Okataina Geology
The Okataina Volcanic Centre covers a roughly circular area some 30km in diameter, around 3 times the size of the adjacent Rotorua Volcanic Centre and dominated by the impressive Mt Tarawera in the south. Mt Tarawera, however, is not the only volcano in the Okataina Volcanic Centre and is in fact just one of many - it just happens to be the most will known because of its impressive size, rift crater, and well documented recent eruptive and destructive history.
The Okataina Centre includes the large but young volcanoes of Tarawera and Haroharo, with others at Mt Edgecumbe, Okareka and Rotoma. Unlike other well known New Zealand volcanoes such as Ngauruhoe and White Island, which have relatively small eruptions every few months or years, the volcanoes in the Okataina Volcanic Centre have erupted at intervals varying between 700 and 3000 years. When such eruptions do take place at Okataina, however, they are 100 to 10,000 times larger than those at White Island or Ngauruhoe. It is the size of most Okataina eruptions, despite their relative infrequency, that presents a significant volcanic hazard to the Bay of Plenty region.
Statistically, the probability of an eruption in the Okataina Volcanic Centre is about 1 chance in 20 for any given 100 year period. However, when (not if) the next eruption occurs it will be extremely damaging and probably affect much of the Bay of Plenty region between Tauranga, Opotiki, Murupara and Tokoroa. The craters you will visit and view in the course of completing this earthcache resulted from eruptions that occured about 3750 years ago.
Eruption Types
There are four types of magma ejected from volcanoes, and these are defined by the amount of Silica content, which affects their colour and viscosity. The four types of magma are listed below along with their properties and effects on the violence of eruption:
- BASALT has less than 53% Silica content, has a low viscosity and is associated with low to moderate violence eruption activity. Basalt is usually black or red in colour.
- ANDECITE has 53-63% Silica content, has a medium viscosity and is associated with moderate eruptive violence.
- DACITE has 63-68% Silica content, has a medium to high viscosity and is associated with low to high violence eruptions.
- RHYOLITE has greater than 68% Silica content, has a high viscosity and is associated with highly violent eruption activity. Rhyolite is typically white or grey in colour.
Although eruptions have occured within the Okataina Volcanic Centre for at least the last 400,000 years, most is known about the activity within the last 20,000 years. The volcanoes in this region have erupted mostly two main types of magma (molten rock).
This Earthcache
To claim a find on this earthcache, please complete the tasks outlined below. Answers to the questions should be emailed to the cache owner and not mentioned in your online log.
1. From either of WP1 and WP2 where you can view the crater forming Lake Rotoatua and referring to the text on this cache page, determine the type of magma erupted here.
2. At WP3, where you can see Lake Rotongata, describe the feature of the crater that you are standing on.
3. Describe the topography of the two craters you can see from WP1, WP2 and WP3 in terms of shape, depth, steepness, etc.
4. (Optional) Please take a photo somewhere along the trail including your GPS device in the photo if possible. Upload this photo with your online log, but please do not give away any of the answers for questions 1, 2 or 3, above.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
Happy Earthcaching!!!
squirrel nutcase