At the end of the film Total Recall\((1990\) ), Arnold Schwarzenegger's character presses an alien button, the martian volcanoes start spewing, and within a few minutes the martian sky is blue, the atmospheric pressure is Earth-like, and the atmosphere is totally breathable. (Probably you can find the scene online.) What, scientifically, is wrong with this scene? That is, why would volcanic gases not quickly create a breathable atmosphere on Mars?

Mars' volcanic gases are mostly water vapor (H₂O), carbon dioxide (CO₂), and smaller amounts of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S). These gases are quite different from what you breathe on Earth. While volcanic activity can certainly produce gases, it can't turn Mars' air into breathable air like we have on Earth.
It would take a long, long time for these gases to change the atmosphere significantly.
Scientists call this timescale 'geologic time,' and it could mean millions of years. So, the idea that volcanic gases could make Mars' air breathable in just minutes, as shown in the movie Total Recall, is just not scientifically possible.
It’s a good story, but it's science fiction!

Atmospheric pressure on Mars is much lower than on Earth.
Mars' atmosphere is less than 1% the density of Earth's atmosphere. This low pressure is one reason why liquid water can't exist for long on the surface—everything either freezes or boils away.
This means that even if you managed to introduce more gases into Mars' atmosphere, it would not magically become Earth-like. Getting the atmospheric pressure to Earth-like levels would require vast amounts of gas. Mars doesn't have that many resources at its disposal to make such rapid changes.
The laws of physics mean that if you want to increase atmospheric pressure, it has to happen gradually.

Understanding the chemical composition of Mars' atmosphere is key to realizing why it can't quickly become like Earth's.
Mars' atmosphere is mostly carbon dioxide (CO₂)—about 95%. There's only a tiny bit of oxygen, just 0.13%. Compare that to Earth's atmosphere, which has 21% oxygen. The rest of Mars' air is mostly nitrogen and argon.
This composition makes Mars' atmosphere very different from ours. Any sudden change in the balance of gases would be incredibly unlikely. Humans need a lot of oxygen to survive, but the gases from Martian volcanoes don't include enough oxygen to make the air breathable.

Earth's atmosphere has about 21% oxygen, which is what we need to breathe and survive.
Mars' atmosphere has almost no oxygen. The volcanic gases there mostly release carbon dioxide, which isn't useful for breathing. There's no quick way to add oxygen on Mars in a short amount of time. Volcanic eruptions would release gases, but mostly water vapor and carbon dioxide.
Even if Mars had a lot of volcanoes spewing gases, it wouldn't produce nearly enough oxygen to make the air breathable for humans. Creating enough oxygen on Mars would require complex chemical processes that take a lot of time and engineering.

Mars' atmospheric dynamics are very different from Earth's.
The thin atmosphere, high radiation, and low gravity all affect how gases move and interact. Unlike Earth's dynamic atmosphere, which is shaped by oceans and strong winds, Mars has an almost static atmosphere with very little movement.
The thin atmosphere means rapid changes are almost impossible. Climate changes on Mars occur over long periods. Any substantial change, like creating breathable air, would also need the atmosphere to retain its new form, which Mars' thin atmosphere struggles to do.
It simply can't trap heat or gases as effectively as Earth's, making rapid climate changes not only unlikely but practically impossible according to current scientific understanding.