A bubble can be defined as:

"A market phenomenon characterized by surges in asset prices to levels significantly above the fundamental value of that asset."

Though there is no fundamental value here, it is all speculative, I think something going from $20 billion to $900 billion in less than a year might fit that definition.

There are often smaller bubbles (or booms and busts) within the context of a larger bubble. I think that is what is going on here.

The larger bubble may reach significantly higher numbers. Some are calling for $20 Trillion.