A basalt fan structure is a

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

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 un ...

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 plates a ...

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