1. Igneous Rocks Chapter 4

2. Igneous Rocks
Igneous rocks form as molten rock (magma) cools and solidifies
General characteristics of magma
Parent material of igneous rocks
Forms from partial melting of rocks
Magma at surface is called lava

3. General Characteristics of Magma
Rocks formed from lava = extrusive, or volcanic rocks
Rocks formed from magma at depth = intrusive, or plutonic rocks

4. General Characteristics of Magma, cont’d
Magma consists of three components:
Liquid portion = melt
Solids, if any, are silicate minerals
Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

5. General Characteristics of Magma, cont’d
Crystallization of magma
Cooling of magma results in the systematic arrangement of ions into orderly patterns
Silicate minerals result from crystallization in a predictable order
Texture - size and arrangement of mineral grains

6. Igneous Textures
Texture - overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals
Several factors affect crystal size

7. Igneous Textures Factors affecting crystal size Rate of cooling
Slow rate = fewer but larger crystals
Fast rate = many small crystals
Very fast rate forms glass
% of silica (SiO2) present
Presence of dissolved gases

8. Types of Igneous Textures
Aphanitic (fine-grained) texture
Rapid rate of cooling
Microscopic crystals
May contain vesicles (holes from gas bubbles)

9. Types of Igneous Textures
Phaneritic (coarse-grained) texture
Slow cooling
Large, visible crystals

10. Types of Igneous Textures
Porphyritic texture
Minerals form at different temperatures
Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

11. Types of Igneous Textures
Glassy texture
Very rapid cooling of lava
Rock is called obsidian or pumice

12. Types of Igneous Textures
Pyroclastic texture
Fragmental appearance produced by violent volcanic eruptions
Often appear more similar to sedimentary rocks

13. Types of Igneous Textures
Pegmatitic texture
Exceptionally coarse grained
Forms in late stages of crystallization of granitic magmas

14. Igneous Compositions
Igneous rocks are composed primarily of silicate minerals
Dark (or ferromagnesian) silicates
Olivine, pyroxene, amphibole, and biotite mica
Light (or nonferromagnesian) silicates
Quartz, muscovite mica, and feldspars

15. Igneous Compositions, cont’d
Granitic versus basaltic compositions
Granitic composition
Light-colored silicates
Termed felsic (feldspar and silica) in composition
High silica (SiO2) content
Major constituent of continental crust

16. Igneous Compositions, cont’d
Granitic versus basaltic compositions
Basaltic composition
Dark silicates and Ca-rich feldspar
Termed mafic (magnesium and ferrum, for iron) in composition
Higher density than granitic rocks
Comprise the ocean floor and many volcanic islands

17. Igneous Compositions, cont’d
Other compositional groups
Intermediate (or andesitic) composition
Contain 25% or more dark silicate minerals
Associated with explosive volcanic activity
Ultramafic composition
Rare composition that is high in magnesium and iron
Composed entirely of ferromagnesian silicates

18. Igneous Compositions, cont’d
Silica content as an indicator of composition
Crustal rocks exhibit a considerable range--45% to 70%
Silica content influences magma behavior
Granitic magmas = high silica content and viscous
Basaltic magmas = much lower silica content and more fluid-like behavior

19. Naming Igneous Rocks Granitic rocks Granite Phaneritic
Over 25% quartz, about 65% or more feldspar
Very abundant - often associated with mountain building
The term granite includes a wide range of mineral compositions

20. Naming Igneous Rocks, cont’d
Granitic Rocks
Rhyolite
Extrusive equivalent of granite
May contain glass fragments and vesicles
Aphanitic texture
Less common and less voluminous than granite

21. Naming Igneous Rocks, cont’d
Granitic rocks
Obsidian
Dark colored
Glassy texture
Pumice
Volcanic
Frothy appearance with numerous voids

22. Naming Igneous Rocks, cont’d
Intermediate rocks
Andesite
Volcanic origin
Aphanitic texture
Diorite
Plutonic equivalent of andesite
Coarse grained (phaneritic texture)

23. Naming Igneous Rocks, cont’d
Basaltic rocks
Basalt
Volcanic origin
Aphanitic texture
Composed mainly of pyroxene and calcium-rich plagioclase feldspar
Most common extrusive igneous rock

24. Naming Igneous Rocks, cont’d
Mafic rocks
Gabbro
Intrusive equivalent of basalt
Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
Significant % of the oceanic crust

25. Naming Igneous Rocks, cont’d
Pyroclastic rocks
Composed of fragments ejected during a volcanic eruption
Varieties
Tuff = ash-sized fragments
Volcanic breccia = particles larger than ash

26. Origin of Magma Generating magma from solid rock Role of heat
Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer
Rocks in the lower crust and upper mantle are near their melting points
Additional heat may induce melting

27. Origin of Magma, cont’d Role of pressure Role of volatiles
Increases in confining pressure increases a rock’s melting temperature
When confining pressures drop, decompression melting occurs
Role of volatiles
Volatiles (primarily water) cause melting at lower temperatures
Important factor where oceanic lithosphere descends into the mantle

28. Evolution of Magmas
A single volcano may extrude lavas exhibiting very different compositions
Ice freezes at a single temperature, magma crystallizes through 200 degrees of cooling. If we look at this example from Mt. Mazama, we can see lite colored silica-rich ash near the base grading to dark-colored rocks at the the top. Geologists believe that the magma body had begun to segregate as the less dense, silica rich magma migrated to the top of the magma chamber. The zonation is seen because a sustained eruption tapped deeper into the magma chamber. The rock is an inverted representation of the magma body.

