VOLCANOES

A few volcano basics…• Active – a volcano that has erupted

recently (geologically speaking)

• Dormant – (sleeping) has erupted within the past few thousand years – will likely erupt again

• Extinct – hasn’t erupted in 10’s of thousands of years; unlikely to erupt again

Mt. Rainier - active

Mount Baker - dormant

Shiprock Peak – extinct volcano

magma: molten rock inside the Earth

lava: magma that has emerged on the Earth’s

surface

er

                              

magma

lava

hot spot: area of volcanic activity independent of

lithospheric plates

ex. Hawaii

Different types of volcanoes are categorized based on the viscosity of the magma.

viscosity: a fluid’s resistance to flow

high viscosity = high resistance to flow

ex. honey is more viscous than water

•Low viscosity magmas allow gases to escape easily (they are hotter!)

•In higher viscosity lavas, gas pressures build up and erupt explosively

Viscosity is determined by silica content and

temperature.

The higher the silica content, the higher the

viscosity.

The hotter the magma, the less viscous it is.

TYPES OF VOLCANOES

1. Shield2. Composite3. Cinder cones

Shield Volcanoes

- largest volcanoes (tallest when measured from the ocean floor)- shaped like a gentle arch or shield-basaltic lavas (low silica content)

                                                    

- usually non-explosive eruptions- mainly lava flows- found in Hawaii (Mt. Kilauea) and Iceland

                                                    

lava tubes: caves left behind after lava flows

•

                        

        

Composite Volcanoes- coned shaped; sometimes called stratovolcanoes- lava contains mostly andesite or rhyolite (higher silica content)- more viscous

-combination of explosive activity (pyroclastic) and lava flows- responsible for most deaths- ex. Mount Saint HelensMt. PinatuboMt. FujiMt. Vesuvius

Mount Saint Helens, Washington State

Mt. Saint Helens, after eruption

Mount Pinatubo, 1994

                                                

    

Mount Fuji, Japan

Mount Vesuvius / Ruins of Pompei

Cinder Cones

- smaller volcanoes- form quickly -are active for a few years and then usually go dormant

                                                    

Wizard Island

ex. Paracutin, Mexico

- 1943, hole in ground- 2 weeks prior, small tremors- 1st day grew 10 m- By 5th day it was 100 m tall- After 2 yrs, it was 400 m tall- went quiet after 9 years

Ring of Fire•Area of the world where volcanoes are common

•Here is what a volcano on the ring of fire looks like:

Features of Volcanoes

crater - depression found at the top of a volcano; formed by the explosion of the upper portion of the cone

hot springs: water is heated by the hot rock

and reaches Earth’s surface

geyser: heating and circulation of water forms a pattern (RARE!)ex. Old Faithful

                        

        

caldera: a giant crater that can be more than 12.5 miles in diameter

- formed by rare, very violent eruptions- none have occurred in recorded history (they are 1000 times more violent than Mt. St. Helens!)- Yellowstone caldera was formed 600,000 years ago!

Crater Lake, OR

Long Valley, CA

                                                    

Aniakchak, AK

Volcanic Hazards

Primary Effects

Lava flows: eruption of magma at Earth’s

surface

Lava flows vary based on the composition of the

magma. We are specifically going to look at

basaltic lava flows.

                                                

    

pahoehoe: fast moving; low viscosity

- smooth ropy texture when it hardens- move at a rate of up to 20 mph

                        

        

                        

        

                                                

    

aa: slow moving; higher viscosity

- blocky texture when hardens- move at rates of a few meters per day-found further away from volcano

                                                    

Pyroclastic Activity: explosive volcanism where tephra is physically blown into the atmosphere

tephra: any material that is blown out of a volcano (mostly ash)

                        

        

ash fall: huge quantities of rock, glass and gas are blown high into the air

- kills vegetation- contaminate water- structural damage- jet engine “flame out” - respiratory irritation

lateral blast: explosions of gas and ash from the side of the volcano; destroys part of the mountainex. Mt. St. Helens

Ash flow (nuee ardentes)

- move very quickly- destroys everything in its path

Poisonous Gases

- CO2, carbon monoxide, sulfur dioxide, and hydrogen sulfide- can cause acid rain; contaminates water supply and vegetation

                                                

    

Secondary Effects

lahar: debris flows and mudflows

-volcanic debris becomes saturated with water-ex. Mt. Pinatubo, Philippines

                                                

    

Buildings buried after lahars at Pinatubo

Aerial View, Pinatubo Aftermath

Predicting Volcanoes

1. Monitoring of Seismic Activity- often the earliest sign2. Thermal monitoring - measuring ground temperatures

3. Topographic Monitoring - mountain may tilt and swell before an eruption

4. Volcanic Gas Emissions - changes in composition of gases may indicates rising of magma

5. Geologic History - geologists map and date rocks around the area