Understanding Basalt: An Extrusive Igneous Rock

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.

7. Basalt • Often in lava flows, shield volcanoes - associated with MOR eruptions to create oceaniccrust (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

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

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 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

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

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 20-30 degrees • Dips decrease higher up in the sequence

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

17. Plagioclase • 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 • Note the high order interference colors and the minor secondary calcite

19. Basalt Photomicrographs • Both photographs above 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

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

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

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 a 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

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 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. Fragment size Fragment name Rock name >265 mm Block Breccia 64 - 264 mm Bomb Agglomerate 2 - 64 mm Lapillus Lapilli Tuff < 2 mm Ash Tuff Pyroclastic size classification

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