Laura Harms' 74th challenge is to use concentric circles for strings.
Here's Maria's contribution:
If you are new to Zentangle, this may look a bit complicated. However all the Zentangle patterns and creations you see here and around the web are each done one simple stroke at a time in a particular sequence.
It's also how those tree rings appeared: one at a time as each ring eccentrically concentrically aura'd the one before it.
Click image for larger view.