You cannot do this with "an explosion" for the reasons illustrated by @HalfThawed.
You would need the equivalent of a fizzle cannon shot - dig a very deep pit, have a very large "boulder" (a mountain actually) inside, have some reaction create a sufficient overpressure in the pit that the mountain is launched towards Earth but not so quick and violent that it disintegrates. It must also not rotate, since centrifugal forces would quickly disintegrate the chunk anyway.
Then, there is no easy ballistic trajectory from the Moon to Earth: everything on the Moon has an orbital velocity of roughly 6.28*384 thousand km per 28 days. I'd say about one kilometer per second radial. We need to neutralize the Moon's attraction, and the orbital velocity. We need 1.4 km/s upwards and 1.0 km/s counter-orbitwards ("widdershins" maybe, since orbit-wards is sunwise?), so the chunk must be ejected at about 1.7 km/s at an angle of 55° from the horizon (35° from vertical) and at the appropriate point of the Moon's orbit.
To reach a terminal speed of 1.7 km/s (1700 m/s) with an acceleration of 1G, supposing constant acceleration, it takes 170 seconds, so the mountain has to cover a distance (i.e. the depth of the pit) of s = 1/2*g*t^2 = approx 0.5*10*170*170 = 144 kilometers. With a 10 times higher acceleration, pit depth decreases by the same factor, so about 15 km. I am totally not sure of this, but it seems to me that such a large chunk of space-sintered regolith would probably crack under a stress of 10G, turning the "gun" into a space shotgun.
If it did not, we would have a mountain in L1 with zero speed, starting to fall towards Earth from a distance of a bit more than one light-second. The kinetic energy it acquires is the same as the energy required to leave Earth and attain L1, which should not be too different from the energy required to reach infinity, which is Earth's escape velocity and known to be about 11.2 km/s. So, our mountain would gain the same velocity falling down, which means an impact at about 10 km/s after a fall on the order of one day.
If we reduce the horizontal component a bit (allowing for the pit to be more vertical), the chunk would (from the Earth's standpoint) assume a more and more curved trajectory until it missed Earth completely, entering a different orbit (much lower than the Moon's) with a higher and higher perigee. When perigee is below some 400 km, the chunk would experience a significant aerobraking, making the orbit decay and impact after some difficult-to-predict time.