Among the Moon’s many geological oddities, the Gruithuisen Domes stand out as puzzling exceptions. Situated near the western rim of the Imbrium Basin, these rounded volcanic domes (notably Mons Gruithuisen Gamma and Delta) contrast sharply with their basaltic surroundings. Their composition, form, and origin challenge many assumptions about the Moon’s interior—and even its volcanic history.
On the one hand, scientists accept that they are likely formed from silicic, viscous lavas (rich in silica) that piled up rather than flowed outward. But that immediately raises a cascade of questions: without water, plate tectonics, or typical Earth-style magmatic differentiation, how could the Moon generate enough silicic melt to build such domes? Even more, how did those domes remain intact over billions of years, resisting collapse or resurfacing?
The “impossibilities” of the Gruithuisen Domes are as follows:
1. How Do Silicic Lavas Form on a “Dry” Moon?
On Earth, silicic volcanism (andesite, dacite, rhyolite) is often linked to processes like partial melting in crustal plates, fractional crystallization, and the role of volatiles (notably water). These processes are uncommon or nearly absent on the Moon:
- The lunar interior is thought to be relatively water-poor, limiting the volatile components that help melt silica-rich magmas.
- The Moon lacks plate tectonics and a thick crust that can be reworked, a common setting for silicic magma evolution on Earth.
- For the Moon to generate a silicic melt, some unusually efficient crustal melting or differentiation must have occurred—perhaps via heat from radioactive elements or late-stage magmatic residuals.
Putting it plainly: the conditions to produce large volumes of viscous, silica-rich magma on the Moon seem unfavorable — making the existence of these domes feel “impossible” under many standard models.
2. Viscous Domes That Stayed Standing
If one manages to produce a viscous lava dome, you then have to keep it from collapsing, flowing, or being eroded. The Gruithuisen Domes have steep flanks and maintain distinct topography, implying that:
- The magma must have extruded slowly, building height rather than spreading wide.
- Cooling must have been rapid enough to prevent collapse but slow enough to avoid internal fracturing or deformation.
- Over billions of years of meteoroid impacts, thermal cycling, micrometeorite bombardment, and lunar quakes, the domes survived — which is surprising given their apparent fragility compared to the massive mare basalt flows around them.
This long-term survival suggests either that the domes are more rigid, stronger, or more resilient than we normally expect for silicic lunar rock — or that something preserved them in unique ways.
3. Volume, Scale, and Distribution
The volumes, sizes, and spacing of the domes also defy easy explanation:
- Their domes range in volume from tens to hundreds of cubic kilometers, meaning substantial magma reservoirs must have been involved.
- They’re localized near one sector of the Moon rather than widespread, implying either unique local conditions or a rare, late-stage volcanic event.
- Given how basaltic lava floods tend to erase or bury older topography, these domes must either post-date major basalt flows or persist despite them.
Reconciling those scales with the limited magmatic energy and thermal budget of the lunar interior is nontrivial.
4. Compositional Contrasts & Thermal Evolution
The domes’ lighter color, different thermophysical signatures, and distinct spectra suggest a non-basaltic composition (i.e. enriched in silica or incompatible elements) unlike the mare basalts around them.
To sustain such composition differences at depth over time, the Moon’s thermal and magmatic evolution must have allowed local differentiation, remelting, or concentration of heat-producing elements in the crust. Many lunar interior models don’t easily support such concentrated, late-stage melt generation in just a few spots.
What to Do About the “Impossible”?
Scientists aren’t ignoring these oddities. Proposed missions (e.g. Lunar-VISE) aim to land on the Gruithuisen domes and sample them directly, measuring composition, structure, and regolith details to test hypotheses about how they formed. The hope is that ground truth will clarify which mechanisms — rare silicate melting, crustal heating, latent volatiles, or other exotic processes — made these features possible.
Until then, the domes remain “impossible” in the sense that they compel us to reconsider simplistic models of the Moon. They remind us that even a relatively small body like the Moon can harbor surprises, and that lunar volcanism may have richer nuance than once thought.