Sousaki volcanic area

If you travel from Athens toward Corinth, your eye will soon catch a strikingly colored gorge on the right side of the road near the large oil refineries. This dramatic valley hides part of the long-extinct Sousaki volcano, active roughly 2–4 million years ago — but still geothermally alive today.

Although the volcano has not erupted for millions of years, there are still active fumaroles (vents where hot gases escape) and mofettes, which release invisible carbon dioxide. These gases can accumulate and displace oxygen, creating a serious suffocation hazard. You cannot smell CO₂ — so never enter deep hollows or caves, especially when the air is still.

In the past, the area was mined on a small scale for sulfur and alum. Along the gorge you can see hydrothermal deposits: rust-red zones where iron and sulfur have altered the rocks. Small crystals of pyrite, chalcopyrite, and sometimes the toxic arsenopyrite can still be found. Secondary minerals like vivid blue chalcanthite form where water interacts with old ore veins. Near the entrance, shimmering gypsum crystals line parts of the slopes.

Modern geologists have drilled in the Sousaki area to test its geothermal potential; underground temperatures and gas emissions show that there is still low-level magmatic heat at depth. The entire region sits on a zone of complex tectonic faulting, connected with the larger Corinth Rift, one of Greece’s most geologically active areas.

Nearby, at Loutraki, you can spot impressive dacitic dykes — ancient magma conduits now exposed at the surface — and hydrothermal deposits of jasper, along with occurrences of opal and agate created by silica-rich fluids circulating through the rocks.