Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They mostly form the upper part of 'Layer 2' of normal oceanic crust and may form layers of thousands of meters thick.
The presence of pillow lavas in the oldest preserved volcanic sequences on the planet, the Isua and Barberton greenstone belts, confirms the presence of large bodies of water on the Earth's surface early in the Archean. Pillow lavas are used generally to confirm subaqueous volcanism in metamorphic belts.
Pillow lavas are created when magma reaches the surface but, as there is a large difference in temperature between the lava and the water, the surface of the emergent tongue cools very quickly, forming a skin. The tongue continues to lengthen and inflate with more lava, forming a lobe, until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting oblate shapes that are pillow-like in cross-section. The skin cools a lot faster than the inside of the pillow, so it is very fine grained, with a glassy texture. The magma inside the pillow cools more slowly, so is slightly coarser grained than the skin, but still classified as fine grained.
Pillow lavas are used as way-up criterion in geology. There are three key ideas that can be used as part of this, and that a pillow lava will show if it is the correct way-up:
- Vesicles will be found towards the top of a pillow as the gas will be less dense than the surrounding rock.
- The pillow structures will show a convex upper surface.
- The pillows will have a tapered base downwards as they have moulded to the underlying pillows during their formation.
The pillow lavas are not actually visible from the geosite and you need to walk down the road towards the border gate to see them. The panel give clear indication where to look. Take care, there may be traffic. The pillows are heavily weathered but you can see the shapes. There is a good solid one, although very small, at the Makhonjwa Lebombo view/picnic site.
To answer question 1 you can either cross the road to study the road cutting or make use of the specimen at the Makhonjwa Lebombo view/picnic site.
To log this EarthCache you have to answer the following questions and e-mail the answers to me:
1. Examine the surface of the rocks in site 1 or 2 in the cutting or the specimen at the other site and describe what you see.
2. Pillow lava rock formations are usually to be found on the bottom of the sea. How do you think it is possible for them to be in this mountains?
3. When standing at the look out point you have an awesome view in the direction of Msauli and the Songimvelo Nature Reserve. According to the display panel, how large is the nature reserve combined with the adjacent nature reserve in Swaziland?
Uploading photos to the cache page is the best way to say thank you to the cache developer and to encourage others to visit the location but is only optional.
NOTES:
- There are no other facilities along the trail except for very nice picnic spots.
- The distance is about 40km from Barberton on a narrow and winding road up to the Bulembu border post to Swaziland with no or limited cellphone reception.
- You can email me your answers in either Afrikaans or English.
Note that a handy guidebook specially compiled for this route is on offer at Barberton Tourism in town.
Enquiries: 013-712 2880 or www.barberton.co.za
Guided tours by Tony Ferrar on 072-376-2581 and Dave Mourant on 082-923-5366.
Thank you to the Barberton Chamber of Business and all those involved to develop this geotrail.
