EES215

Lecture 3

Plate Tectonics

Basic premise: The crust is made up of plates, which move independently of each other.

à Three types of boundaries:

(1) Divergent boundary: formation of oceanic crust at spreading centers (Mid-ocean Ridges; examples are Mid-Atlantic Ridge; East pacific Rise); basaltic volcanism; will be subducted in collision zones; oldest oceanic crust ~200 MA

(2) Convergent boundary: collision of plates results in subduction of one plate under another; formation of deep-sea trenches and mountain ranges (examples are Andes Mountains; Japanese Island Arc); typically, oceanic plate is subducted, resulting in differentiation of crustal material at subduction zones - andesitic volcanism; formation of island arcs; accretion of island arcs; in general, continental crust is not subducted.

(3) Tangential boundary: Movement of plates along each other; neither formation nor destruction of crust; no volcanism, but strong earthquake activity; example: San Andreas Fault in California.

Bowen’s reaction series     Fig. 1

Formation of crustal rock follows differential crystallization based on Bowen’s reaction series. Two types of igneous rocks: volcanic rocks form at surface of the earth, rapid cooling causes small mineral grains (examples: basalt; rhyolite); plutonic rocks crystallize within the crust à slow cooling allows formation of large crystals (examples gabbro; granite).

Geological cycle                Fig. 2

Evidence for sea floor spreading: magnetic stripes; bi-modal distribution of altitudes à two types of crust; topography of oceanic basins; age and magnetic record of sediments

Distribution of earthquakes       Fig. 3

Earthquakes occur mostly along plate boundaries; ‘Ring of Fire’ around Pacific Ocean is a prime example; deep earthquakes are restricted to subduction zones. Continental collision zones such as the India-Asia zone do not have clear subduction, severe earthquakes, but no volcanism.