Volcanoes

From the heat of the core to the earthquakes and volcanoes of the crust...

We've come a long way in communicating science.

Figure 1.

I remember a group of excited geologists, paleontologists and others viewing the first color film of a volcanic eruption. They were at the Illinois State Geological Survey.

Excited voices, Oohs and Ahas, filled the room as we watched the explosions, red-yellow lava spraying out, thick rivers of red-orange streaming over lips and blocks of black jagged stone banks....

Figure 2.

I was in awe, and could almost feel the searing heat...

But just what is a volcano? I mean we can see the pictures, but how does it work?

Our Earth

Let's step back and look at the structure of our planet. Our earth is made up of 4 main layers:

An inner core,
An outer core,
The mantle, and
The crust.

It's the crust holding up the building we are in, and where we live – It's the surface of the earth we walk across.

Below the crust is the mantle, made up of solids, liquids, gases, and we can use its scientific name, lithosphere.

Figure 3. Note the top layers of the Earth.

As you can see, the lithosphere is made up of the crust and the upper most layer of the mantle.

At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.

The release of heat from the Earth's core creates convection currents in the mantle.

Figure 4.

Convection and the release of heat from the Earth's core drives convection cells in the mantle, which move areas of the crust, called plates.

The study of these currents and their features is called plate tectonics.

Plate Tectonics

Our earth's crust consists of a number of moving plates. And because they move, they are always colliding with or pulling apart from the other plates.

Researchers figure that our Lithosphere consists of nine large plates and twelve smaller ones. The continents are embedded in continental plates, and oceanic plates make up much of the sea floor.

Figure 5.

The study of these plates helps to explain continental drift, the spreading of the sea floor, volcanic eruptions and earthquakes, and how mountains are formed.

The force that causes the movement of the tectonic plates is the slow churning of the mantle beneath. Mantle rock moves upwards to the surface by high core temperatures and then sinks after cooling.

This convection cycle takes millions of years, and produces distinct processes and features.

Continental drift

This explains the changing face of our earth through time. With continents only moving inches per year, it has taken millions of years for them to drift across the surface of the earth.

Figure 6.

Plate Boundaries

These also show us the way the mantle moves specific types of plates, and there are several types of boundaries.

Diverging plates

Where plates pull apart, molten rock emerges as lava, which adds to the plates. The place where this happens is known as a mid-ocean ridge.

Figure 7.

As a result, each great ocean has a mid-ocean ridge, and areas of volcanic and earthquake activity. One of the most well-known examples is the Mid-Atlantic Ridge, which extends between the North and South Poles.

Converging plates

In convergent plates, the crust is destroyed and recycled back into the interior of the Earth as one plate dives under another. Known as Subduction Zones, mountains and volcanoes are often found where these plates converge.

Oceanic-continental Convergence

Figure 8.

When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces.

These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.

Oceanic-oceanic Convergence

Figure 9.

When two oceanic plates converge, one is usually subducted under the other, and in the process, a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as the result of the Phillipine Plate subducting under the Pacific Plate.

Oceanic-oceanic plate convergence also results in the formation of undersea volcanoes. Over millions of years, however, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs.

Continental-Continental Convergence

This is when two continental plates move together. The two plates are too light to sink into the mantle. Therefore, the plates buckle up and form mountains.

Figure 10.

The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.

Transform Boundaries

This is when two tectonic plates move in different directions or at different speeds. Plates are locked together by friction while pressure builds up. Finally, the plate breaks along the fault line.

Figure 11.

Most transform faults are found on the ocean floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

A few, however, occur on land. The San Andreas fault zone in California is a transform fault.

Hot Spots

This is another tectonic phenomenon, and a great example is the Hawaiian islands.

Stretching to the west and to the north of the big island of Hawaii is a string of smaller islands and submerged volcanoes, or sea mounts, 3,700 miles long. Working within the theory of plate tectonics, there is convincing evidence that every one of these islands and sea mounts has been formed in the exact place where Hawaii now stands.

Figure 12.

How can this happen?

Geologists believe that a huge column of upwelling lava, known as a “plume,” lies at a fixed position under the Pacific Plate. As the ocean floor moves over this “hot spot” at about five inches a year, the upwelling lava creates a steady succession of new volcanoes that migrate along with the plate - a conveyor belt of volcanic islands.

Quick Review

Our Earth has an extremely hot center and brittle crust.
Convection currents of rising heat move parts of the earth's crust, called plates.
Boundaries between plates change the face of our planet.
Plate movement produce earthquakes and volcanoes – essentially the where and why of volcanic eruptions.

So, let's look more closely at the volcanoes themselves: How they erupt and what they look like.

Types of Eruptions

Depending on the pressure that forces the magna up, and depending on how liquid the magma is (the viscosity) plus the amount of gas in the magna, eruptions vary in size, explosiveness -- and danger.

Plinian eruptions are the most dangerous and explosive. The magna is high in viscosity and gas content, and can cause a lava plume over 10 miles high in the air. The Pompeii explosion of Mount Vesuvius is an example.

Figure 13.

Effusive eruptions are defined as those which have lava outpouring onto the ground. These eruptions are less dangerous, and the magna has lower viscosity and levels of gas.

Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge, slope of the ground over which the lava travels, and duration of eruption.

Figure 14.

Strombolian eruptions are characterized by the intermittent explosion or fountaining of lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly.

Figure 15.

Hydrovolcanic eruptions - When volcanic eruptions occur near oceans, saturated clouds or other wet areas, the interaction of water and magma can create a unique sort of eruptive column.

The hot magma heats the water so it becomes steam. This rapid change of state causes an explosive type of expansion in the water, which breaks apart the magma, creating a fine ash.

