Two pairs of overlapping volcanic domes are shown above, located 150 km southwest of Alpha Regio on the smooth plains of Tinatin Plantia. Each of the four domes are approximately 25 km in diameter and range in height from 190 to 440 m. Each dome, therefore was formed from the extrusion of 90 to 220 cubic kilometres of lava (Pavri et al., 1992) . Their close proximity to one another suggests that they may have been formed during an episode of volcanism whereby the magma source at depth (a hot plume-like source) moved relative to the surface. Looking at the southernmost, complete dome, one notices its remarkable circular outline. The bright radar backscatter on the southwest side of the dome and gradual darkening inward indicates a relatively steep margin that levels off toward the centre. This perimeter area is dominated by radial fractures. The central portion, in contrast, is dark except where radial and concentric fracturing has disrupted the surface. A small circular summit-pit is visible in the centre of the dome. These fracture patterns are indicative of swelling and/or subsequent collapse of the dome.
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