To understand volcanoes we must first understand what is inside the Earth. The Earth is made of three distinct layers (see Figure 1). The innermost layer of the Earth, the core, is made mostly of iron. The inner core is solid, while the outer core is liquid. The middle layer of the Earth is the mantle. The mantle is a semi-molten layer between the core and the outermost layer, which is known as the crust. The crust is the solid layer on which we live. The crust is much thinner than the other two layers. If you think of the Earth as an egg, the crust would be the shell, the mantle would be the egg white, and the core would be the yolk.

Figure 2. The tectonic plates of the Earth. click for copyright

As we know from studying earthquakes, the crust of the Earth is not one solid piece. It is actually broken into about 15 pieces called tectonic plates (see Figure 2).

Figure 3. The formation of convergent boundary volcanoes. click for copyright

Boundaries between plates are called fault lines. Typically (though not always), volcanoes occur at one of two types of fault lines. One is a convergent boundary, where one plate slides under another plate. As one plate slides under, it is forced down into the mantle where the pressure and heat are extreme. Under these conditions, the crust melts, forming magma, which is forced up through the crust above it, finally reaching the surface and forming a volcano (see Figure 3). Upon reaching the surface, the magma is called lava.

Volcanoes can also form at a divergent boundary, where the plates are moving apart. The plates leave a gap where magma from the mantle can escape to the surface. These faults are usually under oceans and form mid-ocean ridges. Figure 4 shows volcanoes being formed along fault lines, including divergent boundaries.

Figure 4. The numerous ways in which volcanoes form. click for copyright

Another place volcanoes form is over a hot spot. A hot spot is a location in the mantle at which the temperature is abnormally high. In these places, magma forces its way up to the surface forming a volcano. A famous hot spot is the one that is forming the Hawaiian island chain (see Figure 5). The magma from the hot spot forms a sea mount (undersea mountain) that becomes an island once it is taller than the sea level. As the tectonic plate moves over the fixed hot spot, a series of islands form.

Figure 5. The formation of the Hawaiian Islands due to a hot spot under the moving Pacific Plate. click for copyright

A volcano has many parts (see Figure 6). Under a volcano, from a pocket of molten rock known as the magma chamber, hot magma travels up through a fissure, called a vent. While a volcano can have more than one vent to the surface, it usually has one central vent. Magma exits the volcano at the crater, becoming lava. The crater is typically a bowl-shaped opening located at the top of the cone. If the eruption is violent, ash and rock are launched into the air. This material is collectively known as tephra. Much of the material ejected from the volcano collects on its sides. After many eruptions, enough layers, known as strata, build up to form a volcano's recognizable cone shape.

Figure 6. The parts of a volcano. click for copyright

The three main types of volcanoes are cinder cone, shield volcano and stratovolcano. A cinder cone is the most common. Also known as scoria cones, they are usually small and made of volcanic fragments from previous eruptions, called cinders or scoria. They have steep sides and typically a bowl-shaped crater (see Figure 7).

Figure 7. (left) Cinder cone Pu`u ka Pele near Mauna Kea on the island of Hawaii. (right) Cut-away diagram showing the shape and interior of a cinder cone. click for copyright

Another type of volcano is a shield volcano, which is formed almost entirely of liquid lava. The lava flows out of a vent and slowly slides down the side of the volcano. As the lava cools, it forms a broad cone of basalt. The slopes of a shield volcano are usually very gentle (see Figure 8).

Figure 8. (left) Shield volcano Mauna Loa on the island of Hawaii. (right) Cut-away diagram shows the shape and interior of a shield volcano. click for copyright

And last, stratovolcanoes have the tall conical shape most often associated with volcanoes (see Figure 9). They have very steep sides and often form during violent eruptions. They are made of tephra and solidified lava.

Figure 9. (left) Stratovolcano Mt. St. Helens, WA, before its 1980 eruption. (right) Cut-away diagram shows the shape and interior of a stratovolcano. click for copyright

To give people advance warning that a volcano is going to erupt, engineers and scientists are creative in designing and building devices to detect natural indicators of volcanic eruptions. Warning signs include earthquakes, gas emissions, change in magnetic field, and the swelling of the volcano itself. To monitor active volcanoes, seismometers detect the vibration of earthquakes, tilt meters detect even slight changes in the shape of the mountain (see Figure 10), and other devices monitor and measure escaping gases.

Figure 10. Tilt meters are used to predict volcanic eruptions. click for copyright

Explosive ash eruptions from volcanoes can shut down airports, disrupt air routes, and temporarily stop air supply service to remote areas. Some of the equipment that engineers design to monitor volcanoes help people in the Federal Aviation Administration and airlines make better decisions on flying during volcanic eruptions.

Class Poll: Before the lesson, ask the entire class the questions below. Have students raise their hands to answer. Write their answers (or key facts) on the board, and summarize (in percentages or number of students) how many answered the same or similarly. Ask the students:

• What is a volcano?
• Can you name any famous volcanoes?
• Do you know where volcanoes usually occur?

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Tally the votes and write the totals on the board. Give the right answer.

• True or False: Lava is magma that has reached the surface. (Answer: True)
• True or False: All volcanoes are the same. (Answer: False. There are three different types of volcanoes.)
• True or False: We cannot detect when a volcano is going to erupt. (Answer: False. While no one can detect exactly when a volcano will erupt, we can usually determine that one is due to occur.)

Engineering Design: Engineers often work with scientists to design instruments to detect if a volcano is going to erupt so that people near the volcano can be warned and evacuated. Working in teams, have students brainstorm ways that engineers might be able to tell if a volcano is swelling (getting bigger) and that an eruption might be about to happen. Have them sketch or design an instrument that could help detect an eruption, and present their designs to the class.