UKC

Enjoyed a few routes at this splendid little crag today and spent some time wondering about the rock. Despite what the ukc page says it was clear to me that it was a course-grained igneous rock but for the life of me I could see absolutely no quartz in it so my only conclusion was that it is gabbro. Can anyone confirm or deny?

In reply to Rog Wilko:

I heard it was pillow lava, and it certainly looks like it with the waves and breakable crust.

There are some great videos of pillow lava being formed on YouTube

In reply to Rog Wilko:

pictures? or at least a location? were not mind readers!

if its coarse grained then its not going to be pillow lava, so one of you is wrong!

In reply to Dan Arkle:

Pillow lava is made of basalt, so not that.

In reply to Webster:

It's in the ukc database. I couldn't make the link mechanism work on my tablet.

In reply to Rog Wilko:
Craig Y Tonnau

In reply to Rog Wilko:

According to Wikipedia :
"Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite, dacite or even rhyolite are known"

That would presumably make Craig y tonnau a rhyolitic pillow lava? The stone is similar in composition to nearby rhyolites on the Moelwyns but with a waved surface which gives the crag it's name.

In reply to Rog Wilko: I wouldn't assume a rhyolite composition even if there are rhyolite lavas locally. I don't have a map to hand but things there are complex and varying rapidly.

How coarse grained is this 'coarse grained' rock? How many mm are crystals? All crystals or large crusts in a finer matrix?

In reply to Rog Wilko:
I'm guilty of not having updated the crag, I was recently told it is a marine wave modified volcanic ash deposit.

In reply to scott titt:

That's tuff (a geological joke).

In reply to Ron Rees Davies:

I had already seen the UKC page, but didn't agree with the geological information on there.

In reply to Ron Rees Davies:

I think you've missed the main point which is that lavas are always fine-grained as they cool quickly so no visible-to-the-naked-eye crystals can form. Large crystalled igneous rocks (sometimes called plutonic rocks, and include granite and gabbro) cool slowly at depth giving time for large crystals to form. If you've been to Craig y Tonnau I'm sure you'll agree it is anything but fine grained basalt (or rhyolite).

In reply to scott titt:

It's not like any tuff I've ever seen before!

In reply to Rog Wilko:
Climbed there a number of times.

Largely rhyolitic pillow lava with tuffs of the Capel Curig Volcanic Formation these were deposited from ash-flows into a submarine environment forming isolated pods of tuff enveloped by marine sediments.

If you really want all the detail get hold of a copy of The Geology of the Dolwyddelan Syncline (North Wales)
Howel Williams and Oliver Meredith Boone Bulman
It includes details of the Snowdonian lavas of the syncline in comparison to Capel Curig lavas which underlie them along the southern slopes of Moel Siabod.

In reply to wbo:


I wouldn't say it was rhyolitic in composition as absolutely no quartz is visible. It is obviously not rhyolite in texture.


It has the texture of a coarse grained granite with crystals of felspar (plagioclase?) about 10mm + or -. This is what it looked like at the top, which is where I spent most of my time belaying looking at it. Lower down the crag the texture looked perhaps a little porphyritic with grains like large seeds in a finer groundmass but all a similar dark colour. It was very like I remember some of the gabbro on Skye where the most amazing holds seemed to be created by a strong external crust to the rock. I'm going to have a look at some pictures I took yesterday and see if anything is worth putting up here.

In reply to Rog Wilko:

having seen the pictures on the crag page, it does look to have a hint of cross bedding and definitely doesn't look like pillow lava, although there are a few possible pillow like structures near the top of the crag...

a lava (or tuff) can have larger crystals - megacrysts or phenocrysts which formed prior to the eruption. I could be that the op has observed these.

impossible to be certain from the photos, but given the location, cross bedded/reworked volcaniclastic sediments (i.e. the "wave washed ash" as mentioned above) is highly plausible. there are vast quantities of this in north wales of varying compositions.

In reply to starbug:

That's very interesting, but I'm still struggling with the concept of large crystals in pillow lavas, and certainly the rock I was sitting on and looking at on the belays was much more like a granite in texture with decent sized felspar crystals. It's clearly well-beyond my O-level standard geological knowledge and experience!

In reply to Rog Wilko:

I've now uploaded a picture showing the typical crusty nature of the rock and holds on the lower half of the crag. I'll post the link when it appears. It also shows the rock texture quite well which does look rather pyroclastic in nature. I'm beginning to realise that perhaps the lower and upper parts of the crag are totally different, where most of the holds in the lower part are these crusty plates whereas higher the holds are more conventional with cracks providing the purchase.

In reply to Rog Wilko:

This website is useful for getting background info at least.
http://mapapps.bgs.ac.uk/geologyofbritain/home.html

Also available as an App, which is fantastic BTW!

In reply to Samuel Wainwright:

Thanks for the link. I have used it before but had lost the link somehow.

In reply to Rog Wilko:

the large feldspar crystals will almost certainly be phenorysts and you cant see any quartz because its 'glassy' ie too small to see by the naked eye. this means that rhyolite tends to look black on a fresh surface despite being felsic instead of mafic. and those pictures you posted of the bottom of the crag look very much like well weathered rhyolitic tuff such as that which you will find on tryfan.

In reply to Webster:

Thanks. I'm still a bit confused, but will bow to your greater knowledge.