Sunday, August 03, 2014

Geology walk at Santubong National Park


CRUSTS, caramel, nuts in porridge – they sound more like a cooking lesson than a walk to learn about geology.

However, these are the striking analogies Hans – a geologist with Shell in his former life – and a dedicated nature lover and photographer, used to explain to us about the ways rocks have behaved over the geological time scale.

Santubong is a striking coastal sandstone mountain reaching an 810-metre elevation.

Some brave, and fit, souls actually climb to the summit, but last month, about 30 MNS members took a much gentler walk from the newly operational Santubong National Park headquarters as far as the waterfall with the canopy bridge.

We walked slowly as Hans explained about continental and ocean crusts.

Continental crust consists largely of granitic (mainly quartz and feldspar) rocks that form the continents.

Oceanic crust, on the other hand, is located under the oceans and mainly composed of different materials known as basaltic rocks, which are mostly dark in colour.

Over the aeons, different crustal slabs slide or ‘dive’ under one another, releasing enormous forces that often push up continental rocks (but sometimes oceanic rocks are pushed up).

That is part of the story of how, Hans explained, Gunung Santubong, which seemingly arises almost directly out of the sea, was formed. Another piece of the puzzle may be found in the intrusive rocks found at Santubong.

How can we detect where these slabs of crust (known as tectonic plates) have dived under one another? One important clue, Hans explained, is when we see dark-coloured basalt rocks, usually hidden under the oceans, at the land surface.

One of the big boundaries between crustal slabs in Sarawak corresponds approximately with the Lupar River.

One can see dark-coloured basalt rocks on a bank of the Batang Ai hydroelectric lake. Interestingly, though not surprisingly, some tree species are only found on one side of the Lupar valley dividing line, while others are only found on the other.

So how about the caramel? The cooks among us will know that to make kueh sarang semut, one first has to make caramel.

This we do by heating sugar carefully in a pan until the mass of white crystals begins to melt to a shiny liquid and darken in colour.

In the same way, Hans explained, showing us a sandstone rock that was pale brown on one side and dark brown on the other, some of the sandstone at Santubong has been heated up by hot magma injected into narrow cracks in the sandstone, and has undergone a similar change, with the melted part not gritty like normal sandstone but smooth and also darker in colour.

After melting, the rock is known as hornfels, (a German word for horn rock) as smooth as the horns of a cow.

This is where the nuts in porridge example comes in – an example of rock that was injected as hot paste into cracks of the sandstone is diorite – which cooled very slowly when still deep underground, so that some components made big crystals of mainly feldspar and hornblende (the nuts) floating in a fine-grained mass (mainly feldspar).

Then Hans explained some of the other geological features we can see at Gunung Santubong.

Continue reading (Incl. Pics) at: Geology walk at Santubong National Park
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