Pillow Lava
The term 'Pillow Lava' refers to a volcanic formation of bulbous, sperical or tubular lobes of lava formed by an underwater eruption with a relatively low effusion rate. As the magma erupts and makes contact with the water, a solidified crust forms around the molten core which continues to flow.
Slow extrusion gives enough time for a thick crust to form on all sides of a pillow lobe, and prevents individual pillows from coalescing into a sheet. Internally the pillows are fed via a distributary system of interconnected channels. Pillows are not typically hollow and tend to solidify all the way through and often have lineations or scrape marks on their sides that form during extrusion.
Pillow flows are produced by the piling up of individual pillow lava lobes. As a pillow flow forms, the newest pillows are erupted from the top of the stack and flow outward a limited distance before freezing, a process which tends to produce steep-sided mounds or ridges which can grow to be 10's of meters thick.
Waiareka-Deborah Volcanics
The Waiareka-Deborah volcanic group describes a number of Surtseyan volcanoes that erupted on the shallow submerged continental shelf between 35 and 30 million years ago, in what is now the Oamaru/North Otago region. A Surtseyan eruption is simply one that takes place in shallow seas or lakes and it is named after the island of Surtsey off the southern coast of Iceland.
This type of volcanic activity often includes episodes of violent explosive eruptions caused by rising basaltic or andesitic magma coming into contact with the sea, but it is generally considered that most of the few tens of cubic kilometres of island-building material ejected by the Waiareka-Deborah volcanics occurred over the space of just days or weeks. The pillow lavas generated from the Waiareka-Deborah volcanics, on the other hand, were created over a longer period.
This Earthcache
This earthcache takes you to an easily accessible spot where you can see the cross-section of some pillow lava lobes, where a cutting into the earth has been made for the construction of roading. Since you are on the side of a road and adjacent to State Highway 1, please be very careful and keep a close eye on any children with you.
This is an earthcache and like any earthcache, the purpose of it is to provide a geological lesson at the site you are visiting. This is not a virtual cache and you must visit the site and answer the questions below in order to claim you have 'found it'.
Before claiming your find, you must submit your answers for the questions below to the owner of this earthcache, by clicking on the link to the owner's profile at the top of the page, and sending the owner an email.
When emailing your answers, don't forget to include the name of this earthcache, and choose the option to include your email address so that we can reply to you. Do not include answers in your log on the cache page.
After you have emailed your answers, you can go ahead and log your find. Any problems with your answers, and we'll get back to you.
1. Identify a cross-section of pillow lava in the bank at the published coordinates (either side of Thousand Acre Road). Thinking about how pillow lava is formed, describe the structure of the pillow lava lobe you are studying (in terms of layering, crust, structure, colouring, dimensions).
2. Describe the surrounding rock - how does it differ from the pillow lava? Which came first, and why?
Please note, this is an easy earthcache to complete if you visit the site, and we will therefore reserve the right to delete logs if answers are incomplete, incorrect, or not provided at all.
 |
Big smoke |