1. Igneous Rocks (IR) a.k.a Volcanic Rocks
Ms. Tasneem

2. Rock cycle
Igneous rocks form when the minerals in magma crystallize, or harden.

3. Igneous Rock Formation
There are two types of igneous rocks. These types are based on where magma cools and hardens to form the rocks. Igneous rocks formed deep within the earth are called intrusive. When magma reaches the surface, it is called lava. Lava is the liquid rock that pours out of volcanoes. Igneous rocks formed from lava at the earth’s surface are called extrusive rocks.

4. How do Intrusive IRs Form?

5. Igneous Intrusions What is an igneous intrusion?
Read about them, what is the main difference between them?
Sometimes magma does not flow out onto the surface of the earth. Instead the magma flows upward into cracks in the rocks or spreads out between rock layers. There it hardens under the earth’s crust, forming irregular formations of intrusive rock. The formations called intrusions have many shapes because the magma hardens in many different positions. Intrusions are classified according to their relationship to the older rock surrounding them.

6. Igneous Intrusions-Batholiths
Batholith (from Greek bathos, depth + lithos, rock)
a large inclusion of intrusive (also called plutonic) igneous rock that forms from cooled magma deep in the earth's crust
Enchanted
Rock -->
A batholith is a huge, irregularly shaped intrusion that extends deep into the earth’s crust. Scientists estimate that batholiths extend between 10 and 30 km into the crust. If an intrusion similar to a batholith has an exposed area less than 100 km it is called a stock. Enchanted rock is a great example of an EXPOSED batholith.
How would you explain what happened geologically in order for us to see this outcropping? The surface exposure is larger than 100 square km. Most batholiths are granite formations. Sometimes movements of the earth push batholiths above the surface. As the overlying rocks are worn away, the batholiths are exposed looking like large, steep hills.

7. Yosemite National Park

8. El Capitan
This is at Yosemite national park. It is only an exposure of the Sierra Nevada Batholith most of which remains underground. You can see El Capitan as an exposure to the left. It’s massive and compared to enchanted rock just amazing that these structures exist!

9. Igneous Intrusion – Stock & Sill
Stock = Smaller Batholith with surface exposures around100 km2
Sill = parallel to “country rock” beds and usually horizontal in orientation
Intrudes between
layers of
sedimentary rocks
The surface exposure is smaller than 100km

10. Igneous Intrusion – Laccolith & Dike
Laccolith = dome shaped intrusion between two layers of sedimentary rock
Dike = any geologic body that cuts across other rock formations
From my graduate school days. We were in the Texas hill country and you can clearly see the granite dikes running along the laccolith.

11. How would you group these paperclips?

12. Igneous Rock Classification
Color = (Proxy for silica content)
Size = (Proxy for grain size)
The size of the crystals in an igneous rocks is called its texture. The color of the igneous rock tells us something about the chemical composition of the rock.

13. Intrusive Rock Formation
Intrusive (Plutonic) = crystallized from slowly cooling magma intruded within the Earth’s crust
Granite
Gabbro
The texture of an igneous rock depends on the time it takes the rock to harden. This time is called the rock’s cooling rate. In general, the slower the cooling rate is, the larger the crystals will be. So intrusive rocks, which cool and harden slowly beneath the earth’s surface, have large crystals like you see in the examples on the slide.

14. Composition of IR How are minerals related to rocks?
How would knowing the different types of crystals help you describe how igneous rocks formed?

15. Composition of IR = MINERALS!

16. Compositional Classification
Granite
Compositional Classification
Quartz
Orthoclase
Biotite
Granite
Plagioclase

17. Compositional Classification
Granite
Compositional Classification
Granodiorite
Quartz
Granite
Amphibole
Plagioclase

18. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Plagioclase
Amphibole

19. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Gabbro
Plagioclase
Pyroxene

20. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Gabbro
Pyroxene
Olivine
Peridotite

21. Size: Grain size Color: Silica Content
Coarse Grained = large crystals
Fine Grained = Small crystals
Mafic (Magnesium/Fe) Ma = Magnesium
Fic = latin for Iron
IR rich in dark, heavier colored elements (Mg & Fe) = low silica
Felsic (feldspar/Silica) Fel = Feldspar
Sic = rich in silica
IR rich in silica
Igneous rocks made of large crystals are called coarse grained. If the cooling rate is fast, small crystals will form. Igneous rocks made of small crystals are called fine grained.
The color of an igneous rock is often a good clue to its composition. Most igneous rocks are made of various combinations of six minerals: quartz, feldspar, pyroxene, amphibole, olivine, and mica. Light colored igneous rocks are rich in quartz, feldspare and silica and are called Felsic. Dark colored igneous rocks are rich in pyroxene and olivine and are called Mafic.

