Rauðhólar (Reykjavík)

A branch of the Leitahraun lava flow, is called Elliðaárhraun and was emitted by the shield volcano about 5000 years ago.

The Rauðhólar built up on a location where lava flows entered and covered a small lake just north of today's Elliðavatn.

The heat of the lava causes expansion and boiling,[4] the water content is changed into steam and this initiates hydromagmatic explosions[2] “This often leads to the formation of very regularly shaped scoria craters“.

[2] In this way, these craters have no direct connection with a magma reservoir in the Earth's crust, thence the denomination as rootless cones.

[2] The internal stratigraphy of the cones shows the different phases of the eruption: “They are typically stratified (formed by multiple eruptive events), inversely size graded (showing decreasing explosivity with time), and capped with welded spatter (indicating cessation of explosivity due to volatile depletion).”[7] Some cones have more than one crater.

Scientists have identified potentially similar geological structures on Mars, and if confirmed they could be a sign of the existence of water at the time of eruptions on that planet.