---
basalt:
  id: 01jbq9d3m00000000000000011
title: Basalt
tags: [rock, igneous]
---
# Basalt

Basalt is a fine-grained extrusive igneous rock, the most common volcanic rock
in the Earth's crust and the floor of every ocean basin. It forms when
low-silica magma reaches the surface and cools quickly enough that crystals
have no time to grow large.

Its low silica content makes the melt runny, which is why basaltic eruptions
tend to flow rather than explode, and why basalt provinces are flat and vast
rather than conical.

## Why it cracks into columns

Cooling is the whole story — see [[Columnar Jointing]]. The rock contracts as
it loses heat, and contraction in a sheet has to be relieved somehow.

## Relation to obsidian

Obsidian is the same idea taken further: cooled so fast that it never
crystallises at all and ends up as glass. Same volcanic origin, opposite
texture. This protocol is named for the rock that cools into ordered columns
rather than the one that cools into glass.

Related: [[Giant's Causeway]], [[Start Here]].