Figure 16.

Fissure eruptions occur when magma flows up through cracks in the ground and leaks out onto the surface. These often occur where plate movement has caused large fractures in the earth's crust, and may also spring up around the base of a volcano with a central vent.

Figure 17.

Types of Volcanoes

Volcanoes come in different shapes and sizes, which is often a result of the types of eruptions that occur.

Figure 18.

Here's a brief comment about each of these types of volcanos.

Fissure volcanoes are elongated fractures or cracks at the surface from which lava erupts.

Figure 19.

Cinder cones are the simplest type of volcano.

They are built from particles and blobs of congealed lava ejected from a single vent.

Figure 20.

As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone.

Scoria Cone Volcanoes (also called cinder cone volcanoes) are small cones which are often the result of a single eruption event - usually a strombolian eruption. These types of volcanoes are the most common.

Composite Volcanoes (also called Stratovolcanoes) are the most familiar type of volcano.

Figure 21.

They are tall, symmetrical, and slope steeply towards a small summit crater. They are usually formed by multiple Plinian eruptions, which omit large amounts of lava in violent, tall eruptions which add quickly to the height of the volcano.

Shield Volcanoes are low, wide volcanoes, formed by low viscosity, slow moving lava which spreads over a many kilometers.

Figure 22.

Lava Domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent.

Figure 23.

A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees." Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes.

Caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir.

Figure 24.

The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression.

Volcanoes in Our World

There are over 1,000 volcanoes in our world. These volcanoes can be active, dormant or extinct:

Active volcanoes have erupted within recent history.
Dormant volcanoes have not erupted in recent history.

Extinct volcanoes have not erupted in the last 10,000 years – or are thought to have experienced shifts in lithospheric plates to move them away from any possibility of future eruption.

Volcanoes and Climate Change

As volcanoes erupt, they blast large clouds of gases, particles, water vapor, and aerosols into the atmosphere. As a result, they affect the Earth's atmosphere and global climate.

When large masses of gas from an eruption reach the stratosphere, it can produce a large, widespread cooling effect. This is sometimes referred to as an “Volcanic Winter.” This “winter” occurs because volcanic ash and droplets of sulfuric acid obscure the sun, causing the temperatures to drop.

Here are some examples:

In 1783, Benjamin Franklin blamed the unusually cool summer on volcanic dust coming from Iceland, where the eruption of Laki volcano had released enormous amounts of sulfur dioxide.

The eruption killed much of Iceland's livestock and lead to a catastrophic famine, killing a quarter of the population. Temperatures in the northern hemisphere dropped by about 1 °C in the year following the Laki eruption.

Known as the “year without a summer,” the summer of 1816, unexpected climate changes left countries in the Northern Hemisphere suffering from devastating famine and epidemic outbreaks. These weather patterns were the result of the volcanic eruption of Mount Tambora in Sumbawa, Indonesia, on 10th April 1815.

It snowed in June in the United States and Europe: Crops failed, there was starvation, people lost their farms, and it touched off a wave of emigration that led to the settlement of what is now the American Midwest. In the meantime, hundreds of thousands more starved around the world.

New England and Europe were hit exceptionally hard. Snowfalls and frost occurred in June, July and August and all but the hardiest grains were destroyed. Destruction of the corn crop forced farmers to slaughter their animals. Soup kitchens were opened to feed the hungry. Sea ice migrated across Atlantic shipping lanes, and alpine glaciers advanced down mountain slopes to exceptionally low elevations.

In 1991 explosion of Mount Pinatubo, another stratovolcano in the Philippines, cooled global temperatures for about 2–3 years, interrupting the trend of global warming which had been evident since about 1970.

Another possible effect of a volcanic eruption is the destruction of stratospheric ozone.

Researchers suggest that ice particles containing sulfuric acid from volcanic emissions may contribute to ozone loss. When chlorine compounds resulting from the breakup of chlorofluorocarbons (CFCs) in the stratosphere are present, the sulfate particles may serve to convert them into more active forms that may cause more rapid ozone depletion.

Quick Review

There are general types of volcanic eruptions
There are types of volcanoes that sometimes match the types of eruptions.
Volcanoes can be active or not.
Volcanoes add to the size of our world and are part of our Earth cooling dowm – but most importantly, Volcanoes affect our world, people, livestock, and climate.

Figures & Acknowledgments

We want to thank all of the wonderful sites which helped illustrate this discussion, and we wish that all of you visit them as part of your reading.

We are humbled by the intelligence and grace of our science communities.

Figures

Figure 1. www.fukubonsai.com

Figure 2. www.pbs.org

Figure 3. mediatheek.thinkquest.nl

Figure 4. pubs.usgs.gov

Figure 5. www.uwsp.edu

Figure 6. www.noc.soton.ac.uk

Figure 7. www.sema.go.th

Figure 8. pubs.usgs.gov

Figure 9. pubs.usgs.gov

Figure 10. pubs.usgs.gov

Figure 11. pubs.usgs.gov

Figure 12. oceanexplorer.noaa.gov

Figure 13. www.homeschoollearning.com

Figure 14. www.homeschoollearning.com

Figure 15. www.homeschoollearning.com

Figure 16. www.homeschoollearning.com

Figure 17. www.homeschoollearning.com

Figure 18. www.solcomhouse.com

Figure 19. www.fun-costa-rica-vacations.com

Figure 20. www.fun-costa-rica-vacations.com

Figure 21. www.fun-costa-rica-vacations.com

Figure 22. www.fun-costa-rica-vacations.com

Figure 23. www.fun-costa-rica-vacations.com

Figure 24. www.fun-costa-rica-vacations.com