Formation of lava fields on Lanzarote

[sophiecentaur]

Just got back from Lanzarote and was walking through a recent (300yr old Timenfaya) lava field. Very little rain in 300 years so it's more or less like it started, I guess. A great landscape to investigate the way volcanic landscapes develop.
What I couldn't understand was the structure of the surface in places. It looked as if a giant JCB had been breaking up turning over a layer of concrete - or possibly a plough in a clay field. The surface consisted of large broken fragments (many, at least a metre diameter), with lots of air gaps underneath. I couldn't figure out what mechanism lifted the lumps so far above the mean ground level. The field was more or less horizontal so it wasn't as if they had rolled downhill or been carried like the lumps of granite in Snowdonia etc.

Also, there seemed to be dunes of fine stones - a la Sahara but the wind would need to have been v. strong to form them with such large particles.

Someone must have worked it out. . . . .

 

[matthyaouw]

Photos would be haelpful if you have them.

The lava sounds like a'a lava. It cools in a blocky manner and the solid bits get shifted and tossed about by the parts still flowing. Have a look here: http://www.universetoday.com/guide-to-space/earth/aa-lava/

The dunes of stones could be pumice or scoria: (formerly) airbourne volcanic pebbles that rain down on the landscape after being thrown out of the volcano. They'd be very bubbly in texture and if pumice, so light they'll float on water. I'd bet small pumice stones could be blown about by the wind. Don't rule out them being water deposited though.

I've been to Tenerife, a couple of islands over. Fascinating landscapes, eh?

 

[sophiecentaur]

Thanks for that.
Yes, I heard the term a'a lava used. Also, they referred to the small stones as pilli (spelling?). They are more dense than pumice but have holes in them which are used in systems to catch the dew in the morning; Lanzarote has humid air at times but very little rain.
Lanzarote has so little water that the 16k year old lava fields are pretty much unchanged, with just a thin layer of vegetation in places. I believe Tenerife has much more rain and loads of vegetation so changes are much more rapid.
Here are two photos I took.
Some of it's like Mars apart from the blue sky. Strange.

 

[matthyaouw]

Lapilli, maybe? http://en.wikipedia.org/wiki/Lapilli
They definitely sound pyroclastic.

Tenerife gets lots of rain in the lower altitudes but up on top of the volcano past the treeline it's more like that in the photos.

 

[sophiecentaur]

Lapilli - that's the badger. So, it's just stayed there for 300yrs. It certainly brings the idea of Geological Time into perspective.
Another strange thing. Many of the beaches seem to have typical golden / white sand on them despite all the surrounding cliffs and rocks being very black. These are natural beaches, in inaccessible coves, so the sand can't have been introduced artificially. Where does it come from and why isn't there more sign of black grains in it as on other island beaches? The sea is knocking hell out of the coast all the time!
Compare the two pictures, taken not many km apart. (I am quite pleased with the 'surf and golden sand' picture')

 

[matthyaouw]

Are there any acidic igneous rocks on Lanzarote? I haven't been able to find a geological map. Can you be sure the beach isn't artificial?

 

[sophiecentaur]

I can't say about the geology ( hence my questions) but I can say that a lot of small golden coves are remote and almost inaccessible. The surf beach in the photo is on La Graciosa which is small an non-trippery and there aren't even Tarmac roads. Rocks all seem very black, though. I guess this question applies all round coasts, with some exceptions like S Devon.

 

[matthyaouw]

Tough one. Perhaps the surrounding rocks were light rocks but with a dark weathered surface. Did you see the colour of any freshly broken bits? Did you have a close look at the grains? Were they rocky particles, or fragments of broken shell?

 

[sophiecentaur]

I clearly walked around the place with Engineer's eyes open rather than Geologist's. NExt time, if the re is one, I shall bear in mind your questions, matthyaouw. Thanks for your answers, though. I a a bit wiser about things than I was!

 

[sophiecentaur]

The ipod 'keypad' doesn't always give you the letters you want!