One of the things that tend to focus the mind here in Alaska is the volcanic activity along the Alaska Range and Aleutians. These are mostly andesitic vents driven by the subduction of the Pacific Plate under the North American Plate.
But we here in Alaska sit downwind of a massively active region in the Kamchatka Peninsula which stretches south to the Kuril Islands and on to Japan itself. At any single moment, there are typically four active volcanoes on Kamchatka. The obvious question would be why is Kamchatka so active? What is tectonically driving the activity? This post attempts to introduce the Kamchatka Peninsula from a tectonic standpoint.
The Kamchatka Peninsula of Russia is a 1,250 km long peninsula on the far eastern edge of Siberia. It sits at the junction of a triple plate collision between the North American, the Pacific and the Eurasian Plates. To make things even more interesting, it occupies something called the Okhotsk Block which is being shoved into the Eurasian Plate and borders on the Bering Block which defines the area north of the Aleutian chain of volcanic islands. The final interesting thing about Kamchatka is that there are at least a few papers out there that believe that the Emperor Seamount string is subducting under Kamchatka.
Volcanic activity is mostly subduction driven andesitic eruptions. There are over 160 volcanoes including no small number of caldera structures on Kamchatka. Some 29 of them are thought to be active and are monitored by the Kamchatka Volcanic Eruption Response Team (KVERT). After a rough patch of funding in the late ’00s, they seem to be up and operational. http://www.kscnet.ru/ivs/kvert/index_eng.php. The Kuriles also have a page that is in Cyrillic. http://www.imgg.ru/ru
There are two lines of volcanoes in Kamchatka. The westernmost line has for the most part not been recently active. Activity migrated eastward toward the trench constructing a line of active volcanoes stretching from central Kamchatka south to Japan. Recent volcanic activity does not extend north of a fault line in central Kamchatka. It does extend south along the Kuril – Kamchatka Trench, which plunges some 10 km into the Pacific Ocean. Volcanic islands comprising the Kuril Islands stretch some 1,300 km along the trench until it reaches the northern Japanese island of Hokkaido. Subduction – driven volcanoes parallel the line of the trench.
It appears to me that the crux of the matter with Kamchatka lies with the Okhotsk Block, also sometimes referred to as the Okhotsk Plate. It has two subduction boundaries. The main one on the west under the Eurasian Plate is thought to be inactive, which makes sense as there is no active volcanic activity. Russia’s largest island, Sakhalin is on the Okhotsk east of this boundary. The western boundary is defined by the Kurile – Kamchatka Trench which is the surface expression of the subducting plate driving the volcanic activity.
The southern boundary is not as well defined. Some define it as far north as the southern end of Hokkaido Island, Japan. Others define it as a triple junction plate boundary in central Japan. Still others define the area between the two limits as yet another platelet. Large earthquakes like the Fukushima earthquake of April 2011 help define the boundaries.
The northern boundary is defined by a transform fault called the Ulakhan Fault. The motion along the fault is generally westbound north of the fault.
The Okhotsk Block is thought to be a remnant of the Kula Plate from some 80 my ago. The Kula eventually subducted into the North American Plate under Alaska and was much larger then than its remnants are today. http://www.uwgb.edu/dutchs/platetec/kula.htm
Since that subduction episode, the Okhotsk has been essentially squeezed between the Eurasian, North American and Pacific Plates. This has shallowed the Sea of Okhotsk bit by bit over the millions of years since the initial collision. This squeezing also imparted a small clockwise rotation along the Ulakhan fault.
But the main action remains at the triple junction in the Pacific Ocean off the east coast of central Kamchatka. And it is that triple junction that provides sufficient weakness in the Okhotsk to allow subducted andesites to erupt in significant volume, not unlike the rift on the eastern edge of the Jalisco Block has allowed magma feeding the Colima Volcanic Complex to reach the surface. https://volcanohotspot.wordpress.com/2015/02/24/eruptions-continue-at-colima-mexico/
Kamchatka – Aleutian Islands Triple Junction
The triple junction expresses itself in Kamchatka’s central depression with the Bezymianny Volcanic Complex, which includes several large, active structures very close together. Shiveluch sits a small distance north of this complex. It is also very active. Sa’ke at Volcano Café wrote about the Central Kamchatka Depression in June 2013. I will not belabor her conclusions. https://volcanocafe.wordpress.com/2013/06/03/central-kamchatka-depression/
What I would like to add is the existence of a fault line, the Alpha Fracture Zone that is active in the ocean and tends to not express itself upon landfall. The western end of the Aleutian trench is more transverse faulting than it is subduction, as the plate movement is parallel to the line of the trench. Further proof is the line of Emperor Seamounts moving in parallel to the trench and subducting under Kamchatka itself. http://geology.com/usgs/hawaiian-hot-spot/
One abstract used the term “intra-arc rifting” to describe the magma source below Kluchevskoy, meaning that some geologists believe that a new rift in this region is either possible or underway. http://www.sciencedirect.com/science/article/pii/S0012821X99002885
If you look at the maps, any time you have significant rotational tectonic forces on a block or plate along with known weak spots, rifting becomes a possibility. It is important to note that the vast majority of recent volcanic activity on Kamchatka is at or to the south of the triple junction / Alpha Fault line east-west line.
Tectonics drive everything we see in Kamchatka. And because the junctions are many and messy, there are lots of weak places for magma to make its way through the surface. Add to the festivities the thoroughly hydrated debris of the subducted Emperor Seamounts, and you have the recipe for a very active and dangerous volcanic region.
The Great Assembly of Honourable Dragons has been sitting and issues the following Declaration: Considering seriously and taking into account, and proactive action upon, constructive criticism and suggestions from our Readership, We decided
to change the layout of VolcanoHotspot to improve readability.
This shall be implemented within the next 24 hours.
Given on Wednesday, March 18, 2015.