When the water boils it makes steam creates the bubbles and the bubbles push the water through the opening to form an eruption. The second part of eruption is the middle. This is where the water comes out of the geyser. As a viewer you would first see some bubbles and small bursts of water and steam. Then it would gradually get larger until it reached a height depending on the geyser and the water supply pressure.
Some geysers just bubble and give foot high bursts of water and steam.
Others get so high it sprays its viewers. The height depends on the plumbing system and pressure. The last part of eruption is the end. This is when the geyser stops shooting up water. The eruption ends slowly.