Volcanic Activity
Eight volcanoes have erupted at least sixty times on the Galápagos in the last two hundred years. Major forces of volcanic activity, the movement of tectonic plates and subsequent erosion, persistently shape the islands. Because of the high fluidity of basaltic magma, due to low concentrations of silica and water, massive explosions are rather an exception in the Galápagos. Nevertheless, Alcedo Volcano on Isabela experienced an eruption abnormally rich in silica, which covered the mountain in pumice and obsidian 100,000 years ago. The thicker compounds tend to dampen the release of energy for longer, accumulating trapped gasses, which then results in mighty eruptions. Pumice from the Alcedo’s earth-shattering eruption can be found as far as on Española.
Volcanoes on the Galápagos are typically shield volcanoes, with domed summits and a decreasing slope gradient. A shield volcano is usually built almost entirely of fluid lava flows. A steady accumulation of broad sheets of lava builds up the volcano’s distinctive form resembling a warrior’s shield lying on the ground.
The United States Frigate Essex by J Howard, 1799
Public Domain
Naval officer David Porter, captain of the United States Frigate Essex, provided the first record of a volcanic eruption in the Galápagos in his Journal of a Cruise Made to the Pacific Ocean. He and his crew witnessed an eruption of the Wolf Volcano on Isabela:
“On the 6th June we were abreast the island of Narborough, and in the afternoon saw a thick column of smoke rising rapidly as from its centre, ascending to a great height in the air, where it spread off in large white curls, and presented us a grand and majestic spectacle. We soon discovered that one of die numerous volcanoes had burst forth; but there were various opinions as to its situation. Some supposed it to be on Narborough, others to the east of Narborough, and on the island of Albemarle. I was of the latter opinion, which was confirmed next day, when we had changed our position. At night the whole atmosphere was illuminated by it; and yet we could perceive neither flames nor sparks thrown out by the crater. I am induced to believe the irruption was of short continuance, as, on the night of the 7th, I could perceive no appearance of it, although our distance, I should have supposed, would have admitted of our seeing it, had it not become extinct.”
When molten lava erupts from fissures and vents, it reaches temperatures of around 1,100°C (2,000°F). Steep slopes allow lava to accelerate to speeds of 40 km/h (25 mph). When exposed to air, the molten lava cools, forming a crust. Lava flowing down steep slopes quickly loses its gas content. The cooling crust, riding on the still-molten flow, breaks up into clinkers and stabs, producing a surface texture called ‘A‘ā, pronounced /ˈɑː.ɑː/ or /ˈɑːʔɑː/. This volcanologist term originates from Hawaiian, meaning “stony rough lava”, “hurt”, or “burn”, pronounced [ʔəˈʔaː]. The jagged texture of this lava makes a strong radar reflector.
An urban myth offers another possible origin of the name ‘A‘ā, derived from a dialogue between a Hawaiian parent and a child: “Mum, please, can I go playing in the rough lava?” The answer is simply bound to sound like [ʔəˈʔaː].
© 2017 Josef Litt
Where lava flows more gently, or where it pools over flat ground, the cooling surface takes on a picturesque form with its ripples cooling in situ, often resembling ribs or intestines. Another Hawaiian word has been borrowed by those who study volcanoes, Pāhoehoe, pronounced /pəˈhoʊ.iːˈhoʊ.iː/, which means “smooth, unbroken lava”. Pāhoehoe lavas retain large amounts of gas which keeps the flowing more fluid at temperatures of 1,100°C–1,200°C (2,000°F–2,200°F).
Lava tubes are formed when a river of fluid lava cools down on the surface and forms an upper crust. The crust is an excellent insulator and allows the lava to run underneath in its fluid form. When the eruption continues over a prolonged period of time, and if the lava drains out once the supply has stopped, long tunnels can be created. On Santa Cruz, these lava tubes are up to ten metres wide and several kilometres long. The lava tunnels on Wolf Island turned into partially submerged underwater caves, accessible to scuba divers.
When lava is forced up into a crack in tuff or other material, it forms a volcanic dike. If the surrounding material is softer than the dike, it may erode more rapidly, leaving the resulting formation proudly standing in its volcanic beauty, such as the pinnacle that adorns Buccaneer Cove.
© 2017 Josef Litt
© 2017 Josef Litt
© 2017 Josef Litt
The Galápagos Spreading Centre was a place of another discovery related to its volcanic history: “Black smokers” were first discovered in 1977 at a depth of 2,550 m (8,370 ft) by scientists from the Scripps Institution of Oceanography, using a deep submergence vehicle called ALVIN. These are hydrothermal vents occurring where tectonic plates spread. Here seawater seeping into the earth’s crust is heated under pressure to between 60°C (140°F) and 464°C (867°F). Where is this heated seawater released along the deep ocean floor, contact with the 2°C (36°F) cool sea water causes the precipitation of minerals contained in the heated water, forming a black chimney-like structure. These black smokers can reach heights of 60 metres (200 feet) and nurture extraordinary life forms encountered nowhere else.
All rights reserved. The rights of Josef Litt to be identified as the author of this work have been asserted in accordance with the Copyright, Designs, and Patents Act 1988, Sections 77 and 78. No part of this publication may be reproduced in any form or by any means without the permission of the copyright owner.
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