The asthenosphere, the layer of transition zone before the mantle layer, is composed of soft but not molten rocks. Why is this so?

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The primary reason why the asthenosphere is soft but not molten is due to its temperature and pressure conditions.

1. Temperature: The asthenosphere is located at a depth of about 100 to 700 km below the Earth's surface where the temperature ranges from approximately 1300 to 2200 degrees Celsius. This is hot enough to soften the rock but not enough to completely melt it.

2. Pressure: Despite the high temperatures, the immense pressure at these depths prevents the rocks from melting. The pressure keeps the atoms in the rock closely packed together, preventing them from entering a liquid state.

3. Composition: The asthenosphere is composed mainly of silicate rocks, specifically peridotite. This type of rock requires extremely high temperatures to melt, higher than what is typically found in the asthenosphere.

4. Presence of Water: The small amount of water present in the asthenosphere also helps to soften the rocks. The water lowers the melting point of the rock, allowing it to become ductile at lower temperatures.

So, while the asthenosphere is indeed very hot, the combination of high pressure, specific composition of rocks, and the presence of water keeps it in a softened, or "plastic" state, rather than a molten state.