1918 was a small 24 days eruption of Katla,10.600 B.C.
there was the largest known Eruption of Katla, the so called Sòlheimar-Eruption, an Eruption of Plinian Type [link to en.wikipedia.org] the only known one Plinian Eruption known for that Caldra, 10.600 B.C. it produced 10km³ of rhyolitic Tephra [link to en.wikipedia.org] Lahars flooded Myrdal and also Pyroclastic Flows, scientifically proven and known as loose Material and the so called Solheimar Ignimbrites [link to en.wikipedia.org] .

The Traces of the Material reach far into the South and INTO the sea, and are Signs of one of the largest Eruption ever known happening on Iceland.

The Caldera propably formed at this moment or heavily enlarged itself at the emptying of the Magma Chamber at this opportunity.

PS: Sorry for my bad English!