The western Tartarus Colles cone groups are located on Mars between northeastern Elysium Planitia and southern Arcadia Planitia (25°N–27°N and 170°E–171°E). This region contains >40,000 cratered cones covering a total area >2000 km2. These landforms have been interpreted as volcanic rootless constructs (VRCs) that were produced by explosive lava-water interactions ∼75–250 Ma ago. To better constrain their paleoenvironmental significance, we develop photogeological maps, morphological descriptions, lava thickness estimates, and statistical nearest neighbor (NN) analyses. The VRC-hosting Tartarus Colles lava flow exhibits bimodal thicknesses of 25–30 m and 55–60 m, whereas lava thickness associated with rootless eruption sites are unimodal, with a mean of 58 ± 8 m at 1 standard deviation (σ). Rootless eruption sites occur in 36 continuous domains with >10 rootless eruption sites per square kilometer and population sizes (N) >30. Renormalized Poisson NN analyses show that rootless eruption sites in 26 of 31 domains with N < 3000 are randomly distributed within 2σ confidence limits, whereas four of five domains with N > 3000 exhibit statistically significant clustering beyond 2σ. Regional clustering of rootless eruption sites in ∼60 m thick lava is interpreted to be the result of a minimum lava thickness threshold required to volatilize ground ice and generate rootless eruptions before the lava core solidifies. Ground ice continued to melt after VRC formation and formed pitted terrains through thermokarstification. Widespread VRCs and pitted terrains in the Tartarus Colles region imply the presence of a major fossil hydrothermal system that was generated by lava–ground ice interactions during the late to middle Amazonian.