Igneous and Metamorphic Rocks: Formation of Igneous Rocks

The video explains the formation of igneous rocks, both intrusive and extrusive. Intrusive rocks form when magma cools slowly beneath the Earth's surface, allowing large crystals to develop, as seen in pegmatite. In contrast, extrusive rocks form when lava cools quickly on the surface, resulting in smaller crystals, as in rhyolite. Obsidian forms so rapidly that no crystals are visible, giving it a glass-like appearance. Some extrusive rocks, like pumice, have trapped gases that create air pockets, enabling them to float.

What type of rock is formed when magma cools slowly beneath the Earth's surface?

• Extrusive rock

• Metamorphic rock

• Sedimentary rock

• Intrusive rock

Which of the following is a characteristic of pegmatite?

• No crystals

• Air pockets

• Large crystals

• Small crystals

What happens to the size of crystals in igneous rocks when they cool quickly on the Earth's surface?

• They disappear

• They become larger

• They remain the same

• They become smaller

Which type of igneous rock is likely to have a glass-like appearance due to rapid cooling?

• Diorite

• Granite

• Obsidian

• Basalt

What causes some extrusive rocks to have holes and air pockets?

• Rapid cooling without gas escape

• Slow cooling without gas escape

• Slow cooling with gas escape

• Rapid cooling with gas escape