1. Igneous Rocks
Group 1 of 3

2. Scientists believe that the first type of rock that was on Earth was an igneous rock.
They believe this because most scientists think that the Earth cooled from molten material when the Earth was formed.

3. Igneous rocks are rocks that are made from molten rock.
That molten rock may be either:
Magma – molten rock underground
Lava – molten rock above ground

4. If the igneous rock forms from magma the cooling rate of the magma is slow (like a few degrees per century).
Slow cooling gives the mineral crystals time to grow big. The rock would look like this:

5. If the igneous rock forms from lava the cooling rate of the magma is much faster.
Fast cooling does not give the mineral crystals time to grow big. The rock would look like this:

6. Igneous rocks are classified according to what they cooled from:
Intrusive igneous rocks –
cooled from magma.
also called plutonic (after the Roman god of the underworld)
large crystals – course grained texture
examples: (next slide)

7. Granite

8. Gabbro

9. Diorite

10. 2. Extrusive igneous rocks –
cooled from lava
also called volcanic
small crystals – fine grained texture
examples: (next slide)

11. Basalt

12. Rhyolite

13. Extremely fast cooling leads to gas escaping
thus creating small holes in the rock.
Here are a couple of examples…

14. Scoria

15. The only rock that floats!
Pumice

16. Sometimes the cooling is so fast the rock just forms one big crystal
Sometimes the cooling is so fast the rock just forms one big crystal. Also called volcanic glass.
Obsidian

17. Banded Obsidian
If there is a large amount of iron present reddish bands will form.

18. Snowflake Obsidian
The markings are spherulites (small rounded bodies made of minerals)