The formation of Réunion island
Réunion was formed from two volcanos: In the northern part of the island is the Piton des Neiges, an
inactive volcano some 3 to 4 million years old. In the south of the island is the Piton de La Fournaise
which is much younger (about 500,000 years) and is still active. Its eruptions are relatively gentle and not
dangerous: the lava is very fluid, there are no falls of ash, and in general the eruptions occur inside the
"volcanic enclosure" or in the surrounding uninhabited part of the island. Eruptions outside the enclosure
are rare and spectacular. The 1986 eruption is well known because the lava flow cut off the main highway
and went into the sea. As a result, the island grew by several hundred square metres.
In contrast with the neighbouring island of Mauritius, Réunion has a steep and varied landscape: high
mountains, deep valleys, wild regions, lunar landscapes, luxurious vegetation, lagoons, magnificent
waterfalls and deep pools.
Réunion’s volcano: drawn to scale
sea level
In July 2001 the volcano cut
off the main highway for the
first time since 1986. The road
was closed for two weeks.
People living in the east of the
island had to make a big detour
to get to St Pierre.
1. With each eruption of the volcano, the chamber
containing the magma (molten lava) empties.
2. New layers of lava are added to the surface of the
volcano. This surface has also been fractured and
weakened with subsequent eruptions.
3. The weight of the lava flows causes the volcano to fall in
on itself creating a caldera (a portuguese term meaning
circular depression). A caldera is the first stage in the
formation of cirques and valleys.
You can see in this diagram the series of collapses that have
been responsible in less than a century for the present day
shape of the Dolomieu crater. The Dolomieu crater is a young
crater one and a half kilometres in diameter at the summit of
the Fournaise.
The profile of the volcano will continue to change well into
the future.
The Dolomieu crater in 1998.
Volcanic eruptions took
place outside the
volcano’s main crater
twice in the 20th century:
in 1977 and in 1986.
In April 1977 the lava flowed
over the village of Saint Rose,
destroying about twenty
houses, engulfing a service
station and encircling the
church, before going into the
sea.
Catholics believe that it was a
miracle that the lava didn’t
enter the church. Scientists
explain that the basalt cooled
down and congealed in the
narrow entrance way.
The same scenario was
repeated in 1986, on the other
side of the volcano, this time
uphill from le Tremblet. These
flows destroyed eight houses
and increased the size of the
island by several hectares. The temperature of the lava was 11600 and at times the flow reached
100,000m3 per hour.
Do some research!
1. In the eruption of 1986 the temperature of lava was found to be 11600. How hot is that?
Find the melting point of the following:
basalt
glass
steel
bitumen
brick
concrete
Share your findings with the group.
2. Work out the thickness of lava that flowed from the volcano in 1986 over a four hour period
(assuming that the lava flow was constant).
How do you do that?
To work out the total volume, multiply the flow (100,000m3) by the number of hours.
Use the diagram on the next page to work out the area covered by lava.
Divide the total volume by the area to work out the thickness.
5 km
The Dolomieu The
crater 10 km coast
Do you know how to work out the area of a triangle?
Area = half the base x height
Email your work to your teacher.