What would happen if earth’s mass was gradually cut in half over a period of 75 years? The cause is irrelevant.

Long answer: It’s straight forward enough to calculate what the earth would look like if it lost half its mass. Then all we have to do is run the clock back and figure out what would have to happen to the earth to get to this point.

Assuming a constant density for the earth (not a bad assumption at all), the earth’s radius will be about 80% of its current value after the mass-halving. Surface gravity also goes down by the same factor. Beyond this it’s hard to say what happens because how the planet changes is going to depend where the magic mass-loss is happening. Is mass disappearing from the core? Is it a top down process? Are atoms randomly disappearing all over the planet? Don’t worry about why mass is disappearing – let’s just pretend a space-wizard is doing it.

I love questions where “a wizard did it.” It gives me a blank check to make shit up.

So let’s make some shit up.

If the earth is going to lose 50% off the top over 75 years, then we’re losing 0.66% each year. Coincidentally, the mass of the crust is close to 0.5% of the mass of the earth, so all life on earth would be dead within a year. In fact, the earth’s atmosphere is so sparse that it could all be lost in the first hour. At the end of these 75 years the core will have been untouched, but the earth will have lost a considerable amount of the mantle, leaving a hot magma ball where the earth once lived.

The earth would probably look similar to Venus when it’s all over. Needless to say, this would be bad.

But what about the other way around? Instead of losing mass from the top, what if the earth was losing mass from the core?

In this case I imagine that the earth is going to steadily contract over the years, leaving our planet with about 64% of its current surface area. This tells me that we’re going to need to do more than just play Tetris with plate tectonics. Since the continental plates are granite, which is less dense than the oceanic basaltic plates, I expect that the oceanic plates will subduct at the continental boundaries like they do now, but much more rapidly than if they were driven by natural processes alone. If the plates are currently moving at 5 cm per year, they’d have to subduct a million times faster to keep up with the contraction.

The earthquakes and volcanos would probably kill everyone to death, but I’m not a geologist or a doctor so don’t take my word for it.

Since the earth is losing 36% of its area, and I’m arguing that mostly comes from the loss of oceanic plates which cover 70% of the planet, then we’re effectively halving the area of the oceanic plates. Since the oceans have a lot of water (citation needed), I expect sea level to rise considerably. If the current ocean depth is about 3.5 kilometers, then at the end of the contraction there will be enough water to cover all the continental plates with the exception of the tallest mountains. The Himalayas would be the new Hawaii.