Erupting Volcano

A $100 \text{m}$ long erupting volcano ejects a rock with speed $10\sqrt{10} \text{ms}^{-1}$ at a such angle from horizon that rock gets it's maximum range. Find this unknown angle(in degree).

Details

1. $g = 10 \text{ ms}^{-2}$
2. There is no air resistance and no change in gravitational acceleration.