What causes bubbles in extruded filament? I've noticed this on almost ever print I've ever had. On the initial first line that clears the extruder nozzle tiny little bubbles/craters seem to form on the line. While I don't think these are causing any issue with my prints I'm curious to know the reason why they form at all. 

Is this due to water absorption in my filament that turns to steam, which then bursts through the molten plastic? Is it due to air bubbles in the filament that are cause by the manufacturing process of the filament? Or is this more an indication that my nozzle is damaged or clogged in some way?



This image was made using ABS plastic and a heated build plate. I've noticed these same 'bubbles' appearing using PLA, and Nylon. 

Edit: Nozzle temperature 240°C, build plate temperature 150°C, Nozzle diameter 0.4 mm, filament diameter (measured 1.75 mm) retraction distance 1.7 mm. Using the Makerbot Desktop Slicer.

The first line that my printer extrudes, where I'm seeing these 'bubbles' is a nearly full line. Makerbot starts from the right side of the image, extruding to the left.
Oh yeah, that's simple. You are printing too hot and are literally boiling the plastic. Else you have water. However if it was water you would hear Crackling as it printed. If it is too hot you will not hear nearly as much. I am 87.341% sure you are printing too hot.
Looking at your printing temps you are without a doubt printing too hot.
From this link on 3d hubs.

PLA (Only on Replicator 2) Print temp: 210°C (at 100m/s) Notes: heated
bed optional between 40 and 60°C
ABS (Only on Replicator 2X) Print
temp: 230°C (at 100m/s) Notes: heated bed at 110°C