1. Extrusive Igneous Rocks, part 1
BASALTS, DIABASES, ANDESITES, DACITES and RELATED ROCKS

2. Types of Igneous Rock Extrusive Intrusive Pyroclastic Lava Flow
Hypabyssal
Plutonic

3. Igneous Rock Classification
Igneous rocks may be classified on the basis of what minerals they contain
A distinction is made between essential minerals (those that must be present) and accessory minerals (often present, but not necessary)

4. Igneous Rock Classification
Example: Granite
Essential: Feldspar, Quartz
Accessory: Biotite, Hornblende
Accessory minerals may become part of the rock name: Hornblende andesite, because hornblende is not an essential mineral

5. IUGS Extrusive Igneous Rock Chart

6. Basalt
A general term for dark-colored mafic igneous rocks, commonly extrusive but locally intrusive (e.g. as dikes), composed chiefly of calcic plagioclase and clinopyroxene; the fine-grained equivalent of gabbro. Nepheline, olivine, orthopyroxene, and quartz may be present in the CIPW norm, but not all simultaneously: nepheline and olivine can occur together, as can olivine and orthopyroxene, and orthopyroxene and quartz, but nepheline does not coexist with orthopyroxene or quartz, nor quartz with nepheline or olivine.
The CIPW norm is named for the four petrologists, Cross, Iddings, Pirsson and Washington, who devised it in 1931
See Steven Dutch, for more information.

7. Basalt
Often in lava flows, shield volcanoes - associated with MOR eruptions to create oceanic crust (MORB)
May be hypabyssal in dikes or sills (compare with diabase - columnar basalt)

8. Basalt Occurrence and Mineralogy
Basalt is the most abundant volcanic rock; it underlies ocean floors and forms voluminous outpourings (flood basalts) on several continents; it is also a major constituent of island arcs
Principal mineral constituents: calcic plagioclase and pyroxene (augite) and/or olivine
Basalts are commonly porphyritic
Color dark gray to black (Fe content)

9. Basalt Vesicular basalt
Gas in magma is trapped by rapid cooling, leaving vesicles
Basalt photo from Cinema Expeditions Rock CD-ROM, \extrusiv\basalt\ _.jpg

10. Flood Basalts
Twickenham flows of the Colombia River Basalt Group, Washington State
Note columnar basalt near top of picture
Photo: M.L. Bevier
Photo #mlb9m.jpg

11. Lava flow
Basaltic lava flow
Location probably near Raton, New Mexico

12. Columnar basalts
Dark, fine grained volcanic rocks, chiefly basalt and andesite, are exposed at the northern edge of the Fraser Valley. These rocks formed as lavas, shallow intrusions, and volcanic ash deposits. Most volcanic rocks are resistant to erosion and form prominent hills in the Fraser Valley
Photo #van71m.jpg
Basalt with columnar jointing,
near Whistler, British Columbia

13. Pillow basalts Pillow basalts form during underwater eruptions
This basalt was found in Marin County, California
Photo pillow basalt.jpg

14. Pillow basalt
Pillow lava forms when eruptions are underwater or when lava flows enter a body of water
Abundance of pillows and pillow deltas indicates rivers and lakes were common features during the formation of the Columbia River Flood Basalt Province
Photo by Thor Thordarson
Photo CRB97-F4.jpg

15. Pillow deltas Pillows can be up to 5 m long and about 0.5 to 1 m thick
They are elongate in the direction of flow and dip degrees
Dips decrease higher up in the sequence
Photo by Thor Thordarson
Photo CRB97-F429.jpg

16. Scoriaceous Basalt Photomicrograph
The black oval features in this scoriaceous basalt are vesicles
Acicular, white plagioclase laths throughout
Euhedral, white olivine phenocryst at the lower right
University of North Carolina Virtual Geology project - UNC sample HAW7

17. Plagioclase
University of North Carolina Virtual Geology project - UNC sample W-51
This slide showcases one of plagioclase's very common features: its polysynthetic twinning.

18. Olivine Almost all of the grains in this rock are olivine
University of North Carolina Virtual Geology project - UNC Sample W-44
Almost all of the grains in this rock are olivine
Note the high order interference colors and the minor secondary calcite

19. Basalt Photomicrographs
Both photographs are under crossesd nicols
(Above) Gray and white plagioclase and brightly colored , tiny clinopyroxene grains are in a matrix of dark glass
Below, plagioclase laths and more birefringent orthopyroxene are the principal phenocrysts

20. Clinopyroxene photomicrographs
Clinopyroxene crystal in basalt: crossed nicols above, plane polarized below - field of view 1.5 mm.
Clinopyroxenes are abundant in many intrusive and extrusive igneous rocks, including diorite, gabbro, the peridotite family, basalt, and andesite - They also occur in metamorphosed mafic rocks, and jadeite is a constituent of some blueschist facies rocks

21. Exsolution in clinopyroxenes
The photos above show a clinopyroxene grain in crossed nicols (above) and plane polarized (below)
Exsolution lamellae of orthopyroxene are clearly visible in the upper photo

22. Orthopyroxene Photomicrograph
Enstatite, hypersthene, and ferrosilite are members of the orthopyroxene solid solution series
All have low birefringence (first order red maximum), parallel extinction, and pyroxene 87° cleavage
Pale green, pale red, or pale purple pleochroism occurs in some grains
Distinguished from clinopyroxene by low order interference colors and parallel extinction
The photo shows one cleavage and thin exsolution lamellae

23. Orthopyroxene Photomicrograph
Photo shows fine exsolution lamellae in orthopyroxene - field of view 1.5 mm, crossed nicols
Orthopyroxene occurs with clinopyroxene in many igneous rocks, such as gabbro, diorite, basalt, andesite, and the peridotite family
It is an indicator mineral for the granulite and pyroxene hornfels facies of metamorphism

