I have never really thought about why the moon doesn't have volcanoes. According to scientists, the moon has plenty of liquid magma locked away in its core to produce volcanoes. Scientists believe they have figured out why exactly that magma doesn't bubble to the surface of the moon and create volcanoes such as the ones we have here on earth.
The answer scientists settled on is that the magma inside the moon is so dense that it can't bubble to the surface. The European scientists came to the theory after they took moon rocks collected by the Apollo missions and melted those rocks at high pressures and temperatures. The high pressure and temperatures used in the experiment are believed to be the same as conditions present inside the moon. After melting those rocks down, the density of the resulting magma was measured using X-rays.
Measurements taken were combined with computer simulations that allowed scientists to calculate the density of the magma at any location on the moon's surface. The magma was found to be less dense than the solid surroundings, which is how it is here on earth. However, the big difference between magma on the moon and the magma present on earth comes from tiny droplets of titanium-rich glass that was first discovered by the Apollo 14 mission inside moon rocks. That titanium-rich glass produces liquid magma this is a dense as the rocks found in the deepest parts of the lunar mantle today making it too heavy to move towards the surface. There is a chance at some point in the distant future things on the moon can change, and a volcano could form according to the team.
"After descending, magma formed from these near-surface rocks, very rich in titanium, and accumulated at the bottom of the mantle – a bit like an upside-down volcano. Today, the Moon is still cooling down, as are the melts in its interior. In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings," says Wim van Westrenen from VU University Amsterdam.
"This lighter magma could make its way again up to the surface forming an active volcano on the moon – what a sight that would be! – but for the time being, this is just a hypothesis to stimulate more experiments."