Giant Coral Boulders
When you arrive at the published coordinates of this earthcache, you will see before you the largest example yet discovered in the world of a rock that has been displaced and deposited up hill by a tsunami - one of the largest ever triggered by a volcanic eruption, most likely originating from a chain of sunken volcanoes some 30 km to the west of Tongatapu. By comparison, the tsunami generated by Krakatoa, the most powerful explosion in recorded history, produced 40m waves and moved only boulders up to about one third of this size.
There are seven such "erratic" boulders of significant size in this area (this one is the largest) and each was tossed between 100 and 300 metres inland from the coast. The boulders are made of the same reef material found just offshore, which is quite distinct from the island’s volcanic soil and it is therefore believed they originated as part of the island's reef. Many of the boulders’ coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef. The coral was alive and growing around 122000 to 130000 years ago.
How long ago these boulders were tossed up here by the mega-tsunami is not precisely known. However, it can be estimated by observing evidence around the boulders:
- Thickness of the layer of topsoil that caps the boulders, formed after the boulder has been deposited here.
- Whether a limestone pedestal has formed around the base of the boulders as rain dissolves the coral over time.
- Thickness (or sparseness) of the topsoil layer in the area surrounding the boulders (indicating whether the area was recently scoured by waves).
The boulders are such an unusual part of the Tongan landscape that tales of their origins appear in local folklore. According to one legend, the god Maui hurled the boulders ashore in an attempt to kill a giant man-eating fowl.
In order to log this earthcache as found you need to visit the site and complete some educational tasks. The answers to the following questions should be emailed to the cache owner by using the link at the top of the cache page. Do not include any spoilers to the answers in your online log.
1 Please describe the shape and measure the dimensions of the huge rock located at the earthcache coordinates. Then, using your measured dimensions, and an average coral limestone density of 2000kg per cubic metre, calculate the mass of the rock in kg or metric tons. (Use the formula width x length x height x π / 6 to calculate volume.)
2 Exactly when the giant tsunami occurred and this rock was deposited here is not precisely known. Theories range from around 100 thousand years ago to 7 thousand years ago. Based on the rock and the surrounding terrain, which of these do you believe is more likely to be correct, and why?
3 Take a couple of photos of the boulder, or yourself (or GPS/personal item) near the boulder, and upload these with your online log. Include your GPS device for extra points.
You can log this earthcache as soon as you have emailed your answers. Logs that do not follow such an email, or do not include the required photo, will be deleted.
First to have their world rocked: