Geology: Volcanoes

Volcanoes and Civilization

Humans and volcanoes have coexisted for millennia, for better or for worse. Civilizations have prospered (volcanic soils are highly fertile; excellent for crops) and been erased under volcanoes

Hotspot Case Study: Iceland

Iceland is the only example of a mantle plume hot spot coinciding with a mid-ocean ridge. The island is being torn apart by the divergent boundary.

The Wrath of Vulcan: Volcanic Eruptions

In 79 C.E.Mt. Vesuvius erupted violently with pyroclastic flows that destroyed the Roman city of Pompeii and killed 20,000 people.

Pahoehoe(pa-hoy-hoy)

a Hawaiian word describing basalt with a glassy, ropy texture.

Cinder Cones

-The smallest type of volcano -Piles of ejected lapilli-sized fragments that have built up at the angle of repose around the vent. -They are often symmetrical with a deep summit crater.

Caldera

-a gigantic volcanic depression. -One to 10's kms across -They form when a magma chamber empties and the volcano collapses into the evacuated space.

Eruption Styles

-effusive eruption -explosive eruption

Rhyolite

-has the highest SiO2 and is the most viscous lava. -Rhyolitic lava rarely flows. Rather, lava plugs the vent as a lava dome. -Sometimes, rhyolitic lava domes are blown to smithereens during eruption.

What Is a Volcano?

A volcano is a mountain constructed by the eruption of molten rock from Earth's interior. Volcanoes are a direct consequence of plate tectonics and mantle convection

Mt. Vesuvius erupted in a very violent explosion in 79 C.E. and buried the residents of Pompeii in ash. On the basis of this information, which of the following would you LEAST expect to find at Vesuvius?

Basaltic Lava

Geologic Settings of Volcanism: Oceanic Mantle Plume Hot Spots

Decompression melting creates large volumes of basalt.

Rhyolitic Eruptions

Explosive eruptions generate huge volumes of debris, especially volcanic ash.

Andesitic Lava Flows

Higher SiO2 content makes andesitic lavas viscous. Unlike basalt, they do not flow rapidly.

Plate tectonics theory explains why volcanic activity occurs where it does. Which example describes a geologic setting as it relates to its volcanic activity?

Iceland sits on an oceanic hot spot located on the Mid-Atlantic Ridge.

Stratovolcanoes = Composite Volcanoes

Large, cone-shaped volcanoes with steep slopes made of alternating layers of felsic lava, tephra, and debris

Geologic Settings of Volcanism: Mid-Ocean Ridges

MOR-generated oceanic crust covers 70% of Earth

Magma Chambers

Magma chambers are located in the upper crust. They inflate and deflate as they fill and empty.

Protection from Volcanic Hazards: Predicting Eruptions

Magma movement causes a change in shape that can be measured with tiltmeters or satellite radar (InSAR).

Volcanic Hazards: Threat of Gas and Aerosols

Many volcanic gases are poisonous (H2S, SO2, CO, CO2) and cause respiratory illness.

What is true about volcanic activity in Yellowstone National Park?

The park contains a 72-km-diameter caldera that formed during an eruption 600,000 years ago.

Which statement describes glowing, hot, avalanche-like flows of tephra?

They are called pyroclastic flows, or nuées ardentes.

Mitigating Volcanic Hazards: Danger Assessment Maps

Used for planning, zoning, and emergency preparedness.

Products of Volcanic Eruptions: Lava Flows -Viscosity

Viscosity depends on composition (especially silica content), temperature, and gas content

Volcanoes in Space

Volcanic activity is evident on the Moon, other planets, and the moons of other planets.

Beware of Volcanic Hazards!

Volcanic eruptions cause great harm to humans, resulting in an estimated 250,000 deaths over the past two millennia. More people live in volcano-hazard areas than at any other time in history. Understanding volcanic behavior is the best defense.

Volcanic Eruptions

Volcanoes are unpredictable and dangerous natural phenomena.

Volcano Size and Shape

Volcanoes shapes and sizes reflect magma type. Shield volcanoes are the largest due to the fluidity of basaltic magma. Strato volcanoes are intermediate in size Cinder cones are the smallest.

A'a'(ah-ah)

a Hawaiian word describing basalt that solidifies with a jagged, sharp, angular texture.

A lahar is

a volcanic mudflow that resembles fluid concrete.

Pyroclastic Flows

an avalanche of hot ash, gas, and debris that develops from some silica-rich volcanic eruptions. The eruption cloud collapses and races down the slope of the volcano.

Pillow basalts

are blobs of basalt cooled rapidly by quenching in water.

