Lyttelton's Melting Moment (Canterbury) EarthCache
Hidden : 1/12/2013
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


What are Volcanoes?


Most people's idea of a volcano is a cone-shaped mountain with billowing clouds of ash rising from its top and lava flows pouring down its slopes. This correctly describes a few, but by no means all, volcanoes; they are complex and varied in appearance as well as being highly individual in behaviour.

Well-known cone-shaped volcanoes are Ngauruhoe or Mt Fuji in Japan. Other volcanoes have broader, low-angled cones and are called 'shield' volcanoes. The Lyttelton and Akaroa, as well as most of the Hawaiian, volcanoes are more like these. Conical, or 'central' volcanoes acquire their shapes by the accumulation of lava flows and other products around a single source or vent which conducts magma (molten rock) to the Earth's surface from below. (A second type can result from the opening of a lengthy fissure, along the sides of which lava and spatter accumulate. The Tarawera eruption of 1886 was a fissure eruption.)

The products of a volcanic eruption can be put into two categories: Lava flows are formed when a volcano is in a relatively quiet, non-explosive mood and represent a coherent mass of extruded magma. Pyroclastic deposits (such as ash) are formed of solidified fragments of magma expelled by volcanic explosions. Individual volcanoes may be composed entirely of lava or pyroclastic deposits — or a combination of the two (see Fig. 1). What governs whether magma is erupted as a lava flow or blown sky-high to become pyroclastic material is the gas content of the magma and the physical property known as viscosity: the resistance to flow, or degree of sluggishness, of the material. The viscosity determines both the surface appearance and the internal structure of lava flows. Gas can escape freely from fluid magma but is released explosively from viscous magma, resulting in pyroclastic activity. Most of the Banks Peninsula lava flows are of the aa type. Aa is the Hawaiian word for lavas whose surface has broken up into rough, cindery blocks. The flows are typically a few metres thick and may be traced considerable distances, both across and down the slopes of the volcanoes (see Fig. 2). Another Hawaiian term, pahoehoe, refers to less viscous flows buckled into a series of pleats resembling coils of rope; pahoehoe is also called 'ropy lava'.


Fig. 1 Cross-section through a composite volcano made up of alternating se-quences of lava flows and pyroclastic units. The main pipe conduit feeds magma up from the magma chamber (not shown). Eruptions take place from the summit crater or from parasitic vents on the flanks of the volcano, and a significant volume of magma is intruded as dikes and sills.

Whether the surface is of pahoehoe or aa type, it is the movement of the magma in the hot and relatively fluid interior of the flow which deforms the surface. When magma is very viscous, it is unable to flow away and thus piles up around the vent. This can result in a volcanic dome, in which successive batches of magma have inflated the dome like the blowing-up of a balloon. Since magma often flows as a series of thin sheets moving parallel with one another, this pattern can therefore be used to map out the original shape of the volcanic dome. There are a number of excellent examples of volcanic domes on Banks Peninsula. Castle Rock on the Summit Road above Lyttelton and Panama Rock, off Cameron's Track east of Akaroa, are both trachyte domes. Conical Hill on the Summit Road, Gebbies Pass, is a rhyolite dome.


Fig. 2 Cross-section through a typical aa basalt lava flow. The top and bottom surfaces and the flow front break up in response to movement within the flow, as shown by arrows; masses of mobile lava may be pushed up ('ramped'). On cooling, the interior becomes a coherent solidified mass of lava sandwiched between layers of clinker.

Magma that reaches the Earth's surface is called extrusive, but large amounts of magma solidify below ground, either within the superstructure of a volcano or at greater depth. These result in intrusive igneous rocks — that is, rocks formed when magma has pushed into pre-existing rocks. Beneath many volcanoes, magma is stored in a large reservoir or magma chamber a few kilometres down. Periodically it rises and may feed an eruption at the surface. New magma drawn from the source zone where rocks are melting (perhaps many tens of kilometres down) may rise to top up the magma chamber. When melting of the source rock ceases, the supply of new magma is cut off. Eventually the magma in the reservoir and in the plumbing systems connecting the reservoir to the volcano crystallises to form coarse grained plutonic rocks, and the volcano becomes extinct. These rocks are only revealed when the superstructure has been well and truly eroded and can be seen, for example, in the peaks of Tapuaenuku in the Kaikouras. Intrusive rocks solidifying within the cone itself cool more quickly and are fine-to medium-grained as a result. Magma often intrudes to form thin sheets of rock: When this occurs between and parallel to layers of pre-existing rock, the intrusion is called a sill. A more-or-less vertical sheet cutting across pre-existing strata is called a dike. Dikes often occur in groups or swarms. Dikes within a volcano can form a radial pattern: as the central area of the volcano is forced upwards into a dome, cracks develop, radiating outwards. Magma fills these fissures, and the resulting radial dike swarm is eventually exposed by erosion. Lyttelton volcano has one of the best developed and exposed radial dike swarm in the world. The majority of the dikes did not reach the surface, but some certainly did: Where these intersected the volcanic slopes, lava domes grew or lava flows were fed. See this cache for a good example of a dike, Crikey Dike (Canterbury).

In the embankment along Bridle Path Road, at GZ, is exposed a sequence of aa basalt lava flows. The flows have rubbly tops and bases and are inclined towards the north. Ramping structures (see Fig. 2) are present and the tops of some flows have been ???(colour omitted see below) owing to the baking by the overlaying lavas.
At GZ you'll be required to identify 'clinker' and 'coherent interior', I have supplied photos below for you to have a reference to work from. I found this fascinating I hope you do to.

Logging Requirements
  1. At GZ you'll see a good example of an Aa flow, comment on the colours of the layers.
  2. From GZ walk directly across to the lava flow and take the vertical measurements of the layers of clinkery top, coherent interior, clinkery base and (different coloured - tell me the colour) zone.
  3. How many of these flows can you see?
  4. Optional: Photo with flow in the background
  5. Have fun!
Go ahead and log your find after you have emailed me the comments on the above tasks


The following book was used extensively to make this cache:
Extinct Volcanoes, a guide to the geology of Banks Peninsula.
Stephen Weaver, Rod Sewell, Chris Dorsey
1985, Geological Society of New Zealand.

Additional Hints (No hints available.)



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