There was no activity reported for Pagan Volcano this week, so we are going to look at the island itself this week. I think Its an excellent example of an island arc volcano.
This is a diagram of an island volcanic arc. See? We are back to the structure of the earth, like I said its always important. In order to produce an island volcanic arc, you will have an oceanic/oceanic subduction zone. The oceanic crust that is subducted is the older thicker and denser of the two oceanic plates.
Once the older oceanic crust is subducted to around 100km the water will be driven out of the rocks and lower the melting temperature of the surrounding mantle. That is the 101 explanation of what is going on and is pretty accurate. This produces melt that becomes buoyant and rises. When this magma breaks through to the ocean floor it starts to build volcanoes.
Once these volcanoes start to build they produce underwater eruptions until they have built up enough mass to break the ocean surface. Then it starts to build islands. These islands are volcanic arcs.
That is how Pagan Volcano was built.
As you can see Pagan Island is part of a chain of volcanic arc islands that sit above the subducting Pacific plate as it dives under the Philippine Sea Plate. The Mariana trench is also home to the deepest point on the earth the Challenger Deep. It is 10,898 m (35,755 ft) to 10,916 m (35,814 ft) below sea level.
Here is Pagan Island rising from the seafloor with a vertical exaggeration of 3
Next time we will talk about the island itself and how it formed!
References, more reading and image sources
Pagan Island Wikipedia page
Mariana Trench Wikipedia page
Volcanic Arc Wikipedia page
Pagan Volcano! How cool of a name is that!?
Pagan Island is one of the islands in the in the Commonwealth of the Northern Mariana Islands (US) and it is actually the 4th largest of the Northern Marianas. The island is comprised of two stratovolcanoes (Think Mt Fuji and Mt St Helens, cones and tend to erupt more violently then shield volcanoes like the ones that make up the Hawaiian Islands). The volcanoes are named North and South Pagan. (Guess which one is the northern one and the southern one….)
While both Northern and Southern Pagan have been active, the majority of historic eruptions (since 17th century) have come from North Pagan Volcano. Thanks to the numerous eruptions from North Pagan Volcano, Pagan Island is one of the most active of the Mariana islands. The largest historical eruption from North Pagan occurred in 1981 and led to the evacuation of the island.
North Pagan Volcano is currently at aviation color code yellow and is monitored by the USGS.
(Yellow: Volcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.)
According to the most recent weekly report there was a small low altitude ash cloud on the 10th of July, and drifted around 50Km NW and reached a high of around 1.5km or 5,000 ft above sea level.
References used and other useful links
1) Pagan (Island) wikipedia page
2) Pagan (volcano) page from Global Volcanism Program
3) Current volcanic activity report from Global Volcanism Program
4) USGS monitoring page on Pagan (and current report on activity)
Episode 7: Tsunamis, Volcanoes and Magnetite!
Sorry about the chair creaking! oops…
Indonesia Tsunami and volcanic eruption
Map: the area we are talking about, the blue marker to the east is Merapi
and maker to the west is the earthquake location
Warning Systems Often Don’t Help Tsunami Victims
More Merapi info: