How does lava burn without oxygen?

Because lava is not burning. It is hot, yes, but not on fire. Deep underneath the surface of the earth, rock is under extreme temperature and pressure, which causes solid rock to become liquid. Its heat is what makes it appear to be burning. read more

Non-lava objects in contact or in close proximity to lava will combust as the lava provides the thermal energy required for ignition, but lava itself is not on fire. It only glows because the heat is lost in various other forms of energy. read more

Lava is molten (lava and it contains several minerals and metals in liquid/fluid form burning with oxygen and other gases and ready to erupt like a jet of liquid releasing the gases upto a height of about 20 kms above the earth- refer vesuvious. As said, it is not on fire. This is a common misconception. read more