This glacier tongue is an outlet of the immense Glacier Vatnajökull, Europe's largest glacier, which ice is 1,000 years old.
Glaciers are made from fallen snow instead of frozen water, as some may think. But how does the snow turn into ice then? Simply by compression due to the weight of the snow as it keeps accumulating. It's just like taking a handful of fluffy snow and squeezing it into a hard snowball and doing so for hundreds of years. The ice eventually gets so compressed that most of the air is forced out of it. This effect is what causes glacial ice to appear blue.
A glacier is a river of ice. It flows. That's because the highly compressed layers of ice are very flexible. At locations where a glacier flows rapidly, giant cracks called crevasses are formed. This is why it's so dangerous to walk on a glacier - the crevasses sometimes get covered by snow. Your guide may show you the ice ax spots that may look like solid ice and snow but are steep drops just covered in a light dusting of snow.