Question: During a battle between two extremely powerful space wizards, a beam of magical lightning has blasted a perfectly circular hole around 10 feet in diameter directly through an Earth-sized planet. In and out like a gunshot through paper. How would this impact life on the planet after this event, if at all?

NOTE: The incident occured about 150 miles south of the planet's north pole.

Answer: 
Depends on the side effects of the weapon.

* If a 3 m wide, 12,750 km long plug through the center of the Earth simply **disappears**, at worst there will be two smallish volcanoes and a couple of earthquakes. The total material is roughly a cube 450 m to a side.
* If sufficient force is applied to **punch** such a plug out of the Earth by conventional means like a bullet, the amount of energy that is imparted will cause major destruction.
Other answers have addressed the qualitative details of what happens, I want to try and put some numbers to it.

If the hole is filled by material flowing in from the sides, then the amount of energy released by the expanding material is roughly equal to its pressure times the volume of expansion. Looking at the plot from [this answer on Physics SE](https://physics.stackexchange.com/a/184039), the average pressure in the hole is, let's say, 200 GPa. Then the energy release is:

> 
> (π/4 × (10 feet)2 × 12 000 km) × 200 GPa = 1.7 × 1019 J
> 
> 
> 

We can convert this energy to a value on the [moment magnitude scale](https://en.wikipedia.org/wiki/Moment_magnitude_scale) of about 7. Now since most of the energy release will happen deep in the core and mantle of the Earth, people on the surface nearby will experience less than the equivalent of a magnitude 7 earthquake.

I guess that only the energy release within the first few hundred kilometers of the surface significantly affects the perceived earthquake magnitude. Looking back at the same plot, the slope near the surface looks like around 30 GPa per 1000 km. Taking a wild guess and setting the cutoff of 500 km, the amount of energy released "near" the surface is roughly:

> 
> (π/4 × (10 feet)2 × 500 km) × (1/2 × 30 MPa/km × 500 km) = 2.7 × 1016 J
> 
> 
> 

(The 1/2 accounts for the fact that the average pressure is equal to half the maximum pressure for a linearly increasing pressure.) This is an amount of energy equal to roughly 5 on the moment magnitude scale, which is not that big; globally, more than a thousand of magnitude 5 earthquakes happen every year.