The island of Savai'i is a massive basaltic shield volcano,
which rises from the seafloor of the Pacific Ocean. Most of the
volcano (97%) is under water. Over thousands of years numerous
volcanic eruptions have seen layer upon layer of lava oozing from
the ground, building the island higher and larger in area.
The lava that forms a shield volcano flows out of a fissure in
the ground and has a low viscosity, allowing it to form layers that
build up slowly to form the gradual slope. Similar volcanic
processes built the neighbouring islands of Upolu and Ta'u
(American Samoa), as well as the Hawai'i Islands to the north.
The formation of the Samoa islands is thought by some to be an
example of the classic hotspot model. This theory suggests that
there are certain locations in the Earth's mantle that are hotter
than their surrounds. These hotspots sometimes penetrate the crust
immediately above, pushing magma to the surface. Techtonic movement
pushes the crust resulting in a chain of volcanoes forming on the
surface. Examples of hotspot activity include the islands of
Hawai'i, Tahiti and Iceland.
For sometime, this model was rejected when scientists discovered
conflicting eveidence. For example, the volcanic islands and
seamounts further away from the Samoa hotspot (east of American
Samoa) should be progressively older. But, Savai'i and Ta'u - at
opposite ends of the Samoan chain of islands - both erupted in the
last century.
In 2005, tests on samples from the deep flanks and rifts of
surrounding Savai'i showed rocks that are about five million years
old, which fit the hotspot model.
The Mount Matavanu crater
Mount Matavanu is one of several locations on the island of
Savai'i where lava has broken through the surface. Between 1905 and
1911, lava flowed towards the central north coast and destroyed
villages including Saleaula and Magua.
Despite being active only 100 years ago, flora has revegetated
the crater and some trees almost rise from the floor to the rim. On
a clear day you can see lava fields that make their way down to the
coast.
To reach the crater, turn off the main island road at Safatu, to
the village of Paia. From there it is an eight kilometre road to
Seu's first fale, which is suitable for 4WD or high-clearance 2WD
vehicles - otherwise you'll need to walk. It's then a bumpy
kilometre to the second fale and another kilometre to the car park.
From here you walk about 400 metres to the crater rim.
Lava impact
Between August and November 1905, fast-moving 'pahoehoe' lava
moved across the landscape covering the land at depths of up to 120
metres. The lava travelled 12 kilomtres to coast, covering
approximately 100 square kilometres in the Gaga'emauga district,
including the villages of Sale'aula, Salago, Mauga and
Samalae'ulu.
At the time, the colonial German administration acquired land on
the main island of Upolu and resettled villagers at Salamumu and
Leauva'a. Today, some of the descendents have re-built homes upon
the lava fields where there is sparse volcanic vegetation.
At waypoint 4, you will find the ruins of the LDS church in
Mauga. Here you can explore the impact of the lava and how nature,
and the local people, are adapting to new environment.
Note: Most of the land in Samoa is under customary
ownership. Seu charges an admission of ST10 for entry to the
crater. The Mauga lava field walk costs ST5 per person, which
includes a guide.
Logging requirements
To log this EarthCache you must send us a message with the following information:
- At the crater (WP2) look closely at the rock and describe its
characteristics, such as colour and structure? Can you identify
what type of rock it is?
- What is the elevation according to your GPS receiver at the
crater rim lookout?
- At the LSM church (WP4), what unusual feature can you see on
the surface of the lava inside the building?
- What is different about the rock at the church (WP4) compared
to the rock at the crater (WP2). What is similar?
You can log your visit along with your find immediately,
however, you must send us a message with the answers to the questions above.
Failure to comply with these instructions will result in your log
being deleted.
Your log should indicate how many people were in your party and
any additional information or experiences you gained from this
Earthcache. Although not required, a photo of you at one of the
locations is appreciated.
References