Basalt flows

are often thin and fluid, which facilitates long flow distances. (up to 30 km/hr)

Basaltic (mafic) lava flows

are very hot, low silica, and low viscosity.

Lahars

are water-rich debris flows carrying ash and large blocks. They can move very fast (~50 km per hour) and very far (~tens of kms),

Solidified flows may contract with vertical fractures that are hexagonal in cross section. This feature is

columnar jointing

Lava Tubes

conduits for basaltic lava.

A caldera forms by

draining of a magma chamber beneath a volcano during an eruption.

Which of the following is responsible for the broad, dark areas of the Moon, known as the maria, pictured here?

flood basalts

Volcanoes erupt large quantities of

fragments —volcaniclastic deposits. This material includes: pyroclastic debris, preexisting rock, landslide debris, and lahars

Low-viscosity lava

has low silica content.

Tuff

lithified ash, which may or may not contain lapilli

Which of these types of volcanic settings produced 70% of the Earth's surface?

mid-ocean-ridge volcanoes

Lava is

molten rock that cools and hardens on the Earth's surface.

Lahars

mudflows that result when water moves ash. This material is very much like wet concrete, very thick and dense.

Jupiter's moon Io is thought to be volcanically active because

of enormous tides that keep its mantle hot

Which of the following is a form of lava?

pillow

Rhyolite lava

tends to erupt explosively.

Tephra

the term to describe deposits of pyroclastic debris of any size.

Volcanic ash is

tiny particles of pulverized volcanic rock and glass

A volcanic debris flow is

wetted debris that moves downhill These occur where volcanoes are covered with ice and snow or are drenched in abundant rain

Volcanic Architecture

◼A magma chamber ◼Fissures and vents ◼Craters ◼Calderas ◼Distinctive profiles: ◆Shield volcanoes ◆Cinder cones ◆Stratovolcanoes

Protection from Volcanic Hazards: Recurrence Interval

◼Active: erupting, recently erupted, or likely to erupt ◼Dormant: hasn't erupted in hundreds to thousands of years ◼Extinct: no longer capable of erupting

Volcanic Gas

◼Gases are expelled as P drops with magma rise. ◼Gas bubbles in magma may be trapped in the cooled volcanic rock. These gas bubbles are called vesicles. ◼The style of gas escape controls eruption violence. ◼For low-viscosity basalt, gas escape is easy, leading to more mellow eruptions. ◼For high-viscosity rhyolite, gas escape is difficult, resulting in violent release.

Volcanic History: Krakatau

◼Krakatau was a volcano located between the large islands of Java and Sumatra in Indonesia ◼Krakatau erupted May 20, 1883, and continued erupting through June and July. ◼Then, on August 27, 1883, at 10 A.M., the island was obliterated in a monstrous explosion that was heard 4,800 km away, blowing the island to smithereens, creating a tsunami that killed 36,000 people on nearby islands.

Volcanic Hazards: Threat of Tsunamis

◼On rare occasions, volcanic eruptions can create giant waves called tsunamis. ◼The phreatomagmatic explosion of Krakatau in 1883 generated tsunamis that killed 36,000 people on Java and Sumatra.

Geologic Settings of Volcanism

◼Tectonic plate motion is a dominant control on volcanism. Volcanic settings are: ◆Mid-ocean ridges: spreading axes ◆Convergent boundaries: subduction zones ◆Continental rifts: incipient ocean basins ◆Oceanic hot spots ◆Continental hot spots and flood basalts

Products of Volcanic Eruptions

◼Volcanic gases: expelled vapor and aerosols ◼Pyroclastic debris: fragments blown out of a volcano ◼Lava flows: molten rock that moves over the ground

Volcanic Hazards: Volcanic Ash and Lapilli

Ashfalls can completely bury landscapes, killing plants and crops. Tephra is gritty; it abrades car and airplane engines.

Crater

A crater is a bowl-shaped depression atop a volcano. Craters are up to 500 m across and 200 m deep. They form as erupted lava piles up around the vent.

Shield Volcanoes

Mauna Loa, Hawaii, is a perfect example. Broad, slightly dome shaped. Low-viscosity basaltic lava, have a low slope

Geologic Settings of Volcanism: Convergent Boundaries

Most volcanoes form at convergent boundaries. Volatiles from sediments on the subducting plate initiate melting

Volcanic History: Mt. St. Helens

On March 20, 1980, Mt. St. Helens, dormant since 1857, awakened On May 18, 1980, 8:32 A.M., an earthquake triggered a massive landslide that sheared off the entire north side and flattened 600km2of forest The blast killed 61 people. Lahars plugged the Toutle River and closed the Columbia River. Ash fell in North Dakota.