Adam C. Simon 

Ph.D., University of Maryland, 2003

 Research Associate

Department of GeologyUniversity of MarylandCollege Park, MD 20742

p: 301 405 0235f: 301 314 9661

e-mail: asimon@geol.umd.edu

Volcanism

Eruption of Mt. Etna in Sicily sends a plume of ash visible from space.

Volcanoes affect our lives in many ways. Today we talk about their construction.

Planetary Volcanism

As we have seen, volcanic activity, or volcanism, is an important factor in shaping other worlds in our solar system.

Io, moon of Jupiter

Olympus Mons, Mars

Igneous Rock Classification

increasing melting temperature

increasing viscosity

PERIDOTITE

Volcanic Products

When magma reaches the Earth’s surface, we call it lava.

Aside from lava, volcanoes may eject a variety of common products:

-- volcanic gases-- pyroclastic debris

Volcanic GasesMuch more gas can be

dissolved in a liquid at high

pressure than at low

pressure.

Liquids that are rich in

gaseous components will

tend to degas (lose their gas

component) as they

decompress.

Here on Earth,

decompression occurs as

magma travels from deep

sources to shallow regions

(like the surface).

Gas-Related Textures

Gas leaving decompressing magma may not be

successfully escape.

Gas trapped in rapidly-cooling lava forms vesicles, or void spaces in the rock.

Extreme examples of vesicular rocks are pumice

and scoria: these frothy rocks are lightweight, since most of their volume is air,

not rock.

vesicular basalt

pumice

Pyroclastics

All of the solids ejected from a volcano is collectively called

pyroclasts or pyroclastic debris.

The smallest pyroclastic material is ash. Realize that

volcanic ash is silicate (glass), unlike ash from burning wood.

Pebble-size bits of quenched lava and

ash are called lapilli.

“Fire fountaining” (as shown

here from the 1969 eruption of

Mauna Ulu, Hawaii) produces

abundant pyroclastic material,

including tiny glass beads

(Pele’s tears).

Pyroclastics

Pyroclastics

Larger chunks of lava, from grape size up to practically car size,

are called bombs.

Note that on impact these do not explode in flaming carnage, as

seen in certain extremely-unrealistic Hollywood productions.

Volcanic/Sedimentary Rocks

Pyroclastic materials can form their own rocks, with hybrid

volcanic and sedimentary components.

A tuff is a rock formed from pyroclastic debris.

Welded tuffs are hard rocks, which form when pyroclastic

material is still very hot when it accumulates on the surface.

Molten components cause pyroclasts to ‘weld’ together.

Poorly-welded tuffs form from airfall material far from the

volcanic source. They carry little thermal energy, since they

cool during long travel in the atmosphere.

Lava Flows

Smoothly-oozing, low viscosity basalt flows commonly form ropy structures, called pahoehoe (a Hawaiian word).

Lava Flows

Basalt lava that has lost much of its gas will be more viscous. This lava will form rough, blocky flows called aa.

The type of volcano that forms depends on: viscosity of lava, proportion of lava/ash, lava flux.

-- flood basalt

-- shield volcano

-- cinder cone

-- composite volcano (stratovolcano)

-- lava dome

Volcano Types

increasing viscosity

increasing violence

Large Igneous Provinces

At various points in Earth history massive basaltic eruptions

have taken place, producing what are sometimes called

flood basalts, or large igneous provinces.

These eruptions have left lasting marks of the Earth’s surface

and may have had significant effects on global climate.

activity on Hawaii, except

that much greater volumes

are involved and eruptions

last for much shorter time

periods.

Why these massive eruptions occur is poorly understood, but they are fundamentally like hot spot

Columbia River

Deccan Traps

Ontong-Java plateau

Columbia River Flood Basalts

This is the largest

large igneous province in

North America.

Deccan Traps, India Flood Basalts

512,000 cubic km of lava (Mt. St. Helens erupted ~1 km3). The eruption about 65 Myr ago is strangely coincident with a

global extinction event (i.e., dinosaurs).

