In the early days of the solar system, many planetesimals orbited the Sun. Theory has it that one large planetesimal the

size of Mars struck a glancing blow to the molten Earth. How did this event contribute to the formation of the Moon?

This event led to the breaking away of a part of earth and the part was held on by earth's gravity and this part became the moon.

Explanation:

The formation of moon was after the formation of the earth. In the initial years of the formation of earth it was affected by several asteroid hits and comets. Small planets referred to as planetesimals also kept continuously affecting the earth. Before 4.5 billion years ago a planetesimal which was about the size of Mars hit earth.

This hit led to the breaking away of a part of earth. The part which was sent to space by the hit was caught by the gravity of the earth and the part started orbiting around the earth. The similarity of the rocks in moon to that of earth is attributed to the fact that moon was actually formed from earth.

The moon and earth keeps influencing each other. The moon's effect reduced the speed of earth's rotation and the moon keeps moving away from the earth due to the influence of earth.