Hjálparfoss
The river fossá flows through the Þjórsávalley, unites with the river Rauda and falls here spectacularly devided in two arms into this beautiful basalt rock formed basin.
The landmarks named Hjálp is located beneath Skeljafell and this in turn comes from the fact, that in earlier times, after crossing the dangerous, desolate inland stretch of Sprengisandur, people finally found water and fodder for their horses here again. So they got help.
If you stand on the wooden platform, you will see basalt formations in a wide variety of shapes: Candle-straight, horizontal, diagonal. Why is that?
The lava field Búrfellshraun fills the valley. It flowed from craters of Veiðivötn about 3,500 years ago. At the waterfall, the Fossá River has eroded part of this lava field, exposing the interior of pseudocraters as well as fan-shaped lava columns formed by very rapid cooling of the lavas under the influence of water. So the basalt columns formations around the waterfall were formed from the cooling lava.
Basalt is formed, when thin, low-silica magma escapes at the earth's surface and cools relatively quickly to form basalt lava.
Basalt is usually dark gray to black. It consists for the most part of a fine-grained groundmass. Coarser, naked-eye inclusions are relatively rare, but may be more common in some basalt varieties.
The appearance of cooled basaltic lava depends mainly on two factors. Erupted lava cools quite rapidly to a coherent rock structure, that solidifies as pahoehoe lava or as ʻAʻā lava, depending on temperature and gas content. However, if cooling is delayed, the contraction often results in polygonal meter-long basalt columns, that form perpendicular to the cooling surface. Thereby hexagonal columns are formed preferentially.
All basalts have a basic composition, i.e. they are poor in SiO2. Predominant mineral groups are plagioclase (mostly labradorite) and pyroxenes, predominantly as augite. Olivines and foids occur in some basalt deposits; however, they may be absent. Biotite and hornblende may also occur. Alkali feldspar and quartz, on the other hand, are absent or subordinate in basalts. Overall, basalts prove to be extremely variable rocks in terms of their chemical composition, which is in contradiction to their rather uniform appearance. Depending on the zone of formation, the mineral composition can vary greatly.
When the basaltic lava solidifies at a temperature of approx. 840 to 890 degrees Celsius, there is a volume loss, which leads to the formation of stress cracks (contraction). First, a network of mostly hexagonal stress cracks forms on the surface, which cools down fastest, similar to the cracking on a mud surface. With increasing cooling and thus also increasing contraction, the stress cracks grow further into the deeper layers and lead to the typical basalt column formation. Thus, the cracks form perpendicular to the surface, from which the cooling originates.
The slower the lava cools, the more uniform the columns.
If basalt columns form in vertically ascending magmatic veins (dykes), they are tilted 90 degrees, because the cooling surface in the case of a vertical vein is the long side of the intrusion. Rosette- or fan-shaped basalt columns, on the other hand, are formed in lava caves and horizontal veins.
Chevron-shaped columns are formed, when the lava has more than one cooling surface and the columns spread from each surface.
Rosettes are formed, when the lava is ejected from a specific point (location) and has a dome-like cooling surface.
Fans are formed, when the lava flows into a valley.
Basin patterns form, when isotherms correspond to the topography beneath the lava.
Scource: Wikipedia, https://www.vulkane.net, https://www.mineralienatlas.de/lexikon/index.php
Already from the platform you can see many of the basalt formations just described.
With a little imagination you can discover other things in some formations: T-Rex, a huge Archeopterix or a Stegosaurus. But even if they look like that, they are only basalt formations. 😀
And now it's your turn:
Task 1:
Here you can see 4 characteristic basalt formations we discovered in the area:
Explore the area on the trails marked in red on the map below to find the location of each of the formations . Note the letter of the area marked green on the map, where you found the formation. At the end you have a series of letters ordered by the formation numbers as your answer for this task; for example FAGB.
Task 2:
Explain in your own words how each of the 4 formations from task 1 could have been formed.
Task 3:
Take a photo of yourself, your GPS device or an object with your cacher name with the waterfalls in the background. Add the photo to your log. This is a log condition.
Send us your answers via messenger or email for verification.
If you like, add a photo to your log, where you discovered a special phantasy object in the basalt like we did with the "dinosaurs" :-)
You can log immediately after answers are sent to us. If there are any questions about your answers, we will contact you.
Logs without previously sent answer message or without foto corresponding task 3 will be deleted without any further notice.
Please do not include pictures in your log, that may answer the questions. But there is so much to see here, you will find suitable motivs for more photos. Have fun!