-- low silica, basaltic lava: low viscosity, flows readily

-- gentle slope

-- non-explosive (in general)

Shield Volcano

Mauna Kea on Hawaii is 10.2 km above the seafloor: the highest mountain on Earth (Everest is only 9.2 km high!).

Cerro Negro, Nicaragua, 1968

Fernandina, Galapagos, 1991

-- low to moderate silica lava

-- steep slopes, generally symmetrical

-- dominantly pyroclastic material

Cinder Cones

Composite Volcanoes

(stratovolcanoes)

Mayon, Philippines

-- moderate to high silica lava: high viscosity-- steep slopes, generally symmetrical-- layered lava flows and pyroclastic material-- explosive!

Fujiyama, Japan

Volcanic Landscapes

Kluchevskoi volcano, Kamchatka, Russia

This satellite image shows how volcanoes can dominate the landscape in some areas. The large ones are stratovolcanoes.

Novarupta, Alaska

Lava domes are usually small features,

constructed of lava flows of high viscosity.Many volcanoes return

to life after hiatuses and form resurgent

domes.

Wilson Butte, CADomes

Panum Crater, CA

Phreatic Eruptions

How to make a volcano more destructive than it is

on its own?Just add water!

A phreatic eruption is one that is

triggered or aided by groundwater or

surface water.

When this water infiltrates the hot

volcanic system, it converts to steam,

thus expanding, leading to explosive

circumstances.

The 1883 eruption of Krakatoa in

Indonesia was a famous

phreatic eruption.

Plinian Eruptions

In 79 AD Pliny the Younger

carefully noted the destructive

eruption of Vesuvius in Italy

(which, among other things,

destroyed Pompeii and

Herculaneum,

and killed Pliny the Elder).

Violent, ash-rich eruptions have

since become known as plinian

eruptions.

Comparison of Scales

Diatremes

Features such as

Ship Rock (NM)

and

Devil’s Tower (WY)

are diatremes:

the eroded remains

of extinct volcanoes.

Extinct Volcanoes

In spite of their volcano-esque shapes, they actually are part

of the volcanic conduit that was underground when the

volcano was active.

Airfall Distribution

6850 yr ago, Mt. Mazama, OR, erupted, sending a recognizable

layer of ash throughout the

northwest.

We now know this volcano as Crater Lake.

Big Pyroclastic Eruptions of

North America

This illustration shows the Bishop Tuff, which was

deposited 760,000 yr ago,

the most widespread ash layer of the last hundred million years in North

America.

The Bishop Tuff

This eruption scatter ash hundreds of kilometers away, and the area close to the eruption was treated to

a thick blanket of hot ash!

Ash Layers

Distant deposition

of volcanic material

is important to

dating sedimentary

rocks, as we will

see next class.

ash layers

10-100,000 yr old lake

sediments at Mono Lake,

CA, showing several distinct,

regionally-recognized ash

layers.

Waiting for the Big One

Area one is the Pacific

Northwest, from northern

CA into BC.

Continued oblique

subduction sustains lava

supply at depth.

Let’s not forget what

happened in 1980.

There are three particularly

good choices for which area

will have the next big

volcanic eruption in the

continental US.

The Next ‘Big One’?

The second choice is Long Valley, CA.

This was the source of the Bishop Tuff and has seen cinder cone construction

as recent as ~250 years ago.

north

caldera

Long Valley in Cross-Section

This interpretation was produced by examining surface geology, deep drill holes, and seismic data.

The Next ‘Big One’?Choice three is

Yellowstone, WY. This caldera has blasted out gigantic pyroclastic

eruptions in the last couple million years.

(honorable mention:

Valles caldera, NM)

We will talk about the implications of volcanic

eruptions for society later in class.

Credits

Some images in this presentation come from: Plummer, McGeary and Carlson, Physical Geology, 8th

ed.; NMNH, Global Volcanism Project; Geological Society of America (Geology); USGS; Hamblin and

Christiansen, Earth’s Dynamic Systems, 8/e; D Swanson, USGS; EOS; Univ. of North Dakota’s Volcano World