Petit spots

Fun geology term for the day: petit spot
 

Hirano et al. (2013) use the term to “petit spot” to describe young (~8 Ma to 50 ka), small (<1 km³) basaltic eruptions on the downgoing plate just before it enters the Japan trench. As the lavas are on the wrong side of the trench for subduction related volcanism, and thousands of km from a divergent margin, the authors suggest some unusual explanations for their presence: bending (flexure) of the downgoing plate just before it arrived to the trench created fractures which allowed asthenospheric melts to rise to the surface. High ⁴⁰Ar/³⁶Ar ratios are attributed to an upper mantle source. The problem of why the asthenosphere melted in the first place is not delved into too much, although they suggest that these petit spots provide evidence for the long-debated hypothesis that the low velocity zone in the asthenosphere is a reservoir of long-lived partial melts.

 

Source: Volcanism in Response to Plate Flexure, 2006, Naoto Hirano, Eiichi Takahashi, Junji Yamamoto, Natsue Abe, Stephanie P. Ingle, Ichiro Kaneoka, Takafumi Hirata, Jun-Ichi Kimura, Teruaki Ishii, Yujiro Ogawa, Shiki Machida, Kiyoshi Suyehiro, Science 313, 1426 (2006); DOI: 10.1126/science.1128235