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Divergent Boundaries
Divergent boundaries occur between two plates moving away
from each other. Magma forms as the plates separate, decreas-
ing pressure on the mantle below. This molten material from the
asthenosphere rises, cools, and adds new crust to the edges of
the separating plates. The new crust tends to move horizontally
from both sides of the divergent boundary, usually known as an
oceanic ridge. A divergent boundary is thus a new crust zone.
Most new crust zones are presently on the seafloor, producing
seafloor spreading (Figure 18.14).
The Mid-Atlantic Ridge is a divergent boundary between
the South American and African Plates, extending north between
the North American and Eurasian Plates (see Figure 18.13). This
ridge is one segment of the global mid-ocean ridge system
that encircles Earth. The results of divergent plate movement
can be seen in Iceland, where the Mid-Atlantic Ridge runs as
it separates the North American and Eurasian Plates. In the
northeastern part of Iceland, ground cracks are widening, often
accompanied by volcanic activity. The movement was measured
extensively between 1975 and 1984, when displacements caused
a total separation of about 7 m (about 23 ft).
The measured rate of spreading along the Mid-Atlantic Ridge
ranges from 1 to 6 cm per year. This may seem slow, but the pro-
cess has been going on for millions of years and has caused a tiny
inlet of water between the continents of Europe, Africa, and the
Americas to grow into the vast Atlantic Ocean that exists today.
Another major ocean may be in the making in East Africa,
where a divergent boundary has already moved Saudi Arabia
away from the African continent, forming the Red Sea. If this
spreading between the African Plate and the Arabian Plate con-
tinues, the Indian Ocean will flood the area and the easternmost
corner of Africa will become a large island.

FIGURE 18.12 Formation of magnetic strips on the seafloor. Convergent Boundaries
As each new section of seafloor forms at the ridge, iron minerals Convergent boundaries occur between two plates moving
become magnetized in a direction that depends on the orientation toward each other. The creation of new crust at a divergent
of Earth’s field at that time. This makes a permanent record of
reversals of Earth’s magnetic field. boundary means that old crust must be destroyed somewhere
else at the same rate, or else Earth would have a continuously
expanding diameter. Old crust is destroyed by returning to the
asthenosphere at convergent boundaries. The collision pro-
LITHOSPHERE PLATES AND BOUNDARIES duces an elongated belt of down-bending called a subduction
The strong evidence for seafloor spreading soon led to the zone. The lithosphere of one plate, which contains the crust, is
development of a new theory called plate tectonics. Accord- subducted beneath the second plate and partially melts, then
ing to plate tectonics, the lithosphere is broken into a num- becoming part of the mantle. The denser components of this
ber of fairly rigid plates that move on the asthenosphere. Some may become igneous materials that remain in the mantle. Some
plates, as shown in Figure 18.13, contain continents and part of of it may eventually migrate to a spreading ridge to make new
an ocean basin, while other plates contain only ocean basins. crust again. The less dense components may return to the sur-
The plates move, and the movement is helping to explain why face as a silicon, potassium, and sodium-rich lava, forming vol-
mountains form where they do, the occurrence of earthquakes canoes on the upper plate; or they may cool below the surface
and volcanoes, and in general the entire changing surface to form a body of granite. Thus, the oceanic lithosphere is being
of Earth. recycled through this process, which explains why ancient sea-
Earthquakes, volcanoes, and most rapid changes in Earth’s floor rocks do not exist. Convergent boundaries produce related
crust occur at the edge of a plate, which is called a plate bound- characteristic geologic features depending on the nature of the
ary. There are three general kinds of plate boundaries that materials in the plates, and there are three general possibilities:
describe how one plate moves relative to another: divergent, (1) converging continental and oceanic plates, (2) converging
convergent, and transform. oceanic plates, and (3) converging continental plates.

464 CHAPTER 18 Plate Tectonics 18-10