29. Bowen’s Reaction Series
Minerals crystallize in a systematic fashion based on their melting points
During crystallization, the composition of the liquid portion of the magma continually changes
So we know that magmas evolve. How does this happen? A geologist named N.L Bowen performed lab studies that demonstrated…

30. Evolution of Magmas, cont’d
Processes responsible for changing a magma’s composition
Magmatic differentiation
Separation of a melt from earlier formed crystals
Assimilation
Changing a magma’s composition by incorporating surrounding rock bodies into a magma
Magma mixing
Two chemically distinct magmas may produce a composition quite different from either original magma

31. Partial Melting and Magma Formation
Incomplete melting of rocks is known as partial melting
Formation of basaltic magmas
Most originate from partial melting of mantle rocks at oceanic ridges (decompression melting) or at subduction zones (presence of water)
Large outpourings of basaltic magma are common at Earth’s surface
Remember that crytallization occurs over 200 degrees. That means that melting also occurs over 200 degree. As rock begins to melt—which will melt first? The last to crystallize—the ones with the lowest melting temperatures. If melting continues, minerals with higher melting points begin to melt, then the composition of the magma will steadily approach the composition of the rock from which it was derived. Most times the melting is not complete—Partial Melting. A process that produces most if not all magma. The minerals with the lowest melting point are the granitic minerals—quartz and K-spar. So how do basalts get tot the surface? Partial melting of peridotite, a major constituent of the upper mantle. Peridotite is an ultamafic rock composed mainly of olivine

32. Partial Melting and Magma Formation, cont’d
Formation of andesitic magmas
Produced by interaction of basaltic magmas and more silica-rich rocks in the crust
May also evolve by magmatic differentiation

33. Partial Melting and Magma Formation, cont’d
Formation of granitic magmas
Most likely form as the end product of crystallization of andesitic magma
Granitic magmas are more viscous than other magmas – tend to lose their mobility before reaching the surface
Produce large plutonic structures

34. End of Chapter 4

35. Lava flows are typically finer grained than intrusive igneous rocks
Lava flows are typically finer grained than intrusive igneous rocks. Why?
04.01
Intrusive magma is cooler because it is well insulated by the surrounding rock.
Intrusive magma flows onto the Earth's surface and cools very slowly, allowing many small mineral grains to grow.
The extrusive magma cools quickly so the mineral grains do not have time to grow.
The extrusive magma, because it is deep below the surface, cools very slowly, producing very small mineral grains.

36. What does Bowen’s Reaction Series describe?
04.02
What does Bowen’s Reaction Series describe?
The pressures that different minerals are formed in metamorphic rocks
Which minerals are recrystallized in a sedimentary rock
The temperatures at which different minerals crystallize out of a melt
All of the above
None of the above

37. Which of the following describes best the difference between magma and lava?
04.03
Upon eruption, magma becomes “lava,” the form of the magma that flows out of the volcano
When magma gets to the top of the volcano, it releases gas found within the magma, and the lava that escapes the volcano has therefore lost those gases.
Lava is the name given to the ejected magma from a passive (fluid) volcano, but it is still called “magma” if it is erupted explosively, like at Mt. St. Helens.
If the magma has no crystals or gases within it, it is called “lava.”
Both 1 and 3
Both 3 and 4
All of these.

38. Which of the following rocks is likely to have the most quartz within it and why?
04.04
Granite; intrusive rock that formed from cooling of relatively high silica magma.
Rhyolite; extrusive rock that formed from cooling of relatively low silica magma.
Diorite; intrusive rock that formed from the cooling of relatively intermediate silica magma.
Granite; intrusive rock that formed from cooling of relatively intermediate silica magma.
Basalt; extrusive rock that formed from cooling of relatively low silica lava.
Basalt; extrusive rock that formed from cooling of relatively high silica lava.

39. What is the most important factor for whether magma cools slowly or quickly?
04.05
Pressure of the environment – higher P = slower magma cooling.
The presence or absence of volatiles (gases) – less gases = slower magma cooling.
Temperature of the environment – lower T = slower magma cooling.
The presence or absence of volatiles (gases) – more gases = slower magma cooling.
Temperature of the environment – higher T = slower magma cooling.
Pressure of the environment – lower P = slower magma cooling.

40. Igneous rocks are produced largely by ________.
04.06
Igneous rocks are produced largely by ________.
the changing of a rock from one set of minerals to another
the compaction of metamorphic rocks
the melting of sedimentary rocks
the cooling of magma
Both c and d are correct.

41. Glassy igneous rocks form when the magma _______.
04.07
Glassy igneous rocks form when the magma _______.
cools so fast that mineral grains cannot crystallize and grow
cools so slowly that only one mineral is formed
is composed of basalt
is a rhyolitic type
cools at an extremely high temperature

42. 04.08
Quartz is ________.
resistant to weathering and is an important component of sand in river beds and beaches
a main constituent of many igneous rocks
a main constituent of many sedimentary rocks
the most stable of all minerals at Earth’s surface temperatures and pressures
all of the above