Rangitoto (Auckland) EarthCache

The iconic landmark of the Auckland Harbour. Just 600 years old - scarey stuff.

Future finders please note the tasks you must complete to log this EC and the tools you’ll need to complete these tasks. I have had far to many finders sending incomplete info when clearly they have just turned up and hoped for the best. Earth caches are there for a reason, or learn something. All the info about earth caches is here. I will not be accepting finds on this earth cache if you do not supply the correct info.

Earth caches aren’t there just to pad your ‘found it’ numbers.

Geology

• Height: 260 m
• Age: Formed about 600 years ago (ca 1400 AD)
• Volume lava: about 2,300 million cubic metres (equivalent to 468,000 Olympic sized swimming pools)
• Volume tuff/ash/pyroclastics: about 19 million cubic metres
• 

  Rangitoto is the largest, most recent and least modified volcano of the Auckland Volcanic Field. It forms a near symmetrical cone at the entrance to the Waitemata Harbour. A so-called shield volcano Rangitoto has shallow sloping sides. The name derives from a translation of "Skjaldbreiður", an Icelandic shield volcano whose name means "broad shield," from its resemblance to a warrior's shield. Shield volcanoes are formed by lava flows of low viscosity — lava that flows easily.

  One of the 49 volcanoes forming the Auckland Volcanic Field, Rangitoto was formed by a series of eruptions between 600-700 years ago. Scientists are in dispute about the length of the eruptions, which are thought to have lasted (with interruptions) for 10 to 200 years.

  The early stages of the eruption would have been excessively violent, due to steam explosions where the molten rock came into contact with the shallow seawater. Large amounts of rock and ash soon smothered Motutapu. Rangitoto finally emerged from the sea as a broad eruption crater with liquid rock building up a number of cones.

  The amount of mass that erupted from the volcano was about equal to the combined mass of all other eruptions in the Auckland Volcanic Field before.

  When the eruptions finally ceased, lava in the base of the cone cooled and shrank. As a result, the entire top of the mountain subsided by 10 to 20 metres leaving a moat-like ring around the summit which may be viewed from a path which goes right round the rim and up to the highest point. These are the small mounds either side of the central cone that gives Rangitoto its nearly symmetrical profile.

  The island is considered especially significant because all stages from raw lava fields to scrub establishment and sparse forests are visible. In some parts of the island, fields of lightweight, clinker-like black lava stones are still exposed, appearing very recent to a casual eye. Visitors walk through the lava fields and may also walk through some of about seven known lava caves - tubes left behind after the passage of liquid lava. The more accessible of the caves are signposted.

  The volcano is not expected to become active again (monogenetic), although future eruptions are likely elsewhere in the wider area of the field.

  

  This information is provided with best intentions. If you know better please advise cache owner.

  Nature

  There are virtually no streams on the island so plants rely on rainfall for moisture. It has the largest forest of pohutukawa trees in the world, as well as many Northern Rata trees. In total, more than 200 species of trees and flowers thrive on the island, including several species of orchid, as well as more than 40 types of fern.

  As lava fields contain no soil of the typical kind, windblown matter and slow breaking-down processes of the native flora are still in the process of transforming the island into a more habitable area for most plants, which is one of the reasons why the local forests are relatively young and do not yet support a large bird population. However, the kaka, a New Zealand-endemic parrot, is thought to have lived on the island in pre-European times.

  
  

  Sources:

  1. Wikipedia http://en.wikipedia.org/wiki/Rangitoto_Island
  2. Rangitoto (from the Auckland Regional Council website)
  3. Hauraki Gulf Islands - Rangitoto Island (from the Auckland City Council website)
  4. Welcome to Rangitoto Island (from the Rangitoto Island Historic Conservation Trust)

  Geo Tasks

  

  At this waypoint you’ll get your first glimpse of the Rangitoto’s crater.

  Nearby there’s an information board that amongst other information tells you the diameter and depth of the crater. Using this waypoint as reference, walk the crater rim and determine practically the greatest diameter relative to this point AND the circumference of the rim

  Questions:

  1. Describe what do you see when you look into the crater?
  2. Compared to the information board what diameter for the crater do you find. Walk the crater rim and use your GPSr (nearest waypoint function). Provide the difference to the nearest 10 metres.
  3. What distance do you find walking the crater rim. Use your GPSr track function
  4. Provide at least two small e-mail sized photos taken from the crater rim looking out across Rangitoto

  For an extra brag point - but not required for find

  Calculate the volume of Rangitoto’s cone using the formula one third times pi times radius squared times depth

  Send answers to cache owner. Do NOT post the answers in a log. GOOD LUCK