Lecture 5// Igneous Rock

Question 1

Igneous rock (1)

Answer 1

-Forms when magma or lava cools and minerals begin to crystallize. Some form from ash (pulverized rock).

Question 2

Intrusive/Plutonic igneous rock (2)

Answer 2

• Magma crystallizes slowly within deep, large intrusions (thousands of yrs) = coarse-grained rocks.
• Minerals crystallize from magma at depth in large intrusions.

Question 3

Extrusive/Volcanic igneous rock (3)

Answer 3

• Lava/shallow magma cools and crystallizes quickly (min/hr/days) = fine-grained rocks.
• Minerals crystallize from magma near the surface in narrow, shallow intrusions, or from extruded lava.
• Explosive materials (ash/pumice) settling on the surface after an eruption.

Question 4

Aphanitic (3)

Answer 4

• Minerals too small to see.
• Fast cooling.
• Example: Rhyolite.

Question 5

Phaneritic (3)

Answer 5

• Minerals large enough to see with unaided eye.
• Slow cooling.
• Example: Granite.

Question 6

Glassy (3)

Answer 6

• Glassy, shiny, and conchoidal fractures. No obvious minerals.
• Generally forms from extremely cooling of the surface of a lava flow.
• Example: Obsidian.

Question 7

Pyroclastic (3)

Answer 7

• Fused, glassy volcanic rock fragments and ash from explosive volcanic eruption.
• Forms from explosive eruptions; mix of dull angular fragments and ash.
• Tuff

Question 8

Vesicular (3)

Answer 8

• Many holes or pits in rock surfaces caused by escaping gas. (Aphanitic with vesicles)
• Vesicles form as gas escapes from a lava flow.
• Example: vesicular basalt.

Question 9

Porphyrytic (3)

Answer 9

-Two distinct mineral sizes.
Forms in two stages:
-1st = slow cooling at depth (phenocrysts).
-2nd = faster cooling at or near surface (groudmass).
-Example: Porphyritic Basalt.

Question 10

7 Types of Igneous Intrusions (7)

Answer 10

1) Volcanic pipe
2) Dike
3) Stock
4) Batholith
5) Laccolith
6) Sill
7) Xenoliths

Question 11

Volcanic pipe (1)

Answer 11

-Subsurface geological structures created by the violent eruption of deep-origin volcanoes.

Question 12

Dike (1)

Answer 12

-Form when magma pushes up towards the surface through cracks in the rock, forming vertical structures of igneous rock.

Question 13

Stock (1)

Answer 13

-Large igneous intrusions less than 100km^2.

Question 14

Batholith (1)

Answer 14

-Large igneous intrusions > 100km^2.

Question 15

Laccolith (1)

Answer 15

-Igneous intrusion that has split apart two strata, resulting in a dome-like structure. (mushroom-shaped)

Question 16

Sill (1)

Answer 16

-Form when magma intrudes between the rock layers, forming a horizontal or gentlydipping sheet of igneous rock.

Question 17

Xenoliths (1)

Answer 17

-Not an intrusion but it’s a foreign rock embedded in magma while magma was cooling.

Question 18

Example of batholiths (3)

Answer 18

• Our coast mountains are uplifted and exposed batholiths.
• These are the roots of ancient volcanoes that were exposed over the past 100 million years as erosion stripped away kilometers of overlying rock.
• Example of exposed intrusions: Granodiorite exposed along the roadway to Cypress Bowl.

Question 19

Bowen’s Reaction Series (2)

Answer 19

• Minerals that crystallize from molten rock do so at different temperatures. (1200°C-750°C
• Color index gets darker and green as it approaches ultramafic rocks,

Question 20

Ultramafic composition (1)

Answer 20

-Dominated by olivine (FeMgSiO) and/or pyroxene.

Question 21

Mafic composition (1)

Answer 21

-Dominated by plagioclase and pyroxene. Small amounts of olivine can be present.

Question 22

Intermediate composition (1)

Answer 22

-Roughly even mixtures of felsic minerals (mainly plagioclase) and mafic minerals (mainly amphibole, pyroxene, and/or biotite). There is little or no quartz.

Question 23

Felsic composition (1)

Answer 23

-Mostly feldspar (especially K-feldspar) also muscovite and quartz.