A basalt fan structure is a

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

formation composed of columnar jointed basalt columns that have slumped into a fan shape. They are most likely created when basalt

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

is intruded into a soft material, such as loose

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...

ash, which is unable to support the intrusion. As the basalt cools and forms columns, the ash collapses, causing the lava spread and form a fan shape. The formations can be vertical or any degree of horizontal.

References

Columnar basalts {{Volcanology-stub