14.11.1 Volcanic shapes Volcanoes come in a variety of shapes and sizes, forming different landforms. There are four main types and each depends on: • the type of lava that erupts • the amount and type of ash that erupts • the combination of lava and ash. Lava rich in silica is thick and slow-moving, while low-silica lava is runny and can flow for many kilometres. Shield volcanoes have broad, flat sides due to runny lava, whereas dome volcanoes have steep sides from thick lava. Heavy ash, like volcanic bombs, settles near the crater, while lighter ash travels further. Cinder cones are steep-sided volcanoes formed from ash falls. Volcanic ash, made of rock, mineral and glass particles, is very hard and can collapse roofs. It also poses a risk to jet engines, often causing flight diversions. Composite volcanoes, the most common type, are built from both ash and lava.
SkillBuilder discussion Geographical inquiry 1. Identify which types of volcanoes only produce lava eruptions. 2. Explain the difference between a composite volcano and a cinder cone. 3. Rank each of the types of volcanoes listed above from most dangerous to least dangerous. Justify your response.
FIGURE3 Four volcanic landforms
Lava flows that have built up over time
Crater
C
Dome volcano
A Shield volcano Shape formed
Shape formed by thick lava that cools and hardens rapidly
Lava flows that have built up over time
Crater
by thin lava that spreads widely before cooling and hardening
Vent
Vent
Magma chamber
Magma chamber
Layers made up alternately of cooled lava and ash
B Cinder cone
D Composite volcano Bodies of igneous rock formed when former channels of magma have cooled
Layers made up of heavier rocks near the vent, and cinders and ash towards the edges
Crater
Crater
Vent
Vent
Secondary cone
Magma chamber
Magma chamber
14.11 SkillBuilder activity COMMUNICATING 1. Conduct research to find pictures of volcanic landforms and materials. These include crater lakes, geysers, calderas, fields of ash deposits, volcanic plugs, lava tubes, hummocks and pumice. You could also find pictures of the two types of lava: aa and pahoehoe. 2. Use your pictures to create a field guide to volcanic landforms. Each page should contain a picture of the landform, a brief description and a place where it can be found — sometimes they are tourist attractions.
TOPIC14 Geomorphological processes and hazards
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