Volcanoes Areas of Earth’s surface through which

magma & volcanic gas passes Creative Forces

forming fertile farmland & large mountains

Destructive Forces Turning mountains into clouds of ash

& rock, destroying forests & homes

Structure of a typical volcano.

Notice the alternating layers of ash and hardened lava.

Why does this occur?

Nonexplosive Eruptions Most common type of eruption Release large amounts of lava Calm lava flow Pacific NW of USA covered from these eruptions

Explosive Eruptions Rare & incredibly destructive Debris/molten rock blown out of volcano

Large debris falls close to volcano Smaller, dust-sized particles (ash) can travel

miles

Inside a Volcano Magma

Body of molten rock (underground) that feeds a volcano

Vents Opening through which volcanic material passes

Magma Molten rock Composition affects how explosive a volcano

is Silica (SiO2), Water, Gases (CO2, water vapor)

When pressure decreases as magma rises, the CO2 and water vapor expand rapidly

Silica Rich (Felsic) Magma Traps Gases

Thicker, viscous Difficult for gases to escape, so pressure builds up

What Erupts from a Volcano? Explosive Volcano

Mainly pyroclastic material Magma that is blasted into the air and hardens

Nonexplosive Volcano Mainly lava

Eruptions can (and do) alternate between lava & pyroclastic eruptions (even within the same eruption)

Types of Lava Aa jagged surface; brittle crust

is torn by lave flow underneath Pahoehoe flows slowly; glassy

surface has roundedwrinkles

Pillow Lava rounded lumps formedunder water

B locky Lava does not travel far; heaps of sharp-edged chunks

Types of Pyroclastic Material Formed when magma explodes and solidifies

in the air Volcanic Bombs

Large blobs that cool & harden in the air

Lapilli Smaller, pebble-like stones that

harden before hitting the ground Volcanic Ash

Most of the material in an eruption Walls of gas bubbles explode into tiny, glass-like slivers

Pyroclastic Flows Produced when enormous amounts of hot ash,

dust, and gases are ejected from a volcano Speeds up to 200 km/h Temperatures over 700 C

Review1)Which of the following factors influences

whether a volcano erupts explosively?

A. The concentration of bombs in the magmaB. The concentration of phosphorous in the

magmaC. The concentration of aa in the magmaD. The concentration of water in the magma

Review2) How is lava classified? How are

pyroclastic materials classified?3) Explain how the presence of silica

(felsic) and water in magma increases the chances of an explosive eruption.

Answers1) D2) Lava is classified by its surface texture; the way

it flows may also be used to help classify lava.Pyroclastic material is classified by its size and how it forms.

3) As the magma moves towards the surface, the water changes to water vapor and expands rapidly; silica rich (felsic) magma is viscous and tends to trap volcanic gases and plug vents, the results in high pressures forming