Why does Moana Kea have so many cinder cones?

Submitted by OlderThanMyParents t3_z0an2u in askscience

Just came back from a trip to Hawaii, and we drove over the Saddle Road, and I was really surprised by how many cinder cones are scattered all over the south side of Moana Kea. By contrast, there are comparatively few on the north side of Moana Loa.

I've spent a lot of time around volcanoes in the PNW, from Mt. Baker to Shasta, and never seen that quantity of cinder cones on any of them (although Adams does have a very distinctive cinder cone on the north side.) I assume at least some of them would be erased by glacial activity, but apparently Moana Kea used to have glaciers.

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Rosevkiet t1_ix517v4 wrote

Mauna Loa and Mauna Kea are both shield volcanoes formed by the Hawaiian Plume, but they are in different phases of life for volcanoes of this type. Mauna Loa is in active shield building, where you have large volumes of silica-poor lavas with low viscosity with frequent eruptions. The eruptions occur both from the central, summit vent, and from fissures along the flanks of the mountain. Mauna Kea is currently dormant and has entered the post-shield phase. During this phase lavas become more silica rich, with higher viscosity, and usually have a higher concentration of water, CO2, and SO4 in the magma. Higher viscosity lavas with higher volatile contents are more explosive, gases the exsolve from lava as it rises cannot escape like bubbles in a pot of boiling water. They grow in the lava, becoming bigger as lava rises to the surface. This makes the lavas less dense, making them ascend faster, and eventually when lava hits the atmosphere, they explode, creating cinders.

You can also get cinder cones during active shield building, but it isn’t as common, and since there are so many lava flows happening all the time, the landscape is constantly being covered by new flows and any surface record of cinder cones are lost. Mauna Kea has not been active since 4000 years ago, though it is likely to become active again at some point in the post-shield stage.

An interesting detail about Hawaii is that there are actually two trends of volcanoes, called the Kea and Loa trends, that can be identified by rock chemistry. On the big island Kohala, Mauna Kea, and Kilauea make up the Kea trend, and Hualalai, Mauna Loa, and Loihi make up the Loa trend.

Edited to add some nots that were lost.

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PBJ_ad_astra t1_ix5nf8z wrote

Can you explain more about the Kea/Loa trends? Kilauea is active (like Mauna Loa), so presumably you are not referring to the temporal evolution of silica content.

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Rosevkiet t1_ix6lxxr wrote

They are observed all along the Hawaiian chain, with Kea on the ne side of the Hawaiian arch and loa on the sw. I’m not sure what the origin of the geochemical differences are between the two, or the reason there are two tracks. The geochemical differences are in radiogenic isotope ratios derived from decay series like Sm-Nd, U-Pb, and Rb-Sr. In mantle derived rocks there are variations attributed to different mantle components, usually considered to represent recycled crust that was carried to the deep mantle, entrained in a plume, and preferentially melted. The kea and loa trends have different proportions of different mantle components.

My reading on the subject is out of date, s may be up to lunch on this, but I think it may mean that the spatial variability of the Hawaiian plume is constant through time, which is pretty neat to think about.

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CrustalTrudger t1_ix7vpui wrote

The two geochemical tracks in Hawaii (and as observed in many other plume related hotspot tracks) are thought to be related to some sort of heterogeneity in the mantle plume itself. As described in the recent review on mantle plumes by Koppers et al., 2021, there are three basic models to explain this:

  1. An unzoned, but heterogeneous plume where the double tracks reflect different components with different melting temperatures.

  2. A concentrically zoned plume with the hottest and densest portion of the plume material in the center.

  3. A bilaterally zoned plume where one half of the plume is more a direct sampling of the source LLSVP and, since most plumes originate from the edge of LLSVPs at the core-mantle boundary, the other half incorporates more "ambient" mantle.

At present, the bilateral model is more favored as a general explanation, but it doesn't explain all of the observations of double tracks at all hotspots, so there may not be a single mechanism. Specific to Hawaii though, the bilateral plume model is the favored one (e.g., Williamson et al., 2019).

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Darkwaxellence t1_ix4z0lb wrote

There are at least 30 cinder cones that have come out of vents in my lifetime there in Hawaii. The volcanic system is very active and over geologic time you would expect to see lots of them. The ones you see at the saddle road area have not experienced ocean erosion and so once they pop up they stay for a long time. You're watching a mountain being formed.

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twitch_delta_blues t1_ix500aw wrote

Because it’s an old volcano, but not so old as to have been eroded. Hawaiian volcanoes go through stages. Kilauea is very active and young. Mauna Loa is also active but not so young. Mauna Kea is much older and went though a late stage resurgence of activity that precedes dormancy. This is the source of the small cinder cones. Kohala is ancient and has been eroded away. Eventually Mauna Loa is go through the same process and make its own small cinder cones.

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pretendperson1776 t1_ix53gyn wrote

To add to this, the PNW range is made from subduction of two or more plates. Hawaii exists due to a Hotspot that the oceanic plate is moving over. Imagine a candle over moving paper, the flame burns through occasionally.

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