Three major kinds of rocks

Igneous - Formed by solidification of materials that were previously molten
Resulting rock types depends on elemental composition of molten material, cooling history of rock and processes that remove previously solidified or molten material from the parent melt.

Sedimentary - Formed by consolidation of materials that were previously loose or dissolved.
Materials are consolidated by either being cementated or compacted together.

Metamorphic - Formed by growth of new minerals out of previously existing rock without melting.
New minerals grow from existing ones when conditions that formed the original rock change. Key triggers include changes in temperature, pressure and/or exposure to the chemical action of fluids. Metamorphism is limited to conditions that do not cause melting of previously existing rock. Metamorphic rocks can form by reorganization of existing substances in a rock, or by reorganization of existing substances combined with the addition or removal of some substances. Mineral types present may or may not involve change in overall elemental composition of the rock.

Observe & Discuss:

Station - Which of these is an igneous rock?

  • Conglomerate - No pieces consolidated, but not originally intergrown as in igneous rocks.

  • Diorite - Yes, individual minerals visible, interlocking crystals that are typical for an igneous rock.

  • Basalt - Yes, individual minerals not visible, but tiny crystals can be seen to be interlocked in microscopic view.

  • Obsidian - Yes, individual mienrals not visible and do not exist, but this rock is igneous. It cooled so fast that individual crystals did not have time to grow.