Lord Howe Island is a World Heritage Area located some 550 kilometres off the coast of New South Wales. Formed seven million years ago, it is a narrow island with beaches, cliffs, and rugged volcanic topography along its 11 km length. It was isolated for long enough to develop unique species in a sheltered environment. Evolution is as apparent here as on Galapagos but LHI is much more attractive. It possesses the most southerly coral reef in Australian waters and Sylph’s Hole, a patch reef in a deep hole, is a good place to see fish and coral. It is off Old Settlement Beach where young green turtles can be seen year round in the sea grass meadows.
Approaching Dawson’s Point one crosses several dykes in the rock; here volcanic matter intruded through cracks in the original basalt some 6.4 million years ago. The dykes are easily recognised by their rigid seam like appearance contrasting markedly with the less structured basalt around them.
An intrusive dyke is a body with a thickness that is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters, and the lateral dimensions can extend over many kilometers. A dyke is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dyke is always younger than the rocks that contain it. Dykes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the latter becomes horizontal.
An intrusion is like a slow-motion volcano. Magma from under the surface slowly pushes up and squirts into any cracks or spaces it can find, sometimes pushing existing rock out of the way. Whereas a volcano takes place over hours or months, intrusions take a million years or more to form. The magma is thick and doughy because it's already started cooling. The rock slowly forms a solid, and the minerals have time to form large crystals (millimeters), instead of being spewed out of a volcano and cooled off quickly into lava flows or an ash cloud (whose crystals are tiny dust particles).
To claim this cache e-mail the answers to the following questions to me. Please DO NOT log the answers. No need to wait for an answer just log your find. A picture from close to GZ is welcome but optional.
Are the dykes vertical or horizontal?
How wide are the dykes?
In which direction do the dykes lie (N-S or E-W or something else)?
The dykes are several hundred meters from the beach but the difficulty rating takes into account that you will have to stay overnight (since there are only 400 visitors and many fewer residents at any time I’m assuming you will be a visitor). You do not need to do more than scramble along the rocks on the shoreline.