Initially it was believed that the Moon was created when a smaller planet called Theia grazed the Earth and broke up, sending a smaller chunk into space where it was caught in Earth's gravity.

But if that was the case the Moon would have a different chemical composition to the Earth, because it would be made up, predominantly, of Theia.

However, after studying Moon rocks brought back by astronauts on the Apollo missions, scientists at the University of California have found that their oxygen isotopes are the same as on Earth.

It means that the collision between Theia and the early Earth was so violent that the two planets effectively melded together to form a new planet, a chunk of which was knocked off to form the Moon.

"We don't see any difference between the Earth's and the Moon's oxygen isotopes; they're indistinguishable," said Edward Young, lead author of the new study and a UCLA professor of geochemistry and cosmochemistry.

"Theia was thoroughly mixed into both the Earth and the Moon, and evenly dispersed between them.

"This explains why we don't see a different signature of Theia in the Moon versus the Earth."

The crash with Theia happened approximately 100 million years after the Earth formed, almost 4.5 billion years ago.

It was thought Theia had collided with Earth at an angle of 45 degrees or more, described by the report authors as a "powerful side-swipe".

The researchers analysed seven rocks brought to the Earth from the Moon by the Apollo 12, 15 and 17 missions, as well as six volcanic rocks from the Earth's mantle - five from Hawaii and one from Arizona.

The key to reconstructing the giant impact was a chemical signature revealed in the rocks' oxygen atoms. More than 99.9 percent of Earth's oxygen is O-16, so called because each atom contains eight protons and eight neutrons.

But there also are small quantities of heavier oxygen isotopes: O-17, which have one extra neutron, and O-18, which have two extra neutrons.

In 2014, a team of German scientists reported in Science that the Moon also has its own unique ratio of oxygen isotopes, different from Earth's. The new research finds that is not the case.

Professor Young's research team used state-of-the-art technology and techniques to make extraordinarily precise and careful measurements, and verified them with UCLA's new mass spectrometer.

Theia, which did not survive the collision - except that it now makes up large parts of Earth and the Moon - was growing and probably would have become a planet if the crash had not occurred, added Professor Young.

Professor Young and some other scientists believe the planet was approximately the same size as the Earth; although believe it was smaller, perhaps more similar in size to Mars.

A head-on collision was initially proposed in 2012 by Matija Cuk, now a research scientist with the SETI Institute, and Sarah Stewart, now a professor at UC Davis; and, separately during the same year by Robin Canup of the Southwest Research Institute.

The new research was published in the journal Science.