The Earth's crust does not have the lightest weight rocks. While the crust is generally less dense than the mantle, it's not because it has the lightest rocks. Here's why:

* Density, not weight: Rocks are measured by density, which is mass per unit volume. A rock's weight depends on its density and its volume.

* Crust composition: The crust is primarily composed of igneous and metamorphic rocks like granite and basalt. These rocks are relatively light compared to the denser, iron-rich rocks of the mantle.

* Mantle composition: The mantle is made up of peridotite, a dense, ultramafic rock.

* Pressure: The incredible pressure from the weight of the overlying layers makes the mantle rocks denser than crustal rocks.

In summary:

* While the crust is less dense than the mantle, it's not because it has the lightest rocks.

* The density difference is due to the composition and pressure on the rocks in each layer.

* There are lighter rocks found in the crust, but they are not necessarily the lightest rocks on Earth.