Iceland lies astride the Mid-Atlantic Ridge where tectonic plates move apart from each other, causing a rift zone. A mantle plume, or commonly referred to as a hotspot, is situated below the country, centered beneath the Vatnajökull ice cap. In South Iceland the interaction of the rift zone and the mantle plume results in complex and diverse volcanic activity.

Volcanic activity and its widespread effect on the area’s nature and landscape make Katla Geopark a truly unique area. The Geopark is in the most volcanically active area of Iceland, and the volcanic systems at Eyjafjallajökull, Katla, and Grímsvötn are particularly active. The region is characterised by central volcanoes, eruptive craters and fissures, rootless cones, lava fields, table mountains (tuyas), and hyaloclastite ridges which trend SW-NE, like the rift zone.

Ice caps are prominent in the landscape, topping the highest volcanoes. Outlet glaciers and glacial rivers flow from them and glacial landforms, like moraines and ice-dammed lakes, are occur in the area. Large floods, usually glacial outbursts (jökulhlaup) associated with subglacial eruptions, have formed extensive outwash plains in the lowlands. The oldest bedrock in the area is about 2,5 million years old, and can be found at the base of Lómagnúpur, an old sea-cliff, standing 671m tall. Other interesting features in the Geopark are fossil-bearing xenoliths, and tephra layers which are useful for dating (tephro-chronology).

Have a look at the geological report of Katla UNESCO Global Geopark. An overview of the most prominent geological features. 

Interesting links, videos and other media:

If you are interested in geology and especially, plate tectonics, this website has a wealth of knowledge in an interactive webpage that is fun to browse through. 

BBC made a documentary called "Iceland Erupts" in 2012 where Kate Humble investigate some of the biggest eruptions in Iceland's history. That includes the 18th century eruption that resulted in a world wide famine, and thought to be one of the deadliest of eruptions in known history. Ash and fumes caused a prolonged period of fog, leading to the deaths of thousands in Iceland and further hundreds of thousands across Europe, Asia and America and ruining crops around the world. An interesting and informative documentary that delves into the volcanism of Iceland in an understandable yet detailed way. 

Surtsey, administratively not (yet) part of the Geopark, though geologically very much so, caused an uproar of volcanism in 1963. During its formation it was already protected as a natural conservation site, and in 2008, UNESCO declared the island a World Heritage Site in recognition of the great scientific value it presented. Below is a 20 minute documentary celebrating 50 years since its formation. A TV-crew was permitted on the island where they delve into the findings of biologists and geologists from 50 years of observations.