Many scientists have asked whether Earth was completely molten when it first formed. It is common to see early Earth drawings representing the original formation of Earth as an ocean of magma, and truth is that no one actually knows for sure.

However, there is evidence for and against your idea. The moon's surface has not really changed since its formation 4.6 billion years ago, and moon rocks brought back from the Apollo missions suggest the early moon was a magma ocean a few hundred kilometers thick. However, for scale, Earth's crust is 30-50c km thick and 2900 km to the outermost core. So a hundred kilometers of magma ocean would not support a completely molten Earth.

Another line of evidence is something called primordial noble gases, mainly helium gas (the gas in balloons that causes people's voices to become high pitched) produced by lava at Earth's surface. If early Earth was entirely molten, all the helium in Earth would have been released and escaped to space, yet there is still helium available on Earth. However, it is possible that helium was trapped as pockets inside the planet, providing the helium available today.

A third argument, in favor of a molten Earth, is the fact that most of Earth's crust is thought to have formed between 3 to 2 billion years ago, with very few rocks older than 3 billion years. One explanation is that Earth's mantle was solidifying (from its molten state) prior to 3 billion years ago, thus preventing earlier crust formation. Scientists continue to research early Earth, and some day there will be a better answer to your question.

About 4.5 billion years ago, Earth formed from lots of particles crashing together in space. The collisions generated a lot of friction, which made Earth extremely hot. Earth has been cooling ever since, but the inner core is still 5,000 - 7,000 degrees Celsius or 9,000 - 13,000 degrees Fahrenheit.

A scientist and mathematician named Lord Kelvin tried to calculate the age of Earth from its cooling in the 1800's, but the age he calculated was too young because he did not know about radioactivity. There are many radioactive elements inside the earth, which produce heat when they break down. This keeps the inside of Earth from cooling off as quickly as it otherwise would.

Earth was molten for a period around 4.5 billion years ago due to constant bombardment from asteroids and other objects, and then melted again when the iron in the Earth sank down to form the planet's core. For about two billion years more after that, there was a solid surface of the Earth but the interior was still molten. Over the past 2.5 billion years, more and more of the Earth has frozen, until now the Earth still has a liquid outer core but a solid inner core below it and a solid, albeit plastic, mantle above it.

Yes, it was! Four and a half billion years ago our solar system was born. At first, instead of planets there were only rocks and dust flying around. Over time, those rocks and pieces of dust started to collide and stick together which grew from a baby planet into the large planet Earth we have today (just like how packing a lot of snow together will make a snowball). This is actually how all of our planets formed! Those pieces of rock and dust were flying around really fast with a lot of kinetic energy before they collided, so when they hit each other they released all of that energy in the form of heat.

If you rub your hands together really quickly you may feel them warm up. What you're doing is also transforming kinetic energy (which is the energy of movement) into heat! So every time a new rock collided with the growing planet Earth, it transformed its kinetic energy into heat and caused the newly-formed Earth to be super hot.

Some scientists even think our newborn planet was so hot that it was a magma ocean. This means the entire planet was one big ball of lava (so if you were playing the floor is lava, you would lose really quickly!) Over the last 4 and a half billion years, the Earth has been cooling off, just like how really hot soup will cool off if you let it sit for long enough.