The arrival of plate tectonics was a scientific revolution: Plate tectonic theory states that the Earth's surface is broken into rigid lithospheric plates that slide on top of asthenospheric mantle. The boundary between these lithosphere and asthenosphere is based on rheology (typically defined by an isotherm ~1280°C- it is not a compositional boundary). Plate boundaries are mainly defined by seismicity (locations of earthquakes)- plate interiors can have EQs but much less frequently

A tectonic plate is made of oceanic lithosphere (which contains oceanic crust: the oldest is only 200 Myrs) and/or continental lithosphere (which contains continental crust: the oldest rocks are ~4 Gyrs old). Some plates are just oceanic lithosphere, some are both. Because asthenosphere flows, continents float like icebergs Basically reflects Archimedes principle

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Continental terminology An active margin is a continental margin that is also a plate boundary Example, west coast of N.A.) A passive margin occurs when continental lithosphere transitions into oceanic lithosphere w/o a plate boundary (example, east coast of N.A.) Because continents (=continental lithosphere) float and do not subduct, ages are as old as >3.8 Ga. Continents have two major parts: 1. Stable craton (basically no tectonic activity) which conatins the Precambrian Shield (oldest rocks), surrounded by stable Platform 2. Orogenic belts-recent mountain building at plate

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Remember: 3 types of boundaries 1) divergent 2) convergent 3) transform

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Geologists define three types of plate boundary, based simply on the relative motions of the plates on either side of the boundary. These basic types– divergent, convergent, and transform plate boundaries–are shown in this three-part animation.  PC version  Mac version PC version Mac version Basic Plate Boundaries Animation

DIVERGENT - solid mantle upwells to replace lithospheric plates that are moving apart. In oceanic lithosphere, this is a Mid-ocean ridge (most common) In continental lithosphere, this is continental rift (much less common) Features: Non-explosive volcanism (basalt) relatively small, mostly tensional earthquakes ~75% of Earth’s magma created at mid-ocean ridges Hydrothermal systems take heat from cooling magma-- producing black smokers--350°C and possibly important for initiation life

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Continental Rift Initiation

E. Africa-best known Cont. Rift

Oceanic crust forms around and above a steady-state magma chamber. As the animation progresses, gabbro forms on the sides, dikes form above, and pillows form at the Earth's surface. Note that although the ridge maintains a consistent size and shape, the sea-floor grows wider.  PC version  Mac version PC version Mac version Formation of Ocean Crust Animation

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Mid-ocean ridge rift zone through Iceland

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