Visualizing Environmental Science

Plate Tectonics and the Rock Cycle 299

Sea

level

Plate 1

Plate 2

Rift zone

(mid-ocean

ridge)

Upper mantle

Crust

Magma

rises

from

mantle.

Oceanic crust

moves

toward

continental

crust.

Magma rises

through cracks

in continental

crust.

Upper

mantle Subduction

zone Earthquake

zone

Plate 1

Plate 2

Oceanic

trench

Crust

Oceanic

crust

Upper mantle

Transform fault

Plate 1 Plate 2

a. Two plates move apart at a divergent plate boundary.

c. At a transform plate boundary, plates move horizontally in
opposite but parallel directions. On land, such a boundary is often
evident as a long, thin valley due to erosion along the fault line.

b. When two plates collide at a convergent plate boundary in the

seafloor, subduction may occur. Convergent collision can also

form a mountain range (not shown).

lava along the Mid-Atlantic Ridge, where two plates are

diverging.

When two plates collide at a convergent plate

boundary, one of the plates sometimes descends under

the other in the process of subduction. Convergent col-

lision can also form a mountain range; the Himalayas

formed when the plate carrying India converged into the

plate carrying Asia. At a transform plate boundary, plates

move horizontally in opposite but parallel directions. On

land, such a boundary is often evident as a long, thin val-

ley due to erosion along the fault line.

Earthquakes and volcanoes are com mon at plate

boundaries. San Francisco, California (noted for its

earthquakes), and the volcano Mount Saint Hel ens in

Washington State are both situated on plate boundaries.

Volcanoes

The movement of tectonic plates on the hot, soft rock of

the asthenosphere causes most volcanic activity. In places

where the asthenosphere is close to the surface, heat

from this part of Earth’s mantle melts the surrounding

rock, forming pockets of magma. When one plate slides

under or away from another, this magma may rise to the

surface, often forming volcanoes. Magma that reaches

the surface is called lava.

Volcanoes occur at three kinds of locations: at sub-

duction zones, at spreading centers, and above hot spots.

Subduction zones around the Pacific Basin have given rise

to hundreds of volcanoes around Asia and the Americas,

in a region known as the “ring of fire.” Iceland is a volca-

nic island that formed along the Mid-Atlantic Rift Zone

as the adjoining plates there spread apart. The volcanic

Hawaiian Islands formed as the Pacific Plate moved over

a hot spot, a rising plume of magma that flows from an

opening in Earth’s crust beneath the ocean or continents.

The largest volcanic eruption in the 20th cen tury oc-

curred in 1991, when Mount Pinatubo in the Philippines

exploded (see Figure 9.9). Despite the evac uation of more

than 200,000 people, several hundred deaths occurred,

mostly from the collapse of buildings under the thick layer

of wet ash that blanketed the area. The volcanic cloud pro-

duced when Mount Pinatubo erupted extended upward

some 48 km (30 mi). We are used to hearing about human

activities affecting climate, but many significant natural

phenomena, including volcanoes, also affect global climate.

The lava and ash ejected into the atmosphere by the erup-

tion blocked much of the sun’s warmth and caused a slight

cooling of global temperatures (0.2–0.5°C) for a year or so.

*>ÌiÊLœÕ˜`>ÀˆiÃÊUʈ}ÕÀiÊ£Ó°Ó

All three types of plate boundaries occur both in the ocean and

on land.