Exercise 4-1*: Volcanoes and plate tectonics A, I
Eruption: Indonesian volcano Karangetan, October 29th, 2007
“Karangetan volcano on the Indonesian island Siao erupted this morning. Hours before, hundreds of villagers were evacuated from the slopes of the 1.700 m high volcano. Villages, farms and trees were thickly covered in ash, but no substantial damage or casualties have been reported. Karangetan is one of the most active volcanoes on the Indonesian archipelago.” (From www.trouw.nl)
a. Mark the volcano on your empty world map.
b. Mark on your map any recent volcanic eruptions. Use www.earthweek.com for more information.
c. Now take a look at GB 192B (GB 174B). When you consider plate boundaries, does anything strike you when looking at the locations of volcanoes that you have found? Are there any exceptions? If yes, can you explain these?
The epicentres of earthquakes are concentrated around plate boundaries. The majority of volcanoes can also be found in these zones (see Figure 4.1 and GB 192B and D (174B and D). In this chapter we will study the relationship between plate tectonics and volcanism. First we will look at the different types and occurrences of volcanoes.
4.1.1 Volcano types
There are many types of volcanoes but the two main types are shield volcanoes and composite or stratovolcanoes. They differ in their form: a shield volcano is flat and wide – hence its name; it looks like a shield that is lying flat. A stratovolcano, however, has a cone shape. Whether a shield or a stratovolcano will form is mainly determined by the composition of the material that is emitted during an eruption.
Shield volcanoes (Figure 4.2): shield volcanoes are formed where eruptions produce mainly lava flows with a basalt composition. Basalt magmas crystallize at a relatively high temperature, contain much iron and magnesium but little sodium and potassium, have a relatively high density, and are relatively fluid (not viscous). This latter characteristic is especially important in determining
Figure 4.1: Active volcanoes in the world: The Smithsonian Global Volcanism Program. Source: http://volcano.si.edu/world/find_regions.cfm
the shape of the volcano. A low viscosity means that basaltic lava flows easily, and will therefore spread over a wide area. This results in a volcano with a relatively flat and wide shape. Basaltic magma is formed when the upper mantle partially melts. This happens especially at divergent plate boundaries and at so-called ‘hotspots’ (more about hotspots in the next section). Examples of basaltic volcanism are found in Iceland (a divergent plate boundary and a hotspot) and Hawaii (a hotspot).
Stratovolcanoes (Figure 4.3): This type of volcano is formed where the magma is composed primarily of andesite. Andesitic magmas contain more silicon, sodium and potassium and less iron and magnesium. They are more viscous, resulting in steeper slopes on the volcano, and have a lower density than basaltic magmas. Another difference is that a stratovolcano is formed from alternating layers of ash and lava (this is why they are also called ‘composite volcanoes’). Because of the high viscosity and the resulting low flowing velocity, andesitic lavas do not travel far but stay relatively close to their source.
Stratovolcanoes are known for their explosive eruptions. Different factors explain this explosivity. Low viscosity causes rising magma to slow down or stagnate, so that outflow can be blocked for extended periods of time. Pressure builds up until it is finally released in an eruption. The magma also contains a lot of gas that cannot easily escape due to its high viscosity. If the gas remains trapped, extra high pressure develops. When it finally erupts, it can be extremely explosive. Some stratovolcanoes are characterised by a caldera. Here the explosive eruption was so voluminous and fast that the whole roof of the underlying magma chamber collapsed, leaving a cauldron-shaped feature in the landscape.
Stratovolcanoes are found mainly in subduction zones (places where an oceanic plate is pushed beneath a continental or another oceanic plate), for example in a string of locations around the Pacific. Here, the magma is also formed in the upper part mantle, but its composition changes from basaltic to (usually) andesitic as the magma travels to the surface. This is a process called magmatic differentiation, and will be explained later. The differences in magmatic composition are elaborated in section 4.2.
Figure 4.2: A shield volcano. Source: Grotzinger et al, Understanding Earth (2005)
Eruption
Room
Figure 4.3: A stratovolcano. Source: Grotzinger et al, Understanding Earth (2005)
Ash layers
Passages
Exercise 4-2**: The Pinatubo eruption A Pinatubo erupts (source: www.nieuwsdossier.nl)
June 15, 1991 – The eruption of Pinatubo volcano in the Philippines was one of the most violent eruptions of the 20th century. The first shocks were felt on March 15th. Seismologists placed
instruments around the volcano and were able to warn the population. The first magma erupted on June 7th. A few days later another eruption followed, whereby ash was spewed up to great heights
(24 kilometres). Eruptions kept coming until the biggest one in June, which lasted for three hours. The ash covered most of the Philippines and Central-Luzon island was completely obscured. The ash travelled as far as Vietnam and Cambodia. 300 people died as a result of the eruption, and afterward the summit of the mountain was 260 m lower than before. Because of the particles in the stratosphere, the average global temperature decreased by 0.4 degrees Centigrade.
a. Is Pinatubo a shield volcano or a stratovolcano?
b. Was the Pinatubo eruption explosive? Illustrate your answer using data from the above source. c. Why was the pressure in the magma chamber so large?
d. Can we expect further eruptions from Pinatubo? Explain your answer.
Hotspots: In exercise 4-1 you learned that the presence of some volcanoes is not related to plate boundaries but to hotspots instead. These are isolated areas where slowly rising material from the mantle (a mantle plume) reaches the Earth’s crust. Enormous amounts of basalt can be extruded when magma from such a mantle plume pierces the crust.
A whole island can be constructed from erupted basalt, which could essentially be the upper part of a giant shield volcano if a hotspot in the ocean has remained active for a long time. If the plate on which the island lies moves, and because the hotspot beneath it usually stays at a fixed position, a chain of volcanic islands will often be formed. A well-known example is the Hawaiian island chain. When sited under land, a hotspot can push up a whole region. If the pressure caused by the hotspot continues for a long time, a whole landmass can start breaking apart. Fault-lines will form, and basaltic mantle-derived magma will reach the surface. Silicic magmas can also be extruded if deeper parts of the continental crust are melting as well. Examples of such continental hotspots are Yellowstone in the United States and Mount Cameroon in Cameroon (Africa).
Exercise 4-3**: Volcanoes and plate motion reconstructions A, G