24. Olivine Basalt Photomicrographs
Upper photo (crossed nichols) shows brightly colored, equant olivine and gray plagioclase phenocrysts in a fine-grained matrix of glass and clinopyroxene
Lower photo is the same in plane polarized light

25. Diabase
In the U.S., an intrusive rock whose main components are labradorite and pyroxene and which is characterized by ophitic texture
As originally applied by Brongniart in 1807, the term corresponded to what is now recognized as diorite
"The word has come to mean a pre-Tertiary basalt in Germany, a decomposed basalt in England, and a dike-rock with ophitic texture in the United States and Canada"

26. Ophitic texture
Said of the granular texture of an igneous rock (especially diabase) in which lath-shaped plagioclase crystals are partially or completely included in pyroxene crystals, typically augite

27. Diabase Hypabyssal - dikes or sills
Generally fine to medium grained, usually no glass
Mineralogy similar to basalt, but pyroxene may show zoning (especially in thin section)

28. Diabase Often found in dikes or sills
Hypabyssal - may contain phenocrysts
Chemically equivalent ot basalt
Diabase photo from Cinema Expeditions Rock CD-ROM, \intrusiv\diabase\ _.jpg

29. Diabase dike
Vertical Triassic diabase cross-cutting Jura-Triassic sedimentary strata
Note dike offset
Ramon Crater, Israel
Photo Samuel Root, College of Wooster
Photo #struc59m.jpg

30. Andesite
A dark-colored, fine-grained extrusive rock (sometimes hypabyssal)
Microcrystalline to glassy textures - often porphyritic (2 stage cooling history)
Color - brown to reddish, may be gray to greenish (not black)
Often associated with stratovolcanoes

31. Andesite Porphyry
Contains phenocrysts composed primarily of zoned sodic plagioclase (esp. andesine) and one or more of the mafic minerals (e.g. biotite, hornblende, pyroxene)
Groundmass composed generally of the same minerals as the phenocrysts, although the plagioclase may be more sodic and quartz is generally present
Extrusive equivalent of diorite

32. Andesite
Andesite grades into latite with increasing alkali feldspar content, and into dacite with more alkali feldspar and quartz
Named by Buch in 1826 from the Andes Mountains, South America

33. Andesite Mineralogy
Major minerals: plagioclase (andesine to oligoclase - more Na than basalt)
Mafic mineral hornblende, biotite, cpx (diopside or augite), or opx (hypersthene - Schiller luster)
The micas and amphiboles are hydrous, indicating a lower temperature of crystallization

34. Andesite Plagioclase phenocrysts, aphanitic groundmass
Andesine photo from Cinema Expeditions Rock CD-ROM, \extrusiv\andesite\ _.jpg

35. Andesite Photomicrographs
(Upper, CN; lower. PP)
Zoned and twinned plagioclase phenocrysts in a matrix of glass and plagioclase microclites
Plagioclase microlites are subparallel, producing trachytic texture

36. Dacite
A fine-grained extrusive rock with the same general composition as andesite but having less calcic plagioclase and more quartz
According to many, it is the extrusive equivalent of granodiorite
Syn: quartz andesite
The name, given by Stache in 1863, is from the ancient Roman province of Dacia (now part of Romania)

37. Dacite Mineralogy
Major minerals: similar to andesite, except that quartz occurs as an essential mineral, often as phenocrysts
Mafic minerals are minor, usually biotite, hornblende, or pyroxene

38. Dacite
Quartz crystals should be visible - this separates dacite from andesite in hand specimen - hard to see here
Dacite photo from Cinema Expeditions Rock CD-ROM, \intrusiv\basalt\ _.jpg
YES, intrusive! He doesn’t know his rock types

39. Dacitic to basaltic pumice
Pumice blocks from airfall deposits of the Bridge River Fm. associated with the 2350 B.P. eruption of Mount Meager
Photo shows one stage of mechanical mixing of basalt into dacite
Photo #ch53m.jpg
Location: British Columbia,
Canada

40. Dacite boulder, Mt. Lassen
Dacite boulder in volcanic mudflow of the Devastated Area mudflow, from the 1915 eruption of Lassen Peak
Angular joint surfaces are evidence that the boulder was emplaced very hot but cooled rapidly in place contracting to form the angular joint surfaces
Hammer for scale
Photograph by R. Forrest Hopson
Location: Mt. Lassen National
Park, California

41. Dacite Photomicrograph
Dacite is a quartz-rich extrusive (volcanic) rock that contains abundant sodic plagioclase
Dacite is a minor constituent of many arc volcanoes
Euhedral light gray plagioclase (the bottom crystal is beautifully zoned) and rounded quartz phenocrysts, in a very fine-grained matrix of the same minerals.

42. Pyroclastic eruptions
High silica magmas typically contain a dissolved aqueous phase
Near the surface this aqueous phase separates (exsolution) (forming a 2 stage system) with a large increase in volume
May create a bomb if pressure cannot escape - Subaerial eruption will result

43. Pyroclastic eruptions
Rock fragments (called pyroclasts or tephra) are produced. These may be either:
Lithic fragments: crystalline
Vitric fragments: glassy

44. Pyroclastic size classification
Fragment size Fragment name Rock name
>265 mm Block Breccia
mm Bomb Agglomerate
mm Lapillus Lapilli Tuff
< 2 mm Ash Tuff

45. Bombs and blocks
Blocks are often angular to subangular - they were solidified before the explosion and simply represent shrapnel
Bombs are rounded to sub-rounded - they were viscous liquids when erupted, and were rounded in flight