Scientists know that Earth was once completely molten because a. the surface is smooth. b. the interior layers are denser. c. the chemical composition indicates this. d. volcanoes exist today.

One significant clue that the Earth was once completely molten is the varying densities of its interior layers. The Earth is composed of three main layers: the crust, the mantle, and the core. Each of these layers has a different density.
The outermost layer, the crust, is the least dense. Below the crust is the mantle, which is denser than the crust. The innermost layer, the core, is the densest of all.
This density gradient is a direct consequence of the Earth being in a molten state in the past. When the Earth was molten, the denser materials (like iron and nickel) sank towards the center, forming the core. Lighter materials moved towards the surface, forming the crust and mantle.
The process of denser materials sinking is called differentiation.
Understanding this helps scientists conclude that the Earth must have been molten to allow such separation and formation of distinct layers.

The chemical composition of the Earth's layers gives us further evidence that the Earth was once completely molten. When scientists analyze rocks from different layers of the Earth, they find distinct chemical compositions that couldn't form under stable, non-molten conditions.

For instance:

• The Earth’s crust contains silica-rich rocks.
• The mantle contains silicate minerals with more magnesium and iron.
• The core is primarily made of iron and nickel.

If the Earth hadn't been molten, these varying compositions wouldn't be as distinct. During the molten phase, chemical differentiation took place. The materials separated based on their weights and chemical properties when the Earth was in its liquid state.

Once the Earth started cooling, these elements crystallized into different minerals, forming the layers we study today. These variations confirm that at one point, the Earth had a uniform composition that underwent significant changes due to its molten nature.

Volcanism provides visible and tangible proof of the Earth's molten past. Volcanoes are essentially vents through which molten rock (magma) from the mantle reaches the Earth's surface. This indicates that there are still molten regions within the Earth.
Here’s how it works:
1. The Earth's interior heat causes rocks to melt, forming magma.
2. Pressure builds up beneath the Earth’s crust.
3. This pressure forces magma up through weak spots in the crust, resulting in volcanic eruptions.

The presence of volcanoes shows us that parts of the Earth’s interior are still molten, hinting at a time when the Earth was entirely in this state.
Volcanic rocks also carry clues from deep within the mantle. Examining these rocks helps scientists understand the temperature and chemical processes that have shaped our planet.
In summary, volcanism is a dynamic reminder and a direct link to the Earth’s fiery, molten origins.