A hypabyssal igneous rock is formed in between the depth of intrusive (also called plutonic) rocks and extrusive (volcanic) rocks. They are formed when magma or lava cools down and solidifies.

The rate at which a rock cools will determine its grain or crystal size. Igneous rocks are widely classified into volcanic, hypabyssal and plutonic. Those that cool deep inside the earth are plutonic and those that cool and crystallize at or much near the Earth's surface are volcanic. The intermediate rocks are the hypabyssal ones. Volcanic rocks come immediately in contact with air or water and get quenched developing finer grained crystals that are best identified under a petrological microscope or a high power electron microscope. But plutonic rocks have a hell lot of time to cool due to their hotter surroundings resulting in well grown big crystals, which can usually be identified by naked eye or with the help of a hand lens. You should have now guessed the hypabyssal rocks will have medium sized crystals.

Intrusive

Extrusize

Plutonic

and Hypabyssal

A volcanic neck is the throat of a volcano and consists of a pipe like conduit filled with hypabyssal rocks. the volcanic Lave from the Magma Chamber comes out through the throat when the volcano erupts. :) hope this helped :)