1. RHYOLITES, LATITE, TRACHYTE AND GLASSY EXTRUSIVE ROCKS
Extrusive Igneous Rocks, Part 2
RHYOLITES, LATITE, TRACHYTE AND GLASSY EXTRUSIVE ROCKS

2. IUGS Extrusive Igneous Rock Chart

3. Silica Content Classification

4. Naming aphanitic rocks
In naming aphanetic extrusive rocks, the name is often based only on a few phenocrysts. This is why a mafic name prefix is often used, for example hypersthene andesite. A better name is hypersthene phenoandesite.
Since the felsic minerals are invisible, the rock could actually be a hypersthene dacite, or hypersthene rhyodacite.

5. Naming aphanitic rocks, cont.
In formal writing, it is often wise to indicate the origin of the name you assign. Examples:
1. The field name of this rock is hypersthene andesite.
2. Based on a careful petrographic examination in hand specimen and thin-section, this rock is a hypersthene dacite.
3. It is understood that field names, particularly of aphanetic extrusives, may be in error. Names based on thin section examination should be accurate.

6. Silica-rich magma
When a magma is rich in silica, it is more viscous than if it is poor in silica
Within the liquid, the individual SiO4 tetrahedra may link to form chains
As these chains lengthen, they twist around each other, and make it more difficult for the magma to flow
Such magmas often crystallize at depth
Occasionally, silica-rich magmas do reach the surface and may erupt explosively
Silica-rich magmas also crystallize at lower temperatures than mafic magmas
Granitic/rhyolitic magmas typically exist to as low as 800°C, sometimes lower

7. Wet magmas In addition, high silica magmas are generally “wet”
At depth, they contain a dissolved aqueous phase
If they get close to the surface, this phase may separate, with a concurrent large increase in volume
This can lead to explosive eruptions

8. Tephra A general term for all pyroclastics of a volcano
A pyroclast is an individual particle ejected during a volcanic eruption, and is usually classified according to size

9. Volcanic ash and tuff
Ash = Fine pyroclastic material (under 2.0mm diameter; under 0.063mm diameter for fine ash)
Term refers to the unconsolidated material
Tuff is used for its consolidated counterpart

10. Tuff
Tuff is a general term for all consolidated pyroclastic rocks, especially those who fragments are less than 2 mm across
The IUGS definition is a pyroclastic rock whose average pyroclast size is less than 2 mm.
Course tuff is 1/16 mm to 2 mm
Fine (or dust) tuff is less than 1/16 mm

11. Tuff texture
Tuffs are explosively erupted volcanic material that is consolidated and lithified after deposition
Tuffs may contain lithic fragments, glass shards, and/or broken mineral grains and have pyroclastic texture

12. Tuff photomicrographs
The photos at left (CN above, PP below) show lithic crystal tuffs containing twinned, broken plagioclase clasts, and altered lithic clasts (right side of photos), in a matrix of very fine-grained material

13. Welded Tuff Extrusive igneous
Siliceous tuff indurated by the welding together of glass particles under the combined action of heat retained by the particles, the weight of overlying materials, and hot gases
Welded tuffs are often banded or streaked

14. Welded tuff photomicrographs
The pictures at left show above a CN view showing extinct glass matrix, lithic clasts, and broken plagioclase grains and (below) a PP view of a highly welded tuff with numerous glass shards and broken plagioclase grains

15. Welded Tuff Viewing
Welded tuffs are formed during violent volcanic eruptions; gas-charged material is expelled and deposited while still partially molten, so that the droplets become attached, or "welded" to one another
Typically, the weight of overlying tuff flattens pumice fragments and produces aligned grains and lithic fragments
Tuffaceous textures are commonly best viewed in plane polarized light, particularly if the matrix is glassy, so that grain outlines can be seen

16. Welded Tuff Photomicrogaphs
The photos at left show welded tuffs with irregularly shaped, light gray glass shards and mashed pumice clasts in a glassy matrix

17. Rhyolite
A group of extrusive igneous rocks, typically porphyritic and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass; also, any rock in that group; the extrusive equivalent of granite
Rhyolite grades into rhyodacite with decreasing alkali feldspar content and into trachyte with a decrease in quartz
Term coined in 1860 by Baron von Richthofen (grandfather of the World War I aviator). Etymol: Greek rhyo-, from rhyax, "stream of lava"

18. Rhyolite, cont. Extrusive igneous , sometimes hypabyssal
Aphanitic, light-colored rock
Color ranges from white or yellow to pink or reddish
Often porphyritic, usually containing phenocrysts of alkali feldspar (sanidine, anorthoclase, or orthoclase) - other phenocrysts include quartz, biotite, rare hornblende or pyroxene
Groundmass may be almost completely glassy; although in older rhyolites devitrification may have occurred, resulting in a fine-grained mixture of quartz, feldspar, and other minerals

19. Rhyolite, cont.
The alkali feldspars phenocrysts formed at high temperature, followed by quick chilling - resulting feldspars are often metastable high temperature forms such as sanidine or anorthoclase.
Smoky quartz, resembling a mafic, may be present

20. Rhyolite Apanitic rhyolite “Mafic” grains are probably smoky quartz
Diabase photo from Cinema Expeditions Rock CD-ROM, \extrusiv\rhyolite\ _.jpg

