Geoscience Reference
In-Depth Information
1. The generation of new plate material occurs by
seafloor spreading
; that is, new oceanic
lithosphere is generated along the active mid-ocean ridges (see Chapters 3 and 9).
2. The new oceanic lithosphere, once created, forms part of a rigid plate; this plate may
but need not include continental material.
3. The Earth's surface area remains constant; therefore the generation of new plate by
seafloor spreading must be balanced by destruction of plate elsewhere.
4. The plates are capable of transmitting stresses over great horizontal distances without
buckling, in other words, the relative motion between plates is taken up only along plate
boundaries.
Plate boundaries are of three types.
1. Along
divergent
boundaries, which are also called accreting or constructive, plates are
moving away from each other. At such boundaries new plate material, derived from
the mantle, is added to the lithosphere. The divergent plate boundary is represented by
the
mid-ocean-ridge system
, along the axis of which new plate material is produced
(Fig. 2.3(a)).
2. Along
convergent
boundaries, which are also called consuming or destructive, plates
approach each other. Most such boundaries are represented by the
oceanic-trench
,
island-arc
systems of
subduction zones
where one of the colliding plates descends
into the mantle and is destroyed (Fig. 2.3(c)). The downgoing plate often penetrates
the mantle to depths of about 700 km. Some convergent boundaries occur on land.
Japan, the Aleutians and the Himalayas are the surface expression of convergent plate
boundaries.
3. Along
conservative
boundaries, lithosphere is neither created nor destroyed. The plates
move laterally relative to each other (Fig. 2.3(e)). These plate boundaries are represented
by
transform faults
,ofwhich the San Andreas Fault in California, U.S.A. is a famous
example. Transform faults can be grouped into six basic classes (Fig. 2.4). By far
the most common type of transform fault is the ridge-ridge fault (Fig. 2.4(a)), which
can range from a few kilometres to hundreds of kilometres in length. Some very long
ridge-ridge faults occur in the Pacific, equatorial Atlantic and southern oceans (see
Fig. 2.2,which shows the present plate boundaries, and Table 8.3). Adjacent plates
move relative to each other at rates up to about 15 cm yr
−
1
.
