Volcanoes are the tip of an iceberg. 90% of the volcano is hidden, down to the magma chamber 10 km or more below the surface. What we see is only the cone on top of the conduit. The perfect cone of Fuji, or even St Helens (before it blew up), is like the hat on top. What lies below the hat is concealed to the eye. But for some volcanoes, not even the hat is fully revealed. Sometimes most of the hat is under water, and only the tip of the hat peaks out above the surface. Most of the Hawaiian volcanoes are like that. And so are some in Alaska.
Bogoslof Island was in the news this week for repeated, unexpected eruptions. It is one of those largely-submarine volcanoes, which peak only a little above the water. And that little is constantly changing.
Bogoslof is one (and sometimes two or three) of around 70 Aleutian Islands, part of possibly the most obvious volcanic arc on Earth. The Aleutian arc links Kamchatka to Alaska. All the islands are volcanic in origin. Here the Pacific Ocean meets the Bering Sea in a titanic collision. The run-in happens at about 7cm per year with the Pacific plate the subducting one.
Bogoslof Island is just north of the main arc, sometimes considered part of a back-arc. It is at the same latitude as Edinburgh, so not as far north as you could be imagined. It is cold, though, with temperatures rarely above 20 C and average temperatures in winter near freezing. Rain happens on most days. The island is not large. The surface area is less than 1 km2, and the highest point no more than 150 meter above sea. It is the tip of a submarine volcano, desperately trying to get above water but every time an eruption raises the peak, wave erosion reduces it again. But it has managed to keep its head above water, so it may be destined for great things. Just give it time. Submarine volcanoes grow normally until they reach the water surface, where erosion is suddenly much more severe: a flat-topped mountain develops, limited in height by wave action, until the island reaches a large peak well above the high water line where erosion again becomes less. Below the surface, the mountain of Bogoslof is about 2 km tall, so indeed about 10% of the height (but much less of the volume) is above water.
The island reportedly emerged above water during an eruption in 1796; at the time this was Russian. However, it clearly is older than that. An island appears (unnamed) on a map from 1768, much smaller than Bogoslof but at the same position. It was also on Captain Cook’s maps of 1778. Wikipedia reports eruptions in 1796-1804, 1806-1823, 1883-1895, 1906, 1907, 1909-1910, 1926-1928, and 1992. Now 2016 should be added to this list. It is not always the same point: sometimes eruptions come from next to the visible island. But it is always the same volcano. The youth and instability explain why the island remains uninhabited.
A minor, steep fault runs across the island. The northwest side has dropped by 1.5 meter. Between 1927 and 1934, the region around the fault near the lagoon appears to have been uplifted by 15 meter. It is not just the eruptions that build these islands. The uplift from accumulating magma is more important.
Each eruption changes the island. The map below shows the outline as it was around the second world war, surveyed by the US department of war. Castle Rock, the steeple seen in the photo, is the remnant of the dome extruded in 1796, also called Old Bogoslof. 500 meter away is a second island, Fire Island, remnant of another eruption. It is tiny, and lost half its size between 1935 and 1947. At one time it was the largest of the two. The maps below show the development of the island, with panel A from 1768 and panel I from 1947. At times there were three separate islands. Ship Rock (called Sail Rock at the time ships still had sails) mostly collapsed in 1890. The 1992 eruption occurred at the north end of the island and changed the map considerably over there.
The current eruption started on Dec 20, 2016 (Tuesday). There is no monitoring equipment on the island and the eruption was first spotted from the air. A second eruption occurred on the next day, detected by satellite, and strong seismic activity was followed by a third explosion on Friday. The wind was blowing the ash and sulphur to the north, away from inhabited areas. The eruptions were brief, with the one on Friday lasting for just over an hour. The ash clouds caused a lot of lightning and this has been detected (there are no reports whether it was detected from New Zealand, as in the case of nearby Okmok). There was yet another explosion on Monday. Each explosion caused an ash cloud to some 10 km height, similar to the previous eruption in 1992. The eruption that year lasted for 2.5 week, and if the current activity is similar there may still be some more explosions.
The eruptions have quickly redrawn the map of Bogoslof Island. The map below shows the immediate changes. The dashed line shows the shore line as it was a year ago. The eruption site appears to have been at the shoreline, not far from where it intersects the fault line. It is almost the exact position where a Navy hut had been erected during the war. The area around this site has collapsed below water, whilst the ejecta created new land further out. One possibility is that the eruption will eventually create a lava dome at the central location, and will enlarge the overall island. At least until erosion tries to even things again. Bogoslof is in a fight with the waves, and the outcome is not yet certain.
Bogoslof Island is important for wildlife, with populations of several thousand Stellar sea lions and northern fur seals (both declining), and close to 100,000 sea birds including guillemots, tufted puppins, a small number of pelagic and red-faced cormorants, and two kinds of kittiwakes. the current eruption will not directly affect these as it is now winter, but the changes in the appearance of the island will have an impact, which could be either positive or negative. It will be interesting to monitor the wildlife over the coming summer. The birds and seals may enjoy the still-warm ash.