volcanoes and eruptions

8SCIENCE

Volcanoes and Eruptions

Date Title Page

16/04/13 Volcanoes and Eruptions

Review

• Where do volcanoes form?– Plate boundaries

• How do volcanoes form?– Heat and pressure from inside the Earth produces

magma and pushes up to the surface

• What is a hotspot?– Very hot areas in the mantle that produce volcanoes

although they are not found on plate boundaries

• • Identify the cause of each volcano below using the words: subduction, rift,

and hot spot.

• a) In 1973, a town in Iceland called Vestmannaeujar was surprised when lava began erupting from a vent or crack in the Earth’s surface. The Earth’s crust seemed to split apart._______________________

• • • b) Scientists feel that the Cascade Mountains, extending from California to

British Columbia, are still forming. Two plates are colliding, forcing magma up from the mantle to form the mountain. _____________________

• • c) Mauna Loa in Hawaii is the “growing mountain”. In 1969 and 1974, it

erupted and poured out lava through a hole in the Earth. This lava called and hardened into rock. Below it, molten lava continues to flow. _____________________

Types of Volcanoes• Shield • Cinder cone• Composite

Introduction to the three types of volcanoes

• Three types of volcanoes• As you view the video clip, try to think of

some of the similarities and differences between the three types of volcanoes.

Shield volcanoes• Quiet eruption that produces lava and spreads out in flat

layers• These layers build to form slightly sloping sides• Ex: Hawaiian islands produces 2 different types of lava:– Pahoehoe (smooth)- A’a (rough and chunky)

Pahoehoe and A’a lava

Pahoehoe

A’a lava

Cinder cone volcano

• Explosive eruptions that throw lava and rocks high into the air

• These bits of rock and hardened lava are called tephra– Tephra layers build up to form steep sided

volcanoes

Tephra lava

Tephra lava erupted from the 1980 Mount St. Helens Eruption in Washington State

Composite volcano

• Produces eruptions that are quiet then violent• Cycles of lava and tephra form different layers• Usually found where one plate is forced under

another(subduction zone)

Let’s review with animations…

• Types of Volcanoes

What causes an eruption to be quiet or violent?

• Amount of water vapor and other gases trapped in magma– High gas content = more violent

eruption– Gases are trapped in magma and pressure builds up– As magma moves closer to the surface, there is less pressure

and the gas can escape

• Amount of silica (silicon and oxygen) present in magma– Low in silica = more quiet eruption

Other volcanic structures• Not all magma flows to the surface• Some may rise from the volcano and cools

underground to form other rock structures• Erosion of the rock layers make these visible

to us today

COLOMBIA

DEATH VALLEY, CALIFORNIA

SHIPROCK, NEW MEXICO

CALDERAS IN OREGON (US) AND PHILLIPINES