The posted coordinates have brought you to the Onawe Peninsula in Akaroa Harbour. The Onawe Peninsula is thought to be the approximate site of the original vent of Akaroa Volcano.
Akaroa Volcano was one of three volcanoes that formed Banks Peninsula between 12 and 6 million years ago (Mid to late Miocene). Unusually for New Zealand, Akaroa and the other Banks Peninsula volcanos are shield volcanos.
A shield volcano is built almost entirely of fluid lava flows. The highly fluid lava erupted travels further than that from more explosive volcano types, building up the shield volcano's distinctive form (resembling a warrior’s shield). Over the course of thousands of eruptions, layer upon layer of lava is built up, forming a broad cone. The eruptions that occur at shield volcanos are sometimes called Hawaiian eruptions, since this type of volcano is very common and very active in Hawaii.
Because of their gradual build up and nearly continuous activity, shield volcanos tend to be the largest ones. Present day examples are usually at least 5-6km in diameter and over 600m in height. The largest shield volcano (and the largest active volcano) is Mauna Loa in Hawaii. Mauna Loa stands at 4,169m above sea level. At its largest, Akaroa volcano is estimated to have had a diameter of 50km and probably reached a height of >1,800m above sea level. Its volume has been estimated at 1,200 cubic kms.
Banks Peninsula volcanic activity occurred over a period of about 6 million years, and ceased about 6 million years ago. Over the intervening years, erosion has taken its toll, and the Akaroa and Lyttelton harbours are now classic examples of an eroded crater. Through erosion, much of the rock from the volcanos has weathered away. During the postglacial world-wide rise in sea level beginning about 15,000 years ago, the craters were invaded by the sea forming the present harbours and accelerating the erosion process.
Typically, shield volcanos are composed of basalt. However, it is simplistic to expect to find only one type of rock formation. On Banks Peninsula, the volcanos have been active through the same period so their lava flows have mingled with each other. In some places this makes it difficult to determine which volcano the rocks have come from. On the Onawe Peninsula, there are igneous rocks such as basalt, but there are also metamorphic rocks present. These are metamorphic rocks of a type called Migmatite, which has been formed by the igneous volcanic rocks being subjected to great temperatures and pressures, causing the rock to partially re-melt and start mixing the minerals. This produces a rock with very interesting swirls and patterns.
To log this EarthCache, you will need to complete the following questions and tasks. Please email the answers, but DO NOT post the answers with your online log! Once you have emailed the answers, you may go ahead and post your online log. No need to wait for a reply.
1. Using your GPS, measure your current elevation above sea level. Calculate by approximately how many meters the Akaroa volcano has been eroded at this location.
2. Looking at the cliff, you will see a triangular shaped dark patch. Measure the length of the base of the triangle. This dark patch of rock is an intrusion of a rock formation of a different age and type.
3. Looking at the cliffs near this spot, you will notice “stripes” of migmatite running through the cliffs and along the ground. Approximately how wide are these “dykes” of metamorphic rock?
4. Move to WP1. You will now be standing in front of some migmatite rocks with amazing swirling patterns. Take a photo that shows off most amazing patterns you can see in the migmatite and post the photo with your online log. There is no need to include yourself or your GPS in the photo.
Optional activity: In your online log, feel free to describe the formations you found and include additional photos of the most amazing patterns or formations you found on the peninsula.
Getting to the EarthCache
The posted coordinates are on the beach, and this spot may be under water at high tide. Check the tide times and plan your visit so it is not right on high tide, or you will get wet feet. It is only a short walk down to the beach from the car park on Onawe Flat Rd.