Why does my smoothie expand in the blender?

[Kermit power]

I've just made myself a smoothie for breakfast. When I put it on the blender, the cup is about 2/3 full, but as it's blending, the volume increases in very distinct steps until the whole cup is absolutely full. Anyone got any idea what's going on??? It seems to be happening when the blades hit frozen fruit, but why that would cause an increase in volume is a bit of a mystery to me...

Obviously I'd expect it to expand somewhat just because there's air being whipped in by the blades, but why does it do it in such distinct steps, why does it do it quite so much, and how come it doesn't settle back down once I've finished blending it?

Ingredients were 300ml Almond milk, 25g Protein powder, 100g Frozen fruits (blackberries, blueberries & cherries) and 40g Porridge oats.

[21TonyK]

It is just air basically and it would "shrink back" if you left it long enough. Either that or you have found a way of creating matter from nothing!

[IanMorewood]

Your ripping the fruit up and injecting air to the mix.

[Kermit power]

21TonyK said:

It is just air basically and it would "shrink back" if you left it long enough. Either that or you have found a way of creating matter from nothing!

The oddness, though, is the fact that it's doing it in such defined steps. Maybe a big frozen blackberry going through the blender blades does something extra strange to air???

[TwigtheWonderkid]

If you stir a cup of tea, the level is higher while you're stirring, as you create a vortex and the space that occupies forces the teas higher up the sides. Same in a blender. The vortex increases gradually making the level higher and higher.

[sherman]

Your milk is turning into whipped milk like you get in a cappucino basically

[mph1977]

IanMorewood said:

Your ripping the fruit up and injecting air to the mix.

this

plus a little bit of the vortex effect when the blades are going and immediatey after you switch off ...