This one was suggested by my husband, a keen birdwatcher (see below for why that’s relevant) and adder of troublesome pairs to my list.
Erupt is perhaps the better-known of the two. To erupt is to forcefully throw out rocks, lava, gas and ash, if you’re a volcano. The next meaning is to break out suddenly, usually used of something like a fight, and similarly, you can erupt into laughter, meaning it happens suddenly and forcefully. Finally, a spot or rash erupts when it appears suddenly on the skin, and a tooth erupts through a gum when it grows in your mouth and becomes visible.
To irrupt, also a verb, means to burst into somewhere, to enter suddenly or even forcibly. The kind of thing people do when they break down a door. Interestingly, I’ve seen people being described as “erupting” in this sense, but let’s use irrupt here if we can, to preserve those two senses, yes? The second (and husband-relevant) meaning is to migrate into an area in large – that’s abnormally large – numbers, and it’s especially used of birds. So when an awful lot of waxwings descended on some trees with berries in a Birmingham car park, that was them irrupting. Or an irruption.