Granite emplacement 1: Megiliggar Rocks

Granite bodies can be extremely large - the Cornish granites are thought to form a connected body underground which extends for many tens of km from Dartmoor out towards the Scilly Islands. They can look quite homogeneous in large-scale exposures, leading to the idea that they are intruded as massive diapiric bodies (rather lige lava-lamp style plumes), but there are significant problems with moving multi-kilometer sized melt bodies through the Earth. Looking across towards Megiliggar from Trewavas Head we can see a series of sills emanating from the side of the main granite bodies and intruding the Mylor Slate country rocks. The formation of dykes and sills can isolate blocks of country rock and allow them to fall through the granite body, allowing the granite to progress upwards through the crust by a process called stoping.
See also:
Granite emplacement 2: 2 generations of dyke at Porthmeor
• Granite emplacement 2:...
Porthmeor granite 2: Aplite-Pegmatite sheets
• Porthmeor granite 2: A...

[@DavidAnderson-vt8iq]

Thank you. I've heard the term 'stope' in reference to mining, but not fully understood what I was looking at. Cheers for the video.

[@OneMinuteGeology]

Hi David, geologists borrowed the term from mining. Stoping is a way to follow a near-vertical seam. You drive a tunnel along the seam and then cut into the ore above and let it fall into the ore bucket - it is quite efficient as it saves having to lift the ore back up from the tunnel floor and was the preferred method in pre industrial mines in the UK. The method was called overhand stoping. Geologically it carries the idea that intrusions into the roof of a magma chamber can allow the country rock to fall into the magma, displacing an equal volume of magma upwards.

[@janetmackinnon3411]

A new term! Thank you.

[@mikelong9638]

Nice video Doc. If one didn't know better it looks just like sedimentary layers from a distance.