The Earth's layer formed by volcanic activity is the crust.

Here's why:

* Volcanoes erupt magma (molten rock) from the Earth's mantle. This magma cools and solidifies, creating new crustal material.

* Most volcanic activity occurs at plate boundaries, where tectonic plates interact and create opportunities for magma to rise to the surface.

* The crust is the outermost, thinnest layer of the Earth, and is constantly being renewed and recycled through the process of plate tectonics and volcanic activity.

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