Scientists would typically use meters (m) or centimeters (cm) to measure the thickness of a rock layer created as a result of volcanic activity.

Here's why:

* Meters: For thicker layers, meters provide a convenient scale.

* Centimeters: For thinner layers, centimeters offer greater precision.

While other units like millimeters or kilometers could be used in specific circumstances, meters and centimeters are the most common and practical units for measuring rock layer thicknesses in volcanic environments.