28 Origin and Morphology of Ocean Basins
The "ring of fire" around the Pacific is closely associated with the trenches:
volcanoes sit on top of the dipping earthquake planes, that is, the downgoing lithosphere. Of 800 active volcanoes, 75 % are in the "ring". Where the lithosphere descends before reaching the continents, the volcanoes fonn island arcs; where it descends under a continent (South America), mountain ranges are formed. The magmas
produced by partial melting of the downgoing slab mix with overlying materials on
their way up and form characteristic volcanic rock types, the andesites -named after
the Andes Mountains (Appendix, Fig. A 6.1).
According to the theory of sea-floor spreading, the trenches are produced by the
subduction of the sea floor. The descending lithosphere, some 100 km thick, finally
disappears into the asthenosphere, that is, the "soft" part of the upper mantle. The
descending lithosphere offers up materials to the continent, by the scraping off of
sediment, and by partial melting. These materials contribute to continental accretion,
that is to the growth of continents.
Much of such growth, apparently, also depends on the accretion of "terranes",
slabs of oceanic or continental-type crust which arrive at the trench, but refuse to go
down. Instead, they become part of the adjacent continent. The trench, now clogged,
must move seaward when this happens. Much of the west coast of the USA is thought
to consist of "terranes" moved in from elsewhere. The pieces of foreign real estate are
mapped as having dimensions on the order of 100 to 1000 km on a side.
Continental accretion is one way in which the endogenic forces oppose the wearing down of the continents by exogenic agents. Thus, the continued existence of
continents which rise high above the sea floor is intimately tied to the processes of
sea-floor spreading.
1.6 Fracture Zones and Plate Tectonics
We have earlier mentioned that the Ridge Crest is not continuous but segmented. It
occurs in more or less straight portions which are offset from each other. The consequence of such offset is that a lateral fault must fonn at the two ends of each crestal
portion (Fig. 1.12). Since there is motion along this fault during active spreading,
there are earthquakes on it. These earthquakes are shallow and define the active part
of the fracture zone, that is, the ridge-ridge transform fault. Beyond this active part,
the fracture zone is the frozen trace of the fault; the scarps subside as the sea floor
ages on both sides of the zone. These extensive linear zones have an unusually
irregular topography with large seamounts, steep-sided or asymmetrical ridges,
troughs, or escarpments (Fig. 1.3).
Some fracture zones connect the end of a ridge crest portion to a trench. These
zones are seismically active and constitute the third type of boundary that defines a
lithospheric slab or plate. The other two, of course, are spreading center and trench.
the fact that these boundaries form "plates" was first pointed out by J. T. Wilson in
1965. On the basis of earthquake distributions and first motion studies (that is, observing which way the ground moves upon initiation of a quake), it is possible to
outline a number of large lithospheric slabs dividing the surface of the globe. Each of
The "ring of fire" around the Pacific is closely associated with the trenches:
volcanoes sit on top of the dipping earthquake planes, that is, the downgoing lithosphere. Of 800 active volcanoes, 75 % are in the "ring". Where the lithosphere descends before reaching the continents, the volcanoes fonn island arcs; where it descends under a continent (South America), mountain ranges are formed. The magmas
produced by partial melting of the downgoing slab mix with overlying materials on
their way up and form characteristic volcanic rock types, the andesites -named after
the Andes Mountains (Appendix, Fig. A 6.1).
According to the theory of sea-floor spreading, the trenches are produced by the
subduction of the sea floor. The descending lithosphere, some 100 km thick, finally
disappears into the asthenosphere, that is, the "soft" part of the upper mantle. The
descending lithosphere offers up materials to the continent, by the scraping off of
sediment, and by partial melting. These materials contribute to continental accretion,
that is to the growth of continents.
Much of such growth, apparently, also depends on the accretion of "terranes",
slabs of oceanic or continental-type crust which arrive at the trench, but refuse to go
down. Instead, they become part of the adjacent continent. The trench, now clogged,
must move seaward when this happens. Much of the west coast of the USA is thought
to consist of "terranes" moved in from elsewhere. The pieces of foreign real estate are
mapped as having dimensions on the order of 100 to 1000 km on a side.
Continental accretion is one way in which the endogenic forces oppose the wearing down of the continents by exogenic agents. Thus, the continued existence of
continents which rise high above the sea floor is intimately tied to the processes of
sea-floor spreading.
1.6 Fracture Zones and Plate Tectonics
We have earlier mentioned that the Ridge Crest is not continuous but segmented. It
occurs in more or less straight portions which are offset from each other. The consequence of such offset is that a lateral fault must fonn at the two ends of each crestal
portion (Fig. 1.12). Since there is motion along this fault during active spreading,
there are earthquakes on it. These earthquakes are shallow and define the active part
of the fracture zone, that is, the ridge-ridge transform fault. Beyond this active part,
the fracture zone is the frozen trace of the fault; the scarps subside as the sea floor
ages on both sides of the zone. These extensive linear zones have an unusually
irregular topography with large seamounts, steep-sided or asymmetrical ridges,
troughs, or escarpments (Fig. 1.3).
Some fracture zones connect the end of a ridge crest portion to a trench. These
zones are seismically active and constitute the third type of boundary that defines a
lithospheric slab or plate. The other two, of course, are spreading center and trench.
the fact that these boundaries form "plates" was first pointed out by J. T. Wilson in
1965. On the basis of earthquake distributions and first motion studies (that is, observing which way the ground moves upon initiation of a quake), it is possible to
outline a number of large lithospheric slabs dividing the surface of the globe. Each of