No one has known when major volcanic activity on planet Mercury stopped until now. According to a research team from the North Carolina State University, major volcanic activity most likely ended 3.5 billion years ago.

The team did not have rock samples from Mercury that they could have used for radiometric dating. Instead, they used crater size-frequency analysis, through which they analyzed the craters’ numbers and sizes through pictures gathered by NASA’s MESSENGER mission.

As stated in the journal Geophysical Research Letters, there are two types of volcanic activity. The first is explosive volcanism, which is a violent form of volcanic activity that includes large ash and debris eruptions. The team cites that the Mount Saint Helens eruption in 1980 is a good example for this type.

The second one is called effusive volcanism. Unlike the explosive type, effusive volcanism only includes widespread flow of lava, which spreads over the landscape. According to researchers, this type of volcanic activity plays a crucial role in how planets form crusts.

Due to these findings, scientists may unveil the mystery about the geological evolution of Mercury. These findings also provide additional information into the consequences when rocky planets, like Earth and Mercury, contract and cool over time.

The effusive volcanic deposits shed light into the geological evolution of Mercury. By comparison, this type of volcanic activity only stopped a few hundred million years ago on Venus and a few million years ago on Mars. On the other hand, effusive volcanic activity is still present on Earth.

The team believes that the volcanic activity stopped because Mercury’s mantle lost its heat earlier than the other planets due to its smaller size. Consequently, Mercury’s surface cooled and contracted, covering any conduits where magma could come out again.

“These new results validate 40-year-old predictions about global cooling and contraction shutting off of volcanism,” says planetary geologist Paul Byrne, who is also an assistant professor at North Carolina State University. “Now that we can account for observations of the volcanic and tectonic properties of Mercury, we have a consistent story for its geological formation and evolution, as well as new insight into what happens when planetary bodies cool and contract.”