Getting to know the rocks in and around Strachur

Gravel coverBased on a collection of rocks assembled and described by Ron Thom
September 2009


Granite To Granite

"What's this rock?"
"It's granite. It's made of quartz, feldspar, and black mica."
"What are they?"
"These are minerals, the ingredients that make up the rock."
"OK, that's what it's made of, but what is granite?"
"It's a plutonic igneous rock."
"What does ‘igneous' mean?"
"It means it was formed from molten rock, or magma."
"Like a volcano?"
"Yes, but the rock crystallised at depth, maybe 5km below the earth's surface. That's what the word ‘plutonic' means."
"How do you know it crystallised at depth?"
"From the size of the crystals. Magma cools slowly at depth where the surrounding temperatures are still reasonably hot, giving crystals time to grow to a large size."
"So how come we see it on the surface?"
"Because it has been exposed by erosion."
"Do you mean that 5km of material has eroded away?"
"Yes, I do. All high ground is constantly being eroded, fragment by fragment, grain by grain. Over millions of years this has a huge effect."
"So where has all the material gone?"
"It has been carried as mud, sand and pebbles by rivers and dumped into lakes or into the sea to form sediments, which harden to form sedimentary rocks."
"So if mountains erode to become sediments in lakes and oceans, why doesn't the Earth's surface eventually become flat?"
"Mountains are renewed in different ways. The Earth's surface is made of enormous plates which "float" on the near-liquid rocks beneath. These plates are constantly on the move, and occasionally two continental plates collide, buckling their edges to form huge plateaus like the Himalayas. Or an oceanic plate can slide beneath a continental plate, wrinkling the edge of the continental plate as it passes under, giving a mountain range like the Andes."
"These oceanic plates which are sliding downwards - where do they end up?" "They keep edging slowly downwards towards the base of the crust, where the temperatures are very high. The plates start to melt to form magmas, which rise up through the crust and can be erupted from volcanoes to form lavas. Or, if the magma fails to reach the surface it may crystallise at depth to form granites or gabbros ... "
"So, we're back to granites again. And what are gabbros?"
"That's not easy to answer without giving you more information. Let me collect a few rocks, and I'll put together a story..."

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