Blábjörg( Blue Cliff ) is a green cliff on the Ring Road. Blábjörg is one of Iceland’s oldest features. It was formed in a violent eruption of overheated gases and volcanic ash around nine million years ago, like the surrounding mountains. Its distinction arises from its geological definition as an ignimbrite, which means that the magma composing it was silicic rhyolite ash. It is unlike the more significant, much younger features made of basalt lava in the middle of Iceland, like the recent lava in Geldingadalir on the Reykjanes Peninsula. Blábjörg is a version of compressed ash. If you look up the hills of the mountain to the east, you can see how the ignimbrite continues upward from the shoreline.(1)
Blábjörg is a part of the Berufjarðar tuff and formed in a pyroclastic flow in a huge phreatic eruption that took place here some 9-10 million years ago. Pyroclastic flows can reach 500 km per hour (310mph) and temperatures of 100 to 800 degrees celsius (approx. 212 to 1500 degrees Farenheit).
Ignimbrite is a pumice-dominated pyroclastic flow deposit formed from the cooling of pyroclastic material ejected from an explosive volcanic eruption. As the pyroclastic material settles it can build up thick layers, and if the temperature is sufficiently high (> 535°C) it can weld into rock. The degree of welding depends on the weight of overlying material, so at the tops of such deposits the rock is commonly vesicular and of low density, while at depth the overlying weight crushes the pyroclastic material forming a dense, non-vesicular rock. While most volcanic rocks are found close to the eruptive source, ignimbrite of reasonable thickness can often be found tens to hundreds of kilometres from the site of eruption.(2)
Blábjörg are part of a fused liparite tuff layer or flaky rock, which has been called Berufjörður acid tuff. In the stratigraphy, it is just above the so-called Hólmatind tuff, which has a sour brand, but somewhat below the pink tuff layer attributed to Skessa. The Berufjörður tuff can be traced up the mountains to the northeast and is, for example, prominent in Berunestindi. It has not been found south of Berufjörður. The feldspar rock was formed in a pyroclastic flood in a liparite explosive eruption. It is mostly composed of fused liparite, but fragments of basaltic rock are also found. The stone chlorite has been formed during the metamorphosis of the rock and gives it a greenish hue.(3)
Blábjörg were designated as a Natural Monument in 2012 and is a protected area. Please stay on the stairs and path to the beach.
At the posted coordinates, you will find a parking area along the Ring Road - Route 1 with an information sign board, a gentle metal staircase that leads you to the shore and viewpoint of the cliffs.
Scattered among the "normal" beach stones are the amazing green pieces.
To Log this Earthcache you will need to visit the location and complete the tasks and answer these questions in a message or email to us. Do not post answers in your log.
1. a. What transforms into solid ignimbrite?
b. What is the green color in the rock is due to?
c. When a pyroclastic flow settles and builds layers, what temperature does it begin to weld into rock?
d. What is the size of this Natural Monument protected area?
2. While walking the beach at the posted coordinates you will find rocks that are different like the photo above. Tell me why you think there are the two different rock types on the beach. Are both types of rocks from the cliff? Do you think they came from the same eruption?
3. Do you think the size of the green crystals is or is not dependent on the speed of cooling of the pyroclastic deposits. Is it dependent on the weight of material above? What factors make larger crystals? Why?
4. Take a picture to upload of you with your GPS and the cliffs. If you don't want to be in the picture, write your Geocache name on a piece of paper for the picture. A second photo of a special beach stone you found would be a bonus.
5. Once these tasks have been completed, you may log your find. Upload the picture when you log the cache.
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(1) https://www.hiticeland.com/post/blabjorg-small-cliff-in-the-east-region
(2) https://rocksminerals.flexiblelearning.auckland.ac.nz/rocks/ignimbrite.html
(3) https://ust.is/nattura/natturuverndarsvaedi/fridlyst-svaedi/austurland/blabjorg/
Congratulations on the First to Find - Antigocache and ette from Nova Scotia, Canada
