The Central Kamchatka Depression is a roughly 100km wide valley between two volcanic belts, the Sredinny Ridge and the Eastern Kamchatka Ridge in the middle of the Kamchatka Peninsula. The Central Kamchatka Depression (CKD) is known as the most active arc volcano system in the world (*1). CKD’s volcanic activity is mostly clustered in the Klyuchevskoy Group, named after its youngest, highest and most active volcano, the Klyuchevskoy Volcano. This volcano has a mean eruption rate of 1 m³/s the last 10 kyr (*1). This made it possible to build a 4800m high mountain within 7000 years. It may come as no surprise that Klyuchevskoy earned the title of most active island-arc volcano in the world (*2). Of course the magma needed for such amount of activity in the CKD have to come from somewhere. Let us take a look what happens underneath CKD.
The Kamchatka Peninsula
The Kamchatka Peninsula is situated at the triple junction of the Okhotsk Plate, the Pacific Plate and the Komandorsky Plate. There are three main volcanic belts, The Sredinny Ridge (SR), the Eastern Kamchatka Ridge (EK) and the Southern Kamchatka Ridge (SK). These are caused by subduction of the Pacific Plate underneath the Okhotsk Plate. As noted above CKD lies between EK and SR. The specific tectonic settings are the most important factor behind the huge amount of magma in the CKD. First of all, CKD and the Klyuchevskoy volcanic group lie at the exact place where the Emperor seamounts subduct.
Also the boundary between oceanic crust and continental comes into play. As you can see on fig 1(*3)
the SK and SR lies on continental crust, while both CKD and EK lies on oceanic crust. This has several important consequences. The first, (the least important for our story), is the differences of the type of lava emitted. EK and SK form one volcanic zone (the Eastern Volcanic Front) but have different types of lava. In fact the lava’s from SK and SR are more similar than the lava’s of SK and EK. Also the lava’s of CKD and EK are similar in style. The second consequence is its influence on the subduction angle of the pacific plate. In the South the pacific plate has to dive immediately under thick continental crust. In the North the Pacific Plate comes first across thin oceanic crust and has only after 300km to dive under continental crust. This shows in the subduction angle, which is 50° in the south and gradually decrease to 35° in the North. The third and most important consequence is the creation of a mantle wedge under CKD. Because the pacific plate has to dive under the very thick continental crust of SK and SR, its mean average depth under the CKD is 170km. The oceanic crust at the CKD is only 30-40km deep, leaving a gap of 120 km between the two crusts. This void, the mantle wedge, is filled with magma.
Schematic figure of the mantle wedge:
So what happens with the magma?
As already noted, the CKD lies above the subduction of the part of the Pacific Plate with the Emperor Seamounts. The emperor seamounts and Pacific plate create high gas-rich (mostly H2O) freshly molten magma as it is pulled down. All this magma fills the mantle wedge. Of course the mantle wedge can only have a limit amount of new magma. At one moment the mantle wedge becomes ‘full’. The magma being a fluid seeks the path out of the least resistance out of the wedge. So which options does it have?
It comes from the East, thus it will likely not going back in that direction. To the West and the South thick Continental crust blocks the path. The magma inside the mantle wedge is above the zone of subduction which weakens the crust through friction. It can’t go down because of the Pacific Plate laying below it. So this leaves only two options: To the North and Up. Now is the Pacific Plate tilted through the difference of subduction angle?
This means that the Pacific plate becomes shallower the more you go to the north. Magma migrating north is more and more squeezed between the Okhotsk plate and the Pacific plate. At one point it gets squeezed too much and it takes the only way left: up. The way up (with or without going North) seems to be the easiest option, the magma has only find a way through thin oceanic crust. Hence, this is the option that most magma takes. The very active Klyuchevskoy Group (from Tolbachik in the South-West to Shiveluch in the North- East) is the result.
There is evidence that within this mantle wedge there’s a lot of melting and mixing of magma’s. As example, the Klyuchevskoy Volcano produced both high-MgO basalts and high-Al2O3 basaltic andesites, even at the same moment. There is also a gradual change from South to North of the type of basalt emitted. Tolbachik, the southernmost volcano, erupt mostly basalts from peridotite melts. While Shiveluch in the North erupts basalt from pyroxenite melts.
It is not clear what drives this melting and mixing processes. Temperature anomalies, flux created by the melt/subduction of the emperor seamount and even a trapped part of oceanic crust within the mantle wedge are proposed. However studies have showed that there is an important anomaly at 100 km depth within the mantle wedge. This anomaly seems to be an important factor in the peridotite/pyroxenite melts:
Schematic version of how the mantle wedge is built up with kuma the anomaly:
What we see at the surface of the earth
Of course we see a very active volcanic region. The 4 active volcanoes of the Klyuchevskoy group (Tolbachik, Bezymianny, Klyuchevskoy and Shiveluch) are together good for 22 VEI 1, 83 VEI 2, 39 VEI 3, 5 VEI 4 and 1 VEI 5 eruptions since 1900. GVP list also 4 eruptions smaller than VEI 1. Tolbachik at the southern end, has moderate fractionated basal-andesite lava. It erupts mostly effusive and because of this it is the only shield volcano of the Klyuchevskoy group. The other volcanoes of the group are stratovolcanoes. Shiveluch at the other end receive silicate-rich ‘primitive’ basalt from depth. Hence he will erupt more explosively. Klyuchevskoy in the middle has the most mixed magmas but is constantly smoking/puffing/erupting with a mean eruption rate of 1m³/s.