## Symmetries of volcanic distribution on Mars and Earth and their mantle
plume dynamics

**Ctirad Matyska**
**, David A. Yuen, Doris Breuer & Tilman Spohn**
### Abstract

The symmetries in the distribution of major volcanic centers on Mars have been
analyzed by employing a technique involving the angular self-similarity of
point fields. The distribution of all central vent volcanoes exhibits an
axisymmetry along with a reflecting symmetry about a plane whose normal is the
axis of axisymmetry. The latter axis is skewed by about 30 degrees from the
rotational axis and is suggested to represent the early axis of symmetry of the
interior dynamics. The high level of symmetry of Mars' volcanic distribution
may be the result of a focusing of seismic energy from a major impact in the
Hellas area that formed the Hellas basin and may have triggered the onset of
volcanic activity in the greater Tharsis area. If the highland paterae near the
Hellas region are not taken into account in the analysis, an asymmetry with
respect to the reflecting plane appears. This is probably representative of the
Late Hesperian and Amazonian, when the once global volcanic activity had
retreated to the Tharsis province. These angular symmetries suggest a simple
pattern of long-wavelength axisymmetric martian mantle convection involving
a large stationary plume, as shown by our three-dimensional
(3-D) spherical-shell simulations. On the
basis of these calculations, we venture to propose that the volcanic history
of Mars reflects the transformation of a bipolar mantle plume structure into
a single megaplume by the process of plume-plume collision acting in concert
with the deep Martian phase transitions. This transformation might have
overcome the original pattern induced by a major impact forming the Hellas
basin. We have repeated the procedure for the Earth's hotspot distribution,
which revealed both axisymmetry and a certain level of reflecting symmetry.
This similarity between the two planets is striking. The symmetries of the
Earth's hotspot distribution are probably due to the interaction of the lower
mantle megaplumes with the endothermic perovskite to gamma-spinel phase
transition.

*J. Geophys. Res.*, **103** (1998), 28587-28597.