A large proportion of landfill is paper and compostable material. This won't necessarily turn to rock, it will break down and a lot of it will leach out - as long as there is a route out of the landfill site. Plastics will also eventually degrade, releasing hydrocarbons.

Anything left over would turn into some form of sedimentary rock; I guess it would be similar to sandstone.

All the best,

Kat Austen (Letters and Comments editor, New Scientist)

Adding to Kat's answer, you'd have to know also what rock-making conditions the material is later subjected to.  If it just sits there buried, it'd likely be sedimentary.  But it might get crushed in magmatic heat deep in the earth, eventually; or (fancifully pondering) blasted by an incoming asteroid.  My point is that until you specify what process the material experiences, the question can not be answered in any definite way

Presumably it'll be quite a good ore for all the metals that we throw away instead of recycling: iron, aluminium, cadmium, lithium, copper etc.

In fact, that raises another question: how long will it be before it's more economic to mine landfills than natural ores? It must be easier to get some materials from waste than from the rock?

Because landfills contain large amounts of organic matter under chemically reducing conditions, some large deposits, suitably deeply buried, might well produce coal over a period of some tens of millions of years.

Considering the origin of that coal however, it would probably be very dirty-burning, high-ash fuel. Palaeontologists among the civilised cockroaches that supersede us might well find our garbology very interesting, but wonder what on earth we thought we had been doing...

So do I sometimes...

Do you?

Jon