We have seen where most volcanoes occur on Earth - along plate boundaries where plates are moving apart or moving together. Now lets look at one of the exciting aspects of volcanoes - how explosive they are - and see how this relates to where they are found.
A. Teacher introduction
Volcanism is the process whereby materials formed inside the Earth come out onto the surface. These materials include lava (molten rock flowing on Earth’s surface); dust, ash, and bombs (collectively called pyroclastics) which shoot up into the air during an eruption, sooner or later to fall to the surface; and gases which are released into the atmosphere. How explosive (exciting!) a volcanic eruption is depends in large measure on what kinds of materials are released during the eruption. Show pictures: gentle eruption with flowing lava (e.g. Hawaiian volcano) vs. explosive eruption with lots of gas and pyroclastics (e.g. Mount St. Helens).
B. Student activity
- Throw-away plates (plastic preferable to paper) - 2 per student
- Paper cups (flat-bottomed) - 2 per student
- Straws - 2 per student
- Milk or juice - ½ cupful per student
- Honey or corn syrup - ½ cupful per student
- Paper towels for clean-up (plenty from earlier demo!)
- Have each student place a cup on each plate in front of them. Pour milk or juice into one cup per student.
- Have students place a straw into cup and make a volcanic eruption by blowing into the straw. Start gently, getting “lava” to bubble up, eventually bubbling over.
- Pour honey or corn syrup into second cup, and place straw into cup. Again, make a volcanic eruption by blowing into the straw, starting gently, eventually erupting lava over the side.
- Is it harder to form bubbles in one “lava” than the other? Why?
- Is it harder to form a “lava flow” from one than the other? Why?
- What is the difference between the first and the second lava?
- Why do they flow differently?
The two liquids have different abilities to flow easily - one, milk or juice flows very easily, spreading quickly over a dinner table and onto the floor, much to parents’ chagrin; honey and corn syrup have greater resistance to flow, being thicker and slower. The resistance to flow is the viscosity of a fluid.
How does the temperature of a fluid affect viscosity? Ask students to think about motor oil on a cold winter morning!
C. Teacher Discussion
Different lava types have different viscosities, so volcanoes will behave differently depending on the type of lava erupted. Volcanoes with very fluid lava (like milk) will have relatively gentle lava flows which flow a long way from the break in the ground. These types of volcanoes are found in oceans, either along the mid-ocean ridges or on islands such as Hawaii. These lavas form deep beneath the surface, underneath the lithosphere. Volcanoes with more viscous lava (like honey or corn syrup) will be more explosive, with lava bursting into the air, and relatively short, slow lava flows. These types of volcanoes are found in volcanic mountain ranges on continental edges, such as the Andes, or on volcanic island chains such as Japan or the Philippines. These lavas form over subduction zones, where oceanic plates dive down under other plates, to heat up and melt.
Show class two volcanic cone photos (not erupting): one of an Hawaiian-type volcano and one of a Mount St. Helen’s type.
Which formed from a more fluid lava and which formed from a less fluid lava? Why?
More fluid lava spreads out away from the vent, and with successive flows, builds up a very gently-sloping cone called a shield volcano. Less fluid lavas flow a short distance from the vent and get covered by layers of pyroclastics from the explosively extruded materials, and with successive eruptions, a steeper-sloped composite, or stratovolcano, cone forms.