The name Avalanche conjures up images of a game featuring skiers and snowy mountains, so it's something of a surprise to find that this contains neither. Instead it's raining gems inside your Vic 20, and you must prevent any of them from crossing the horizontal line at the bottom of the screen.
You make a gem disappear by shooting it with your infinite supply of bullets. The problem is that, unless you match the symbol on your ship with the symbol on the gem before you shoot at it, your bullet will have no effect. So the race is on to flip your ship to the correct symbol (There are five possibilities), duck underneath the gem 'column' closest to the horizontal line and ensure that gem eats lead.
You only have a single life and, if any gem, crosses the line that's game over.
As Avalanche progresses, it gets progressively faster. Something I quite liked about playing it is that, for the first thirty seconds or so, it really does seem to be quite easy - you'll be shooting those gems out of the sky long before they get anywhere near the point of no return. However, the Vic 20's screen, by its very nature, is extremely small and it only takes a short speed increase before those gems which, only seconds earlier, were only making it halfway down the screen are suddenly able to reach spitting distance before biting the dust.
It moves quite speedily and, whilst being an extremely simple game, is surprisingly addictive.