The Solar System

CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 473

This computer model of a mountain with the vertical
scale magnified 10 times appears to have steep slopes
such as those of a composite volcano.

A true profile of the computer model shows the mountain
has very shallow slopes
typical of shield
volcanoes.

If the crustal

plate is moving,

magma

generated by

the hot spot

can repeatedly penetrate the crust to build a chain of volcanoes. Only the

volcanoes over the hot spot are active. Older volcanoes slowly erode away.

Such volcanoes cannot grow large because the moving

plate carries them away from the hot spot.

Olympus Mons contains 95 times more

volume than the largest volcano on Earth,

Mauna Loa in Hawaii.

Volcano Gula MonsVolcano Gula Mons Volcano Sif MonsVolcano Sif Mons

Vertical scale

exaggerated

Hot spot

Hot spot

Old volcanic island

eroded below sea level

Caldera from

repeated eruptions

Time since last eruption (million years)

Mike Seeds

NASA

NASA

Digital elevation map

Radar map

5

Kauai

Oahu

Molokai

Maui

Hawaii

3 1.5 1 0

Active

volcanoes

Newborn

underwater

volcano

Plate motion

Plate motion

Volcanoes on Venus are shield volcanoes. They

appear to be steep sided in some images created from

Magellan radar maps, but that is because the vertical scale

has been exaggerated to enhance detail. The volcanoes of

Venus are actually shallow-sloped shield volcanoes.

2

Volcanism over a hot spot

results in repeated eruptions that

build up a shield volcano of many layers.

Such volcanoes can grow very large.

3

The volcanoes that make up the Hawaiian

Islands as shown at left have been produced

by a hot spot poking upward through the middle of

the moving Pacific plate.

3a

2a

The plate moves about 9 cm/yr and carries older volcanic

islands northwest, away from the hot spot. The volcanoes

cannot grow extremely large because they are carried away

from the hot spot. New islands form to the southeast over the

hot spot.

Olympus Mons at right is the largest volcano on Mars. It is a

shield volcano 25 km (16 mi) high and 700 km (440 mi) in

diameter at its base. Its vast size is evidence that the crustal

plate must have remained stationary over the hot spot. This is

evidence that Mars has not had plate tectonics.

3b