Geology Chapter 2: Plate Tectonics - The Unifying Theory

Rayleigh number

- determines critical temperature for a fluid to convect - incorporates viscosity, thermal expansion, conductivity, gravity

How is Earth's magnetic field generated?

- self-exciting dynamo with a fluid metallic outer core in constant motion because of thermal convection due to Earth rotation - moving fluid passes through weak magnetic fields generated by the Sun, produces electric current, and therefore a more intense magnetic field

Continental vs. Oceanic Crust

1) because continental crust is lighter, it is not as easily recycled back into the mantle as oceanic crust 2) because continental crust is weaker, plate boundaries that involve continental crust tend to be more spread out and more complicated that those that involve oceanic crust

Three types of plate boundaries

1) divergent boundaries/constructive margins 2) convergent boundaries/constructive margins 3) transform faults/conservative margins

How many major plates are there?

13

How much new oceanic crust is added every year?

20km^3 added each year

How much of Earth's crust is underlain by oceanic crust formed at mid-ocean ridges?

70%

Hot spot

= surface expression of mantle plumes rising from deep level in mantle, maybe ~ 1,000 km diameter. These are driven by upwelling plume --> thins lithosphere and causes regional doming. This thinning of the lithosphere and regional doming by the Walvis Hot Spot may have contributed to the split of South America and Africa by causing the brittle crust to fracture. Could have been the initiation of continental rifting and the precursor to ridge formation/oceanic lithosphere spreading

Isochrons

A contour that connects rocks of equal age

Mantle plume

A narrow, cylindrical jet of hot, solid material rising from deep within the mantle, thought to be responsible for intraplate volcanism. This is the exception to the generally passive spreading process with slow and spread-out rising mantle currents.

Example of ocean and continent destructive margin

Andes (South America), Mount St. Helens (Cascades)

Transform Fault: Continental Transform Fault

At transform faults, plates slip horizontally past each other

How did scientists calculate how fast the seafloor was spreading?

By using the ages of magnetic reversals that had been worked out from magnetized lavas on land, they could calculate the relative plate velocity by using the formula speed = distance/time --> distance is measured from the mid-ocean ridge axis and time = seafloor age. Average spreading rate is 50 mm/year

How long are mid-ocean ridges?

Continuous~ 60,000 km long ridge system

Difference between fast and slow spreading ridges

Fast spreading: smooth topography, central ridge shield volcano, summit rift Slow spreading: rugged topography, broad central rift Illustrates that mid-ocean ridge morphology is dependent on spreading rate

Example of continent and continent destructive margin

Himalayas

How did geologists discover continental drift?

In the late 16th and 17th century, scientists began noticing the jigsaw-puzzle fit of the continents. In 1915, Alfred Wegener postulated that a super continent called Pangea had broken up to form the current day continents

Example of ocean and ocean destructive margin

Japan

How old are seafloor sediments?

Jurassic or younger (<200 million years)

How deep does plate recycling occur?

Lower boundary of the convection system could be as deep as 2890 km beneath Earth's surface

What is the driving force of plate movement

Mantle convection --> heat escaping from the Earth's deep interior causes this material to undergo convection (circulation upwards and downward)

Transform Fault: Mid-Ocean Ridge Transform Fault

Mid-ocean ridges are typically offset by transform faults

Magnetic anomaly

One in a pattern of long, narrow bands of high or low magnetic intensity on the seafloor that are parallel to and almost perfectly symmetrical with respect to the crest of a mid-ocean ridge. The magnetic field is imprinted in iron-rich materials of igneous rocks - mineral dipoles align with Earth's dipole

How was the rate of seafloor spreading determined?

Paleontologic and isotopic dating studies showed youngest rocks at ridge with symmetrical increase of age with distance from ridge

What is the principle way heat is lost from the Earth's interior?

Plate recycling

Convergent boundaries

Plate tectonic boundary where plates come together and one plate is recycled into the mantle (plate area decreases)

Divergent boundaries

Plate tectonic boundary where plates move apart and new lithosphere is created (plate area increases): Two types: oceanic spreading center & continental rift zone

Transform faults

Plate tectonic boundary where plates slide horizontally past each other (plate area does not change). These are only active in the offset between ridges

Alred Wegener

Postulated in 1915 that a super continent called Pangea had broken up to form the current day continents. This hypothesis was based on the jigsaw-puzzle fit of the continents and similar rock ages, trends in geologic structures, and fossil/climate data. His ideas were initially rejected since he couldn't explain the speed of the continents' movement and said tidal waves driving their movement were off.

What are relative plate motions?

