….. and it never ceases to amaze.
While we wait for the freight to arrive we have taken a tour around Faial.
Oceanic islands are very amazing places when you think about the processes occurring deep in the Earth that ultimately result in their creation.
In this case, processes that result in the eruption of lava onto the Earth’s surface.
Faial – at least the bit above sea level – is 173 km2 in area and it is just over 1000 m high.
The highest point of the island is the peak of a volcano and it has a crater called Cabeca Gordo. It is 2000 m in perimeter and 400 m deep.
A 400 m elevation change alone is quite significant as this image into the crater shows, and its edges are almost vertical cliffs.
The crater itself has marshy areas, bushy thickets and geological features known as cinder cones which mark current and past volcanically active sites, as do boulders of pumice and other types of volcanic material that was erupted areally as projectiles.
Not all volcanic eruptions result in material being projected into the air, sometimes viscous molten rock flows out of the crater and down the sides as lava flows.
The Azores are quite interesting places geologically – especially so to volcanologist – but also to those that study how the Earth’s rigid outer shell – its plates – move.
The Azores sit at the boundary of three of these tectonic plates – its called a triple junction.
In this case a triple junction between the North America, European and African plates.
The boundary between the European and African plates is what is known as a transform plate boundary, where one plate simply slides past the other, rather than one plate moving away from the other at extensional plate boundaries such as that marked by the Mid-Atlantic Ridge – which at the Azores – to the north is the boundary between the European plate and the North American plate, and to the south between the North American and African plates.
However, at the Azores the sliding – transform – plate boundary doesn’t just slide, it slides slightly obliquely so that a gap also opens up between the two plates – this is known as a leaky transform – and Faial formed volcanically in that gap.
Often islands are named after volcanoes or the minerals that the eruptions produce. And sometimes the naming works the other way around.
In this case the mineral fayalite was named after the island as it was found there first in the mid-1890s. It is an iron silicate within the olivine family – which are most commonly found in the rocks of the upper mantle.
For oceanic plates, the upper mantle normally lies about 8-10 km below the seabed – so finding them at the surface gives some idea of the scale of the Earth processes that occur associated with volcanic eruptions.
Peripheral eruptions, marked by mountainous peaks, contribute to the island’s topography, and such high points are very useful sites for telecomms masts, satellite receiving dishes and the VHF receivers that enable ships to be tracked sailing about the oceans via the AIS system. The masts are visible in the image below on the south rim of the crater.
Faial has a number of craters which show it has formed by a series of major eruptions.
These relic craters form embayments which are sheltered places, often used ……
…. for villages and safe harbours.
When volcanoes erupt lava, the lava can flow quite a long way while it remains hot.
But once it reaches the sea it will be rapidly cooled and it will solidify.
Looking at the coastal cliffs can reveal the extent of individual flows – and that the rocks are quite dark as a result of the minerals they contain. Olivine, for example, is a very dark green mineral.
And there has been a more recent eruption which resulted in lava flows on land and the cracks in the top surface are due to the cooling.
We are here to monitor an eruption that is currently occurring offshore.
The scientists that we support work mostly on things they will never directly see – so to see the results of these processes with our own eyes really does add a different dimension.
And we’ve done a quick back of an envelop calculation for the part of the island above sea level.
Faial is ~21 km long and 16 km wide – so let’s assume it is round with a diameter of 20 km.
The peak is 1 km high.
So the volume of the volcano above sea level, in itself is ~100 km3 – or ~100 trillion m3.
The rocks its made of have a density of somewhere between 2.6 and 2.8 kg/m3.
So the bit of the island above sea level alone weighs ~280 million tonnes.
That’s about 2,800 million of our instrument platform ballast weights.
And then there is the far larger part of the islands that remains below sea level.
Puts the scale of the Earth’s processes into perspective really.