Tectonic and volcanic activity along the East Pacific Rise near Lat. 21#xB0;N is generally restricted to a 3#x2013;4#x2010;km#x2010;wide area centered over the rise axis. The East Pacific Rise is a medium#x2010;rate (60 mm/yr) spreading center characterized by modest (100#x2013;200 m) relief of hills and seapeaks across the crestal region that is typical for such spreading centers. Few tectonic features appear in an axial volcanic zone 600#x2013;1, 200 m wide characterized by fresh, glassy pillow basalt and little or no sediment cover. This volcanic terrain is commonly flanked by tectonic zones where older lavas are cut by numerous normal faults bounding horst and graben systems and open fissures; these tectonic zones are commonly of unequal width on each side of the central volcanic zone and locally may be absent on one side. Bottom photographs and visual observations from a manned submersible indicate that most faults and fissures in the tectonic zones are young. Farther than 2 or 3 km from the axial volcanic zone, recent tectonic activity appears limited to a few faults that bound linear abyssal hills with total offset, suggesting relatively minor extension, so that instrumentation to measure the rate of plate separation along the rise crest will have to span both the volcanic and tectonic zones. The total width of the active plate boundary is at least 20 km, although less than 10 of the separation of the oceanic plates is accounted for by fault displacement and open fissures observed in the tectonic zones and on adjacent rise flanks. The asymmetric widths of the extensional tectonic zones result from migration of the volcanic extrusive zone over time.
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