The 18th of May marks the beginning of the end for the 2010 eruption. On this day, the eruption activity started to decrease rapidly and after the 22nd of May there was hardly any activity left apart from steam production and activity related to it.

Between the 18th and the 20th of May the height of the eruption plume decreased considerably going from 7 km in height down to 5 km, and the amount of magma coming from the crater decreased from about 200 tons per second down to less than 50 tons. There was still explosive activity within the crater, with limited amount of lava flowing from it. After the 20th of May the activity continued to decrease, and the explosive activity and the lava flow ceased. The eruption plume (composed of fine ash and steam) continued to decrease in altitude but from around the 22nd of May the eruption plume was mostly steam. Earthquake activity continued but only with shallow and minor earthquakes like those prior to the eruption.

Tuff forms in volcanic eruptions where the lava cools down rapidly due to interaction with water, which fragments the lava into glassy tephra. The tephra, which is often fine grained and dark in colour (ash), settles and then starts to solidify and forms fine grained tuff.

The Móberg (Palagonite) formation of Iceland is mainly the volcanic material that was formed due to eruptions under glaciers or in the sea in the latter part of the Ice Age, 0.78-0.01 million years ago. The formation covers an area of about 11,200 km² and can be found on all current active volcanic belts in the country. The formation consists not only of palagonite, as the basic units of the formation are also pillow lavas, intrusions, and lava flows. Hyaloclastite ridges and tuyas are common in the formation and many examples of them can be seen within Katla Geopark. There, the different strata of the Móberg formation can often be seen, such as cube jointed basalt, basalt columns, palagonite and palagonite breccia. The different strata can all form in the same eruption, but under different conditions during the eruption itself.

Palagonite is formed when magma cools rapidly during an eruption under a glacier or in the sea, but then the magma fragments and forms volcanic ash instead of lava. The ash then settles around the eruption vent and is then called volcanic tuff. The tuff soon undergoes chemical alteration so that it glues together to form rock, which is then called palagonite. The Palagonite breccia is formed under the same conditions, but when the fragmenting of the magma is not as forceful so that pieces of lava settle in the ash. Cube jointed basalt is formed by the rapid cooling of lava, lava that is either intruded into the tuff deposits or in a lava flow once water can no longer reach the eruption vent. The basalt columns are formed under the same conditions but with slower cooling than when cube jointed basalt is formed, but basalt columns can often be seen either below or on top of a layer of cube jointed basalt.