The NatGeo Explorer did not have far to go for our next day which would be spent on the Iceland mainland in the region called Norðurland eystra or Northeastern. [The names of the regions of Iceland are about as imaginative as the states of Australia!] It was to be a long day in a large coach, but (because it never gets hot here!) the bus had no effective air-conditioning, and it was pretty uncomfortable. Average Summer maximum temperatures in Mývatn are 12-14oC, but for our visit the it was in the 20's and near to the record high of 25.6oC. Climate change for all to see? What's more, the day's itinerary took us to places accessible by large cruise ships, and some of them were very crowded. Today's experience reinforced our long-held beliefs that cruise ship destinations are no place to be, ever, and that long coach trips are to be avoided generally, but what we saw on the day was pretty decent compensation for the discomfort. As our guides explained, the experience we shared today with many others was too good, too important to miss.
Arriving in Husavik, the dominant feature is the wooden Swiss-chalet styled Husavikurkirkja church building (1907). Reforestation projects are evident in the background.
The NatGeo Explorer dropped us off at the port of Husavik ('bay of houses'), at the bottom of the Bay of Skjalfandi, first settled in 870AD by the Swedish Viking and farmer Garoar Svavarsson and the first place in Iceland to be settled by a Norseman. The port was big enough to accommodate our modest ship. Husavik is famous for its whale watching (we saw some humpbacks off Grimsey the day before) and we did get a chance to visit the local Whale Museum (1998) which includes a full-sized Blue Whale skeleton. The museum was especially opened for us early, before the crowds arrived, well done National Geographic!
In the Earth's lithosphere plate tectonic theory (or is it proven?) Iceland is neatly bisected by the Mid-Atlantic Ridge, i.e. the intersection of the America plate and the Eurasia plate. The west of Iceland is drifing west on one plate, with the east half going the other way. Result: never-ending volcanism. Our hot weather day was spent in the hot zones of that rift between the plates of the Earth's crust. Right in that rift is Mývatn, a large shallow lake an hour or so from Husavik formed by a basaltic lava eruption 2300 years ago. (Mývatn means 'lake of midges' and with that reputation, the Expedition Leader equipped us all with mesh face coverings. In the end, the midges were not too bad.) The area surrounding Mývatn is rich with geothermal activity and the remains of it, and provided us with many things to see on this hot day. We were able to walk around parts of the lake's edge, looking for birds, but they must have been hiding in the hot weather. We did see some swans in the distance. At Skutustaoagigar on the lake we saw clusters of pseudo-craters, a rare geological formation created when molten lava flows over water causing it to boil and explode.
A pseudo-crater at Skutustaoagigar on Myvatn, caused by a (big) explosion of superheated water. 
One of the most dramatic reminders of geothermal activity around Mývatn is the 1km diameter Hverfjell crater, one of several well-formed tephra (fragmented material) craters in a row, all part of the ~500BC eruption in the North volcanic zone (NVZ). The Krafla caldera is 10km in diameter and has a 90km long fissure zone. Hverfjell has a tephra cone, and to see into the crater required a scrabble up 80m height on a trail of loose stones. Apparently tephra results from the interaction of magma and water, and it is widely distributed around the lake area.
The Holtasoley (Mountain Avens) is Iceland's national flower. This versatile plant has been used for medicinal purposes and as a substitute for tobacco and tea, is is liked by ptarmigans. 
Nearby was Dimmuborgir, a dramatic field of volcanic eruption lava stacks in a valley, formed when Krafla's molten lava flowed across the ancient waters of Mývatn and 3km down to the sea. The valley was silting up with wind-blown sand, but in 1942 a restoration program commenced enabling the area to be opened up to visitors. And it was! The crowds here were extraordinary, brought to Dimmuborgir in dozens of coaches coming from cruise ships whose closest approach was the port city of Akureyri.
Our Italian-become-Icelander guide Stefano explains how the lava stacks at Dimmuborgir formed. 
Not far away again is Hverir, also called Namafjall, a steamy ochre-coloured lunar-like desert region reeking of hydrogen sulphide and with scattered fumaroles venting hot gases, and bubbling mud pots. Resembling parts of Yellowstone National Park that we have seen, it's fascinating to walk around the trails here, but maybe the most interesting is the sight of tourists nearly burning themselves to get the best selfies. Signage says that the temperature is over 200oC at a depth of 1km. Sulphur was mined in this area 'in previous centuries'.
Visitors are warned to stick to the marked trails at Hverir, lest they get into very hot water. 
And just over the hill in Mývatn is a place that the regular coaches stay away from, because there is just no room, called Grjotagja Cave. It's on private farm land. Only four-six people can clamber into the tight and awkward cave at any one time and be treated to a crystal blue geothermal hot spring which apparently appeared sometime in Game of Thrones, but we don't recall it. It's really dark inside so it helps to have a good camera. The cave sits directly underneath a small gorge that is the Mid-Atlantic Ridge as it passes in this location. Swimming in the spring was popular until 1974-84, when more eruptions increased the temperature to 60oC, too hot to get into. It's since dropped back to 45oC but swimming is banned.
The two tectonic plates of the Mid-Atlantic Ridge moving part, as seen over the hot spring in Grjotagja. 
We had a delicious Atlantic char (a cheaper fish alternative to salmon, both are predatory cold-water fish, but char prefers even colder waters) lunch at Skutustaoagigar. Next door was an ice-cream kiosk which proved to be very popular, given the weather. And we returned to the NatGeo Explorer at Akureyri (it had sailed into Eyjafjordur to pick us up) but on the way we stopped at Godafoss ('Waterfall of the Gods'), one of several on the glacial Skjalfandafljot River. These glacial falls don't depend on rain to be magnificent, but their copious flows are, of course, evidence of climate change. According to Icelandic Sagas, a local chieftan threw his pagan icons into the falls when he was converted to Christianity in about 1000AD, which thus gave the falls their name. In Akureyri, we saw, at least partly, where the crowds had come from. Near the Explorer in the harbour was the Explorer's sister ship, the National Geographic Resolution (same one we were one for Norway and Svalbard), as well as an Adventure Canada expedition ship, rather larger than the Explorer, but way short of the floating apartment blocks we have seen elsewhere.
On the way to Akureyri, our bus avoided a tunnel through the mountains so we could better enjoy the scenery.