How much damage does lava do?

How much damage does lava do?

On contact?

On submersion?

I know being in hot temperatures for long periods of time invokes levels of exhaustion. But the DM’s guide appears to neglect damage for lava, but it does specify damage for “falling” etc.

During the session last night I chose 10d6 for submersion for level 5 characters and that worked for the moment, but now I’m researching what the damage should have been.