TECTONIC PROCESSES AND LANDFORMS - Discovering Physical Geography - page 338

Transform fault

Lebanon

Plate 2

Plate 1

Syria

Mediterranean
Sea

Continental crust

Lithosphere

Israel

Asthenosphere

Dead Sea

(a)

Jordan

Saudi
Arabia

Figure 13.7 Transform plate margins. (a) Plates that meet at
transform margins move horizontally past each other along trans-
form faults. (b) Satellite image of the Dead Sea Fault. This trans-
form fault occurs along the boundary of the Arabian and African
plates and extends from the Gulf of Aqaba on the southern tip of
Israel through the Dead Sea and north into Lebanon. The Arabian
plate is generally moving to the north, whereas the African plate
is moving in a southerly direction in this region.

Dead Sea
transform fault

Gulf of Aqaba

(b)

Plate Divergence
In some parts of the world, lithospheric plates move away from
one another in a process called plate divergence . This type
of movement occurs in regions where rising magma plumes
within Earth move upward and outward between plate fractures,
spreading Earth's plates apart in a process called rifting (or
divergence). A great place to see the process of rifting is on the
seafloor where magma extrudes through fractures in the oceanic
crust (Figure 13.8). This extrusion of magma produces a ridge-
like feature, appropriately called a mid-oceanic ridge , which
lies parallel to the rift zone. The Mid-Atlantic Ridge is perhaps
the best-known oceanic ridge and is located, as its name sug-
gests, at the middle of the Atlantic Ocean along the Atlantic
Rift Zone. If these plates are diverging, what do you suspect is
happening to the width of the Atlantic Ocean? The answer is
that it is slowly increasing every year. This process began when
Pangaea began to break apart about 200 million years ago, and
it continues to this day.
Plates can also diverge within continents, causing a gradual
split in the landmass to occur. One of the best-known locations

Oceanic crust
forms as magma
cools and lithifies

Rift
zone

Spreading
ridge

Lithosphere

Asthenosphere

Magma rises
from mantle

Greenland

North
America

Europe

Mid-Atlantic
Ridge

Rifting The spreading apart of the Earth's crust by magma
rising between fractures in the Earth's plates.

Figure 13.8 Oceanic rifting, seafloor spreading, and
plate divergence. In ocean basins, the seafloor spreads
where plumes of magma interact with the crust, creating a
rift. One of the best examples of seafloor spreading exists
in the middle of the Atlantic Ocean.

Mid-oceanic ridge A ridge-like feature that develops along a
rift zone in the ocean due to magma upwelling.