ChapC1.3

Volcanoes

How Volcanoes Form• volcano: a mountain that

forms when red hot melted rock flows through a crack into the earth’s surface– magma: melted rock inside

the Earth– lava: melted rock that

reaches the Earth’s surface

• volcanoes form ocean floors– plates move apart and magma slowly rises toward

the surface from deep in the mantle– magma gets to the surface - cools, hardens and

forms new ocean floor

• volcanoes form when plates move toward each other– plates collide one plate is forced down into the

mantle - melts partly – magma rises through the vent to form mountains

• vent: a rocky tube that carries magma to the surface

– Andes in South America and Cascade Range in the NW US formed this way

The Andes in South America

Cascade Range in NW US

• volcanoes form when a plate moves over a hot spot in the mantle– fountains of hot rock punch through the

crust to form volcanoes

• volcanic islands of Hawai’i formed this way

Hawaiian Volcanoes

Types of Volcanoes

• Shield volcano: large mountains with gentle slopes– form from runny lava– many volcanoes in Hawai’i are shield volcanoes

• the largest is Mauna Loa on the main island and is about 30,000ft from the ocean floor

• Cinder cone volcanoes: small volcanoes made of hardened lava chunks called cinders– steep sides– usually less than 1000 ft tall

• Composite volcanoes: medium sized moutains– made of layers of alternating layers of lava and

cinders– have steep peaks and gently sloping sides– Mount Fuji in Japan, Mount Pelee on the island of

Martinque, and Mount Vesuvius in Italy were formed this way

Mount Fuji in JapanMount Pele on Martinque

Mount Vesuvius

Volcanoes Build• volcanic eruptions form new crust on continents

– volcanoes add to Earth’s landscape• Crater Lake in Oregon

• volcanic eruptions have good effects on the land around them– soil around an active volcano has minerals that help

crops grow - minerals are added when water flows over the rock

• many types of plants grow in the soil surrounding the slopes of Mout Vesuvius

• fruits and vegetables grow well near the bottom• Hardwood trees grow well in the soils on the slopes

Volcanoes Destroy

• lava and gases that erupt from volcanoes are very hot and often destroy everything in their path– Mt. St. Helens in WA erupted in 1980 - temp of

ashes and gases reached 1470 degrees F– blast blew down trees 15 mi away – 60 people died

• clouds of ash and gases were so thick people couldn’t breath

• 1983 Colo volcano on the Indonesian island of Una Una erupted – scientists were able to warn people– 7000 peole who lived in the area went to

safe places– when the volcano erupted much was

destroyed but nobody died

Summary

• Volcano is a mountain that forms when melted rock called lava flows out onto the Earth’s surface

• Most volcanoes form along plate edges• Volcanic eruptions add crust to the

Earth and can be harmful• Lava flows can make soils rich in

minerals that help crops grow