Crikey Dike (Canterbury) EarthCache
Hidden : 1/27/2013
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Lyttelton Volcano

The first Lyttelton flows poured out about 12 million years ago. Most are dark basaltic rocks called hawaiites, which were erupted as aa lavas. 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. A few lavas and many dikes are of trachyte, which was probably distilled fram a basaltic parent liquid in a magma chamber below the volcano. In 2 million years, a symmetrical cone had been build up. Thought the summit has been completely eroded away, the original height is estimated to have been about 1500m or 5000feet.

Much of the activity would have been the relatively quiet, Hawaiian style of eruption, though evidence exists of more violent Strombolean eruptions, the latter not only from within the main cone but also from parasitic cones on its outer slopes. Air-fall tuffs can be seen in various places, Some eruptions were located along fissures cutting the flanks of the volcano and represented the emplacement of dikes deep within the volcanic pile. Much later, the superstructure domed upwards and cracked in response to the rising magma, resulting in the well-developed radial dike swarm. The centre of the swarm is in Charteris Bay; the summit vent of the volcano was above this area.

As the cone was being constructed, erosion was already under way. Many lava flows were channelled into gullies carved our by developing streams, and lahars accumulated in stream channels.

After eruptions ceased, erosion accelerated swiftly. By 9.8 million years ago, a deep crater had been excavated (under the present position of Mt Herbert). Mount Herbert Volcanics later filled this breach, and a new gap was cut through the southwest wall, forming Gebbies Pass. Drainage through the pass probably persisted until erosion slowed down no reaching the more resistant Cretaceous volcanics and Torlesse rocks, Later, although we don't know when, the northwest sector was breached, and the approaches to Lyttelton Harbour were excavated.

Dikes

Dikes or dykes are discordant tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata, though some dikes are steeply inclined.

Hundreds of dikes can invade the cone and inner core of a volcano. Dikes may occur in swarms of parallel dikes, particularly where there has been crustal extension. In regions of crustal extension, fracturing may open the route for filling by magma from a deep source, or intrusive magma may promote the fracturing and extension of the crust. Outcrops of dikes can range from a few metres to many kilometres in length, and can spread lateral distances from a few centimetres wide to over 100 m. Very thin dikes or dikelets are sometimes called veins.


Below is a screen shot from Google maps and you can see the dike protruding the surface, you can see the 'layers' of dikes.



Tasks

The beauty of the summit road is that in its construction it had exposed many of the hidden features of the Banks Peninsula's volcanic history. There are many along its length that have given us the opportunity to learn about it's history.

The given coords are for a carpark that is on the right as you travel towards GZ from the Sign of the Kiwi. Please be careful at GZ as there is a lot of cycle and car traffic during the weekends and evenings. The picture below is what you will be the area that you need to identify the dikes and rock.

Here a wide trachyte dike with glassy margins has been cut by two thin basaltic dikes. A fourth dike has intruded along the margin between the trachyte dike and basalt lavas (this is on the left of the wider trachyte dike). The dikes are parallel in orientation and form part of the radial swarm of Lyttelton Volcano.
The following tasks are required to log this cache successfully.
  1. Identify which you think is the wide trachyte dike and give the overall measurement of it.
  2. Measure each of the dikes at the location pictured in the above image (picture taken from cache coords and carpark). Hint: you'll need a tape measure at least 5m long.
  3. Identify which dike(s) are either basalt or trachyte
  4. Optional - take a photo of you at GZ and of the dikes
Please email me via my 'Profile' page with your answers and feel free to log the cache. If I have any queries I'll contact your. Enjoy!

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.

This book is out of print but the copy I used was readily accessible from the Public Library.

Additional Hints (No hints available.)



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