The Hverfjall crater is 80-180 meters above the surrounding area with diameter of the rim over 1,000 m. It is among the largest of the tephra craters in Iceland. It was formed about 2500 years ago.
The magma connected with water and that caused some large steam explosions that ripped the magma apart causing the tephra and scoria to form. There were large clouded of ash over a large area and the lava flows reached up to 3 km from the crater.
Having seen Mt St Helens and witnessing the re-birth that has already started there, I was amazed that there is nothing growing anywhere near the crater after 2500 years, there is just the ash.
The views from the top of the crater were gorgeous and well worth the hike.