22. Common Minerals

23. Rock Cycle
Looking at extrusive way now

24. How do Extrusive IRs form?
Volcanic (extrusive) igneous rocks
Cool fast, small crystals
form when molten rock (magma) in the earth’s interior rises to the surface through pipes or fractures in the crust.

25. How do Extrusive IRs form?
Extrusive = or erupted as pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air.
Ash
Pumice
Obsidian
Scoria

26. Classification of Igneous rocks
Texture size of crystals most important
Cooling rates
Surface, fast
Small crystals
Below surface slow
Larger crystals
Which formed at surface?
Both are the same chemistry a) b)
Rhyolite
Granite

27. Chemical Composition Color
Indicative of chemistry and temperatures of formation
Dark colors
High temperatures (1000 to 1200 oC)
Low silica content
Light colors
Lower temperatures
High silica content
Which is the high silica rock?
Both formed below the surface a) b)
Gabbro
Granite

28. How did this form? Intrusive or Extrusive?
Porphyry
Some rocks begin to cool deep within the earth. So the resulting crystals grow large. Then suddenly the magma is carried up near the earth’s surface. Here it cools rapidly, forming small crystals. The result is an igneous rock with large crystals surrounded by small crystals. An igneous rock whose cooling rate has changed and thus has two or more different sized crystals is called a porphyry (por-fuhr-ee).

29. Formation of IR

30. The Legend of Finn McCool
Finn a renown warrior was going about his daily duties on the north coast when one of his adversary, a Scottish giant called Benandonner started shouting and ridiculing Finn's fighting prowess. This angered Finn who lifted a lump of earth and pelted it to Scotland  as a challenge to this giant.  The Scottish giant retaliated with a rock back  to Finn shouting that if he could get his hand on him, he would make sure that Finn would never fight again adding that unfortunately he could not swim the short distance across the Channel, so Finn would be spared that fate. Finn got enraged by this and tore large pieces from the cliffs,  he worked for over a week pushing these into the ocean bed and made a sturdy causeway to Scotland, when he had finished he shouted 'Now you'll had no excuse' to come over and do your best.  Fearing to lose his own reputation and pride the Scottish giant had no alternative but come over the causeway.  Finn was tired, having not slept for the week he worked on the causeway and did not feel ready for fighting. He thought about how he could buy some time and recuperate to face the Scottish giant and came up with an ingenious plan. Quickly he made a large cot and disguised himself as a baby and waited.  The Scottish giant arrived at Finn's house shouting, 'Where is that coward MacCool', Finn's wife said the he was away but sure sit down and have a cup of tea, he'll not be long. the tea arrived with a cake in which Finn's wife had placed some stones. Benandonner took a bite and broke one of his teeth, and thought to himself,  this Finn must be a tough boyo to eat cakes like this, not to be outdone,  he finished off the cake and tea, breaking two more teeth in the process.  He noticed the baby cot and the baby inside it and his eyes widened in fear, he thought to himself, my goodness if this is the size of the baby? What size is the Father?  He reached his hand in to touch the sleeping baby, half out of curiosity and half  as a sign of affection, Finn bit the tip of his finger off.  Benandonner was shocked by this and thought to himself, if this is what the baby is capable of, what must the father be capable of and what could he not do! This thought terrified him, the fear got the better of him and took to his  heels and ran like he had never ran before, back across the causeway to Scotland destroying it as he went.  Well.....that is one version of the story, there are many more to choose from, including one of love and romance.

31. What really happened…
1. Lava flowed into a depression in the ground. It cooled, hardened, shrunk and cracked to form the hexagonal columns we see today.
2. The Giant’s Causeway is made up of layers of rock. The bottom layer is known as ‘Lower Basalt’.
3. During 2 Million years of warm, wet climate the lower basalt weathered to form a deep red rock called ‘Laterite’.
4. More volcanic eruptions occurred and lava poured onto the red rock. It cooled and hardened forming the ‘Causeway’ or ‘Middle Basalts’.
5. ‘Upper Basalts’ formed during a third period of volcanic eruptions. Most of this layer was lost to erosion during the last ice age, 15,000 years ago.

32. More of Ms. Tasneem’s photos

37. Six common Igneous Rocks
Solidifying Temperature
500 C
Increasing Grain Size
Volcanic Rocks
Silica Content
Minerals Present (in order of abundance)
Plutonic Rocks
Basalt
low
pyroxene, olivine, feldspar, & amphibole
Gabbro
Andesite
intermediate
feldspar, amphibole, pyroxene, biotite mica
Diorite
Rhyolite
high
feldspar, quartz, muscovite mica, & amphibole
Granite
Lighter Color

38. 5) Imagine that all minerals found in rocks were the same color
5) Imagine that all minerals found in rocks were the same color. What information would you no longer be able to infer? a. texture b. cooling rate c. composition

39. Analysis Time
Given:
Egg cartons w/ samples
Identify the rocks