Rapid: 18 cm/yr - East Pacific Medium: 3-5 cm/year - Mid Ocean Ridge Slow: 1.8 cm/yr - North Atlantic Avg rate = 50 mm/yr

Where was an example of magnetic anomalies seen?

Reykjanes Ridge - clear evidence of magnetic anomalies and thus seafloor spreading

Divergent Boundary: Oceanic Spreading Center

Rifting and spreading along a mid-ocean ridge (boundary between separating plates) create new oceanic lithosphere as magma wells up into the rifts

Divergent Boundary: Continental Rift Zone

Rifting and spreading zones on continents are characterized by parallel rift valleys, volcanism, and earthquakes. The continents have separated enough for new oceanic crust to form along the spreading axis, creating a deep basis into which the ocean has flooded

Names of active transform faults

San Andreas Fault (dextral) = transform boundary between N. American plate & Pacific plate Alpine Fault, New Zealand (dextral) Dead Sea Fault (sinistral)

How did magnetic anomalies support the seafloor spreading hypothesis?

Since iron-rich lava froze in the direction of the Earth's magnetic field (thermoremanent magnetization), it allowed scientists to determine the magnetic time scale of the past 200 million years and provided evidence that Earth's magnetic field does not remain constant over time. During the period of seafloor spreading, the newly magnetized magma would spread to reach side of the ridge forming two symmetrical magnetized bands, illustrating the period of divergence.

Magnetic time scale

The detailed history of Earth's magnetic field reversals as determined by measuring the thermoremenant magnetization of rock samples whose ages are known. Earth's magnetic field demonstrated by William Gilbert

Seafloor spreading

The mechanism by which new oceanic crust is formed at a spreading center on the crest of a mid-ocean ridge. As two plates move apart, magma wells up into the rift between them to form a new crust, which spreads laterally away from the rift and is replaced continually by newer crust. Evidence: - WWI when echo-sounding devices revealed that the ocean floor was rugged - WWII sonar technology mapped the seafloor and discovered large seismically-active mountain chain - 1947 - sedimentary layer on Atlantic seafloor discovered to be much thinner than thought despite the age of the Earth - seafloor found to be made of basalt, not granite - found magnetic stripes on ocean floor parallel with submarine mountain chains

Subduction

The sinking of the ocean lithosphere beneath over-riding oceanic or continental lithosphere at a convergent plate boundary

Plate tectonic theory

The theory that describes and explains the creation and destruction of Earth's lithospheric plates and their movement over Earth's surface

Oblique boundaries

These plate boundaries combine divergence and convergence with some amount of transform faulting

Walvis Hot Spot

This thinning of the lithosphere and regional doming by the Walvis Hot Spot may have contributed to the split of South America and Africa by causing the brittle crust to fracture. Could have been the initiation of continental rifting and the precursor to ridge formation/oceanic lithosphere spreading

How did ophiolites end up on land?

Were emplaced by tectonic forces at convergent boundaries and thrust above sea level

Convergent Boundary: Ocean-Continent Convergence

When oceanic lithosphere meets continental lithosphere, the oceanic lithosphere (as it is more dense) is sub-ducted and a volcanic mountain belt is formed at the continental margin

What is a triple junction?

Where mid-ocean ridge (R), continental rift (r), subduction zone (trench) (T), and transform fault (F) collide. These often migrate, its stability is dependent on its rate and direction of movement Ex. Afar Triple Junction (Red Sea (R), Aden Ridge (R), East African Rift (r))

Convergent Boundary: Ocean-Ocean Convergence

Where oceanic lithosphere meets ocean lithosphere, one plate is subducted under the other, is recycled by the mantle convection system, producing a deep-sea trench. As the cold lithosphere descends into the Earth's interior, pressure on it increases, pushing water out of it and into the asthenosphere which cause the mantle material to melt and resulting magma produces a chain of volcanoes, called an island arc.

Convergent Boundary: Continent-Continent Convergence

Where two continents converge, the crust crumbles and thickens, creating high mountains and a wide plateau

Active margin

abrupt transition, continent and ocean on different plates --> trenches. Usually the site of tectonic activity

Ring of Fire

area where large number of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean - created from subduction zones

How often do magnetic flips occur?

every 500,000 years; every flip takes about 1,000 years

Transcurrent fault

fault must stretch to infinity

accretionary wedge/prism

formed from sediments that are accreted onto the non-subducting tectonic plate at the convergent plate boundary

Active rifting

initiated by asthenospheric plume

Passive rifting

initiated by tensional forces in lithosphere --> response is upwelling of asthenosphere --> later uplift of rift flanks --> volcanism --> doming/fault movement --> rifting

Continental drift

large-scale movements of continents

Passive margin

smooth transition, continent and ocean part of the same plate - limited tectonic activity, most dominated by weathering and erosion