dust, ash, steam and gases
volcanic bombs
layers of lava and ash from past eruptions
lava
central vent
side vent
Earth’s crust Earth’s crust
magma chamber
Fig 5.4.4 Above 1150°C
1000–1150°C 900–1000°C
500–900°C Less than 500°C
Volcanoes Volcanoes
Volcanic material
Most volcanoes just release clouds of steam, gas (called fume) and ‘smoke’, which is actually made up of fine rock dust or ash and rock.
Magma is molten rock that forms in a magma chamber deep under the surface.
Lava flowing from a volcano Fig 5.4.5
It is lighter than the surrounding rock because it is full of gas. The pressure pushes it up until it bursts out from a vent.
What emerges is called lava. This is made up of magma and gases such as hydrogen sulfide (rotten egg gas) and steam. Lava flows down the volcano at speeds of less than 10 km/h and will later cool to form solid rock.
Hot volcanic ash, steam and gases form a fast-moving (often 200 km/h) cloud that can reach incredible heights. Krakatoa’s cloud is estimated to have reached a height of 80 km! The ash is carried by the winds and eventually settles back to Earth as a thick blanket. The ash from the 1994 eruption of Mt Tavurvur in Papua New Guinea crushed the nearby town of Rabaul. In 79 AD the people of Pompeii, Italy, suffocated on
the ash from Mt Vesuvius and were then buried by it! Ash from Mt St Helens landed up to 500 km away.
Rain often then turns the ash into a lahar, a river of mud that can devastate anything downstream from it.
Volcanic ash can also travel the planet in the jet-stream winds that exist 30 km up. Here the ash blocks the Sun, making the planet cooler and producing spectacular sunsets. Ash from the 1991 eruption of
UNIT UNIT
5.4 5.4
Unexpected turbulence
In 1982 a British Airways Boeing 747 en route from Kuala Lumpur to Perth flew through an ash cloud
from the eruption of Mt Galunggung in Java.
The dust jammed all four engines and the aircraft
dropped without any power for many minutes
before they could be re-started. Volcanic ash is not visible to radar so the pilots had little warning.
Mt Pinatubo in the Philippines is thought to have blocked 4% of the sunlight reaching Earth that year and the dust from Krakatoa changed the colour of the sky in England, 10 000 km away!
Gas explosions can destroy parts of the volcano itself, with large pieces blown out as solid rock, called volcanic bombs.
Volcanic bombs also form when hot lava is thrown into the air, landing great distances from the crater. The rock can also block the vent until the gases build enough pressure to clear it once more with another large explosion.
Fig 5.4.6 Ash from Mt Pinatubo in the Phillipines blocked the Sun for many days in 1991.
Why live next door to a volcano?
Across the world about 500 million people live uncomfortably close to active volcanoes. Why?
Volcanic materials break down to form some of the
most fertile soil on Earth and farming it can give good crops with plenty to
eat and market. Living near an active volcano is
obviously risky but it is often the only livelihood available. For this reason
vulcanologists are constantly trying to to predict future eruptions
of the world’s active volcanoes.
Prac 1 p. 147
5.4
UNIT
[ Questions ]
Checkpoint
Volcanoes everywhere
1 State the number of volcanoes in the world and how many of them are active.
2 State what Krakatoa was, is now, and why it is famous.
3 Explain why erupting volcanoes are sometimes not able to be seen.
4 State the location of an Australian volcano that is probably extinct.
Just like a pimple!
5 Distinguish between a fissure and a vent.
6 Explain why volcanoes are more likely to be found at the edges of tectonic plates than in the middle of them.
Volcanic material
7 Identify five different volcanic products.
8 a Explain why volcanic ash clouds rise.
b Identify three different situations in which volcanic ash can be dangerous.
9 Define the following terms:
a lava d fume
b the magma chamber e jet stream c lahar
10 Identify what a volcanic bomb is and three ways it can form.
Think
11 Copy the following and modify any incorrect statements so they become true.
a Lava is not the same as magma.
b A dormant volcano is a live volcano.
c Volcanic ash moves more slowly than lava.
d White lava is hotter than red.
e Ash clouds do not travel far.
12 Identify what causes the smell that is always around volcanoes and hot springs.
13 The edge around the Pacific is often called the ‘Ring of Fire’. Propose a reason for this.
14 Justify why volcanic areas are also areas of great earthquake activity.
15 The sound of the Krakatoa explosion took four hours to travel 5000 km across the Indian Ocean. From this information calculate the speed of sound. (Remember:
speed = distance/time.)
[ Extension ]
Investigate
1 Examine where Krakatoa (or Pulau Rakata) is located.
On a copy of the map, draw the 5000 km radius circle in which its eruption could be heard.
2 The word ‘volcano’ comes from the Roman god Vulcan.
Research some information about him and where he was supposed to live.
3 Use a scale of 1 cm : 1000 m to construct a diagram of:
a Centrepoint tower (305 m) b Mt Everest (8848 m) c the ash cloud of Krakatoa
d the height at which commercial aircraft fly (10 000 m) Hint: You will not be able to do this in your workbook!
4 Construct a time line of major eruptions in the last century.
5 Research one major volcanic eruption. Imagine yourself as a reporter and present your findings in a newspaper article. Try to find:
a the date of the eruption
b what warnings there were that an eruption was coming
c the type of volcanic products the volcano expelled d the damage it caused
Surf
6 Explore photos and information about volcanic eruptions by connecting to the Science Focus 3 Companion Website at www.pearsoned.com.au/schools, selecting chapter 5 and clicking on the destinations button.
Creative writing
Mt Bigbang
Mt Bigbang is going to erupt in the next day or two. You are a seismologist who is to make a speech to a group of technicians staying in the area of Mt Bigbang to monitor the eruption. Your speech must explain to them what:
a dangers they may encounter
b volcanic products might be coming their way c precautions they should take.