The Solar System

Pacific Ocean

Plate motion Plate motion

Mantle

Plate motion

Subduction

zone

Atlantic Ocean

Midocean rise

S. America Africa

Melting

Ocean floor

Subduction

zone

Subduction

zone

Subduction

zone

Hawaiian-Emperor

chain

Hawaii

Midocean

rise MountainsAndes Midoceanrise

Midocean

rise

Red Sea

Himalaya

Mountains

Ural

Appalachian Mountains

Mountains

Mountains

are common

on Earth, but

they erode

away rapidly

because of

the abundant

water.

A typical view
of planet Earth

William K. Hartmann

Janet Seeds

National Geophysical Data Center

Arift valley forms where continental

plates begin to pull apart. The

Red Sea has formed where

Africa has begun to

pull away from

the Arabian

peninsula.

Subduction

zone

Subduction

zone

Subduction

zone

Hawaiian-Emperor

chain

Evidence

of plate

tectonics was

first found in

ocean floors, where

plates spread apart and

magma rises to form

midocean rises made of rock called basalt, a rock typical of

solidified lava. Radioactive dating shows that the basalt is

younger near the midocean rise. Also, the ocean floor carries

less sediment near the midocean rise. As Earth’s magnetic

field reverses back and forth, it is recorded in the magnetic

fields frozen into the basalt. This produces a magnetic pattern

in the basalt that shows

that the seafloor is

spreading away from

the midocean rise.

Hawaii

Midocean

rise MountainsAndes Midoceanrise

Midocean

rise

Red Sea

Himalaya

Mountains

Ural

Appalachian Mountains

Mountains

Midocean

rise

Red Sea

Midocean

rise

Midocean

rise

Midocean

rise Red Sea

1

1b

Our world is an astonishingly active planet. Not only
is it rich in water and therefore subject to rapid erosion,
but its crust is divided into moving sections called plates.
Where plates spread apart, lava wells up to form new crust;
where plates push against each other, they crumple the crust
to form mountains. Where one plate slides over another, you
see volcanism. This process is called plate tectonics,
referring to the Greek word for “builder.” (An architect is
literally an arch builder.)

Asubduction zone is
a deep trench where
one plate slides under
another. Melting releases
low-density magma that
rises to form volcanoes such
as those along the northwest
coast of North America,
including Mount St. Helens.

1c Hot spots caused by rising magma in the mantle can poke through a plate and cause volcanism such as that in Hawaii. As the Pacific plate has moved northwestward, the hot

spot has punched through to form a chain of volcanic islands, now mostly worn below sea level.
Folded mountain ranges can form where plates push against each other. For example, the
Ural Mountains lie between Europe and Asia, and the Himalaya Mountains are formed by India
pushing north into Asia. The Appalachian Mountains are the remains of a mountain range
thrust up when North America was pushed against Africa.

1a