How Obsidian Forms: Cooling Too Fast to Crystallise

Two things happening at once

Molten rock cooling down is doing two jobs at the same time. Its atoms are trying to arrange themselves into ordered, repeating structures. And the material is losing heat to whatever is around it.

Run it slowly and the atoms win. They find their places, build outward, and you get big crystals you can see without a lens. That's granite.

Run it fast and the heat wins. The material goes solid before any order can establish itself, and you get a solid whose insides are arranged like a liquid, frozen mid-mess.

Obsidian is the second one. It's what happens when lava cools faster than crystal growth can get started.

Why it happens at the surface

Rock underground cools slowly because it's surrounded by other hot rock. Rock at the surface is surrounded by air and water, and it loses heat in a hurry.

So obsidian forms where lava reaches the surface and then cools fast: at the edges of a flow, in thin flows, or wherever lava runs into water or ice. The middle of a thick flow cools slowly and turns into ordinary crystalline rock. The outside of that same flow can come out as glass.

Which is why you can find obsidian and its crystalline relatives a few metres apart, from one eruption.

Why it doesn't last

Glass isn't stable over geological time. Give it enough time and enough water and obsidian slowly devitrifies, which means tiny crystals start growing inside the glass until it isn't glass any more.

That takes millions of years. Short by geology's standards, long by everyone else's. Two things follow from that. Most obsidian you'll ever handle is geologically young. And samples can be dated by measuring how far the devitrification has gone.

About the water

Obsidian usually holds a small amount of dissolved water, generally under one percent by weight. That water is part of why it's a glass at all, since it disrupts the orderly arrangement crystals need.

It's also why obsidian feels slightly different in the hand from quartz. It's a little less dense, and it doesn't come across as cold on first contact the way a dense crystalline stone does.

What that means for a stone you're holding

Every piece of obsidian is the record of a cooling event, hours long at most, from an eruption that happened somewhere specific in the last few million years. The bubbles and the faint flow banding you can see in a finished piece are the last traces of lava that was still moving when it froze.

No two pieces look alike because no two cooling events went the same way.