How can you tell if an igneous rock is formed on the surface of the earth or deep inside it?

Answer 1

An intrusive igneous rock has larger crystals than an extrusive igneous rock. In an extrusive rock the crystals are usually so fined grained that they are not visible by the naked eye. However, the individual crystals in an intrusive igneous rock can be viewed with the naked eye.

Answer 2

On the surface:

Volcanic lava and ash in various forms - some rubble-like, some with wrinkly surfaces, etc. Very fine crystals possibly too small for the naked eye, thanks to rapid cooling. Also vesicles - rounded cavities that had been gas bubbles, revealed by weathering or artificial cutting.

The SW English city of Exeter is built on the remnants of a volcano, and many of its Mediaeval buildings show rough blocks in their walls of both vesicular lava and a very coarse sandstone breccia.

Polygonal columns that form in flood-basalt lava as it cools - e.g Giant's Causeway, Isle of Staffa (Fingal's Cave).

Pillow lava - from undersea eruptions now revealed on land by uplift.

Plutonic rocks:

More noticeably crystalline - possibly very large but rather irregular crystals (phenocrysts) in granite.

Much more solid appearance to the formation generally, but it may be broken by tension-cracks, and an exposed batholith can spall on a large scale, known as 'onion-skin weathering'.

I have seen a fine example, in Norway, of 'onion-skin' layers about 2 metres high, creating short climbs in progressing up the dome-shaped hill.

Also dykes and sills, possibly in erosion or artificial section, showing entrapped xenoliths in varying stages of dissolution; and in granite, rarely, circles that are the sectioned shells of "orbs".

Metamorphism in country rock at side of intrusion.

Answer 3

Extrusive igneous rock (that was formed after being erupted from a volcano) will have a much smaller crystal size than an intrusive igneous rock.

Answer 4

By its texture.