Where Mud Meets Magma
Mud volcanism is widely recognized as an important process in sedimentary systems where large quantities of fine-grained materials are pumped up from subsurface sources as deep as many kilometers and erupt on the surface together with water and other fluids including hydrocarbon liquid and gas. In such systems, enormous edifices can be built up by the erupted mudflows, reaching up to several kilometers in diameter and hundreds of meters in height.
In comparison to mud volcanism in sedimentary settings, that in magmatic settings has attracted less attention in the scientific community despite their frequent occurrences. Its morphological expressions are usually much smaller and less diverse in comparison to the ones in sedimentary settings and it is generally seen as just a minor geothermal phenomenon.
The Goshogake mud volcano field occurs in the Nasu volcanic belt stretching north-south in northern Japan. Its mud volcanism is probably linked to the active composite volcano Akita Yakeyama (summit elevation 1366 m above sea level) rising in close proximity to the west, but another volcano, Hachimantai (1614 m), on the east side may also be influential. The host rock of the Goshogake mud volcano field is a volcanic rock called andesite. The whole area is included within the Towada-Hachimantai National Park.
The active area of the Goshogake mud volcano field covers an area of 82,000 m2. Mud volcano landforms in this field are very diverse and include salsa ponds (water-dominated ponds), gryphons (mound-shaped vent structures of mud and gas, made of extruded sediment), and mud pots (wide vent structures of mud and gas, filled with watery mud). Many of these gryphons and mud pots are active today with low-level emissions. On the west side of the field, you find a spectacular landscape dominated by a large bluish steamy salsa called Oyunuma (English translation: Grand hot pond) and its neighboring gryphon field. Currently, the most active area of Goshogake is a group of mud pots that have formed on the west margin of Oyunuma since 1981-1982. The scene of spewing hot mud and gas from the vents may remind you of a vision of hell. You certainly don’t want to fall into one of them!
Our investigation (Komatsu et al., 2019) focused on characterizing the nature of erupted solid, liquid and gas and their conditions. The measured temperatures of the muddy fluids range from 33 to 98 °C, and they are acidic (pH 2.44–2.94). The mineral species of the deposited solid phase include abundant clay minerals as expected, but there are also high-temperature silica polymorphs, microcrystalline opals and an amorphous form of silica. These latter mineral species are indicative of conditions or processes such as high temperature, hydrothermal circulation, and dehydration reactions at certain depths underneath the Goshogake mud volcano field. The clay minerals can be the products of elevated geothermal processes, namely alteration of host rocks and sediments due to the high-temperature acid water. This type of clay mineral production is called argillization in geology.
The Goshogake mud volcano field is interesting for the emission of both carbon dioxide and methane gasses from its various vents. Carbon dioxide emission is common in magmatic volcanoes and, in fact, the emission of the gas from the nearby Akita Yakayama volcano has been observed. On the other hand, the detection of the emitted methane gas combined with the finding of vitrinite, an organic component in the erupted mud may imply feeding of the mud at least partially from deep fine-grained sedimentary strata that were deposited under an organic-rich environment. Interestingly, presence of an intra-caldera lacustrine sedimentary layer at a depth of about 1000 m below the Akita Yakeyama volcano summit has been hypothesized by other scientists. We suspect that at least some of the sediments erupted at Goshogake have come from the layer.
According to our study, the water emerging from the vents in the field is characterized by the deviations in hydrogen and oxygen isotope ratios from the values of the local meteoric water. This indicates that the water was derived not only from the meteoric water, and the deviations of the values may be explained by processes such as evaporation and mixing of other waters from deeper sources.
Taking into account of the rich mud volcano morphology that requires a significant amount of fine-grained sediment supply, mineralogy, and compositions of gas and water, we hypothesize that the Goshogake mud volcano field has characteristics of both magmatic and sedimentary systems. That’s why we call this mud volcano field a hybrid system.
Regarding the driving mechanisms at the Goshogake mud volcanoes, the buoyancy of muddy sediment, high fluid pressure under overpressured or under-compacted conditions due to rapid sedimentation, together with the presence of a gas phase in the sediment, could be key factors. However, we are still far from the full understanding of how the mud volcanism actually works in Goshogake. For example, the relationship between the Goshogake mud volcano field and earthquakes is unclear. The earthquake triggering of mud eruption has been proposed at some other mud volcano sites in the world. But we have not found clear evidence for the enhanced mud volcano activity triggered by earthquakes despite their frequent occurrences in the area. Interestingly, some gryphons exhibit slow but still notable changing of their shapes documented over the timescale of days to years resulting from a combination of low-level mud eruption and slow erosion due to rain and mass wasting. These minor morphological changes are intriguing for us, and we plan to study them further in the future.
Why are we studying mud volcanism? This is because mud volcanism is considered to provide a window to the subsurface. This means that we can learn a lot about the subsurface conditions and the environment of the past through the investigation of the sediments and fluids brought up on the surface by nature. This point is particularly important for future explorations of planetary bodies where deep drilling is technically difficult. Many scientists have hypothesized that mud volcanism has operated during the long history of Mars. Among such mud volcanism on the red planet, some may have been associated with magmatic volcanism, just as in the case of the Goshogake mud volcanic field. Our terrestrial analog study at Goshogake can hopefully help in assessing the feasibility of conducting a future mud volcano exploration on Mars where the erupted mud may contain information about the subsurface, ancient depositional environment, and even potential habitability of the planet.
Finally, Japan is blessed with the presence of numerous hot springs called “onsen.” The area around the Goshogake mud volcano field is also rich in onsen spots. In particular, the nearby Goshogake Onsen derives its hot water directly from the mud volcano field, and we can bath in the same water bubbling in the mud volcano vents, at lower temperatures of course! Despite the bizarre and hellish look of mud volcanoes, the nature of Goshogake provides benefits to those who visit the area as they can learn science while relaxing in the mud volcano water.
- Komatsu, G., Ishimaru, R., Miyake, N., Kawai, K., Kobayashi, M., Sakuma, H., Matsui, T., 2019. The Goshogake mud volcano field, Tohoku, northern Japan: an acidic, high-temperature system related to magmatic volcanism. Geomorphology, 329, 32-45, https://linkinghub.elsevier.com/retrieve/pii/S0169555X18305464.