, a transform fault as F and a subduction zone (or trench) as T.
• Thus, a ridge-ridge-ridge junction is RRR, a fault-fault-ridge junction is FFR, and so on.
• Sixteen possible types of triple junction.
• One of these sixteen triple junctions is always stable (the RRR junction) if oblique spreading is not allowed.
• Two are always unstable (the FFF and FFR junctions).

Examples of Triple Junctions in Nature

• Azores: EUR, NOAM, and AFR
• Afar Region: African Rift Valley
• Riveras Triple Junctions: Where the North-American Plate meets the Cocos Plate and the Pacific Plate

Absolute Plate Motions

• There is no fixed point on the Earth's surface and plate motions are always related.
• Absolute plate motions describe the motion of plates relative to an imaginary fixed point.
• Hypothesizing that the Earth's mantle moves much more slowly than the plates describes absolute motions
• Absolute motions are calculated from the traces of oceanic island chains or continental volcanism by assuming the plate passed over a fixed heat source with respect to the mantle
• Processes that assume these traits include hotspot references, the measurement of 'hotspot tracks', the measurement of the rates of oceanic and continental plate transition