At some margins, the plate are diverging (moving apart) -- Called rift zones on continental plates -- Called midocean ridges on oceanic plates New seafloor is being created at the midocean ridges This is the answer to Wegener’s dilemma..

The Mid- Atlantic Ridge D. S. Kelley et al., Nature (2001)

A mid-ocean ridge vent (“black smoker”) On-axis vent (~400oC) Black material is FeS (iron sulfide) Kump et al (2000)

Lost City ventfield Off-axis vent (~60oC) Carbonate chimney (8 m) Aragonite and brucite flange Lost City ventfield SEM picture of ‘b’ Filamentous microbial communities Kelley et al. (2001)

How do we know that the seafloor is spreading? Answer: We look at the magnetic patterns on the seafloor

Earth’s magnetic field -- Dipole field (N and S poles), like a bar magnet Kump et al (2000)

Where else does this happen? The magnetic field is not always oriented in the same direction. Every once in awhile, it reverses polarity, i.e. the north pole becomes the south pole, and vice versa.. Where else does this happen?

On the Sun.. Picture of the Sun’s corona showing the influence of magnetic field lines

The Sunspot Cycle Sunspot number goes from low to high and back again every 11 years Solar magnetic field reverses polarity on this same time cycle Magnetic field is created by the solar dynamo

“Butterfly Diagram From G. Albetti, The Sun, 2nd ed. (1969)

Earth’s magnetic field reversals occur over longer time scales and are not as regular Earth’s field is generated by the geomagnetic dynamo Definition: dynamo—generation of a magnetic field by motions within a convecting, conducting fluid

Question: In what parts of the solid Earth does convection occur?

What parts of the solid Earth convect? The outer core This is where the magnetic field is generated The mantle This is what drives plate tectonics

Geomagnetic Field Reversals Normal polarity Reversed polarity Brunhes Matayama Normal polarity Reversed polarity Gauss Gilbert 5 Ma (Epochs)

The geological record of magnetic reversals from lava flow on land Kump et al (2000)

Sea-floor spreading as indicated by magnetic stripes Kump et al (2000)

Internal structure of the Earth, the top 300 km Kump et al (2000)

Three types of plate boundaries (a) divergent (discussed already) (b) convergent (discussed next) (c) transform fault (discussed at the end) Does it make a difference what kind of plates are converging? Kump et al (2000)

Three types of convergent plate boundaries oceanic-continental: - oceanic lithosphere is denser. - subduction of a slab. - deep sea trenches and earthquakes (..Japan) oceanic-oceanic: important dehydration and decarbonization reactions. cycling of water and carbon (more later) continental-continental: continental crust is too buoyant to be subducted. crust crumbles and forms mountains Kump et al (2000)

Transform faults Danger: Earthquake zone! Kump et al (2000)

Final concept: The Wilson Cycle Supercontinents (like Pangea) form and break apart over about a 400-500 m.y. cycle Asia Pangaea (250 Ma) NA Africa SA

Final concept: The Wilson Cycle Supercontinents (like Pangea) form and break apart over about a 400-500 m.y. cycle Sequence of events: Supercontinent forms from collisions of smaller continents Heat builds up beneath the supercontinent, causing it to rift apart Small pieces of the supercontinent drift off in all directions Eventually they run into each other and form a new supercontinent