21. Rhyolite Photomicrograph
The light gray phenocrysts at the top of the photo are sanidine; quartz grain near the bottom edge of the photo
Flow banding is clearly visible in the glassy matrix
Rhyolites are a volumetrically minor component of some volcanic arcs and occur as vast ash flows in some areas affected by tensional tectonics

22. Rhyolite Photomicrograph
Rhyolite porphyry - photomicrograph showing euhedral quartz and albite-twinned plagioclase phenocrysts with a glassy groundmass (Cross nicols; Field of view=5mm)
Photo by S. McKinley
Location : Seneca property, Harrison Mills, SW British Columbia
Photo #sdm1m.jpg

23. Rhyolite Domes Obsidian domes, Long Valley Caldera, California
Photo #mlb5m.jpg
Obsidian domes, Long Valley Caldera, California
Photo M.L. Bevier

24. Rhyodacite Extrusive igneous
Older term for rhyolites whose feldspar content is one-third to two-thirds plagioclase

25. Latite
A porphyritic extrusive rock having phenocrysts of plagioclase and potassium feldspar (probably mostly sanidine) in nearly equal amounts, little or no quartz, and a finely crystalline to glassy groundmass, which may contain obscure potassium feldspar; the extrusive equivalent of monzonite
Named by Ransome in 1898, is derived from Latium, Italy.

26. Latite
Latite grades into trachyte with an increase in the alkali feldspar content, and into andesite or basalt, depending on the presence of sodic or calcic plagioclase, as the alkali feldspar content decreases
It is usually considered synonymous with trachyandesite and trachybasalt, depending on the color

27. Latite texture and mineralogy
Textures may be glassy, felsitic, porphyritic, or vitrophyric
The mineralogy consists principally of sub-equal amounts of orthoclase and plagioclase
IUGS classification is P/(A+P), 0-10 Q
Mafics include hornblende, biotite, or augite
Similar to trachytes but the feldspar phenocrysts range from alkali feldspar to intermediate plagioclase

28. Quartz latite Extrusive igneous
Similar to latite except that quartz is an essential mineral (>10% Q)
Usually porphyritic

29. Mammoth Mountain, California
Composite volcano (11,050 feet) is composed of about 12 rhyodacite and quartz latite domes extruded along the southwest rim of Long Valley caldera from 200 to 50 thousand years ago
Mammoth Mountain is one of the eruptive centers that developed late in the evolutionary cycle of the Long Valley caldera complex
Located in the eastern Sierra Nevada about 250 miles north of Los Angles - Photo by R. Forrest Hopson

30. Trachyte
A group of fine-grained, generally porphyritic, extrusive rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase; also, any member of that group
Extrusive equivalent of syenite

31. Trachyte, cont.
Trachyte grades into latite as the alkali feldspar content decreases, and into rhyolite with an increase in quartz
Etymology: Greek trachys, ''rough'', in reference to the fact that rocks of this group are commonly rough to the touch
There are two types of trachyte, alkali and calc-alkali
Calc-alkali trachyte is more common and is sometimes referred to as normal trachyte

32. Trachyte Mineralogy
The mineralogy consists of perthitic orthoclase or microcline, oligoclase (or, rarely, andesine), diopside or diopsidic augite, and accessory quartz
Alkali feldspar phenocrysts being sanidine, anorthoclase, or soda-rich plagioclase such as oligoclase or albite
Feldspathoid-bearing trachytes contain accessory feldspathoids

33. Alkali Trachyte Mineralogy
Alkali trachyte is rich in sodium - alkali feldspars are strongly perthitic or anorthoclase, and plagioclase is albite or albite-oligoclase
Mafics are iron-rich, ranging from Fe-rich biotite, amphiboles including hastingsite, arfvedsonite, or riebeckite, and pyroxenes including aegirine-augite and aegirine

34. Trachyte Lava Flow, Hawaii
Puu Waawaa is on the north rift of Hualalai volcano - Puu Waawaa cone is the single largest volume (5.5 km3) eruption on the Island of Hawaii
Trachyte lava flow is also associated with the cone (left margin of photo).
Flow is as thick as 900 feet (275 m) and highway 190 climbs its edge
Cone and flow are about 105,000 years old
Photograph courtesy of the U.S. Geological Survey

35. Glassy Extrusive Rocks
If magma is quickly chilled, it will not have time to crystallize - glass will form
Glass is less stable than crystalline material of the same composition
Over time, devitrification may occur - conversion of glass to fine crystals

36. Obsidian Extrusive igneous
Volcanic glass with the composition of rhyolite that breaks with a fine conchoidal fracture and is black, gray, dark brown, red, or (rarely) green
Often flow banded
Nearly free of water, in contrast to pitchstone.
Name from the Latin obsidianus, a rock discovered in Ethiopia by Obsidus, ca. 280 B.C.

37. Obsidian Photo
When obsidian breaks it fractures with a distinct conchoidal fracture
Ancient people throughout the world used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds
Today obsidian is used as a scalpel by doctors in very sensitive eye operations
Text from
Photo from

38. Pearlite Extrusive igneous - Also spelled perlite
Volcanic glass having the composition of rhyolite, and a higher water content (to 4%) than obsidian
Colorless, gray, blue, green, red, or brown
Often shows numerous concentric cracks, with onionlike partings, which impart a pearly luster
May have been derived from obsidian through hydration by meteoritic water

39. Pitchstone Extrusive igneous - exhibits a pitchlike luster
Volcanic glass chemically similar to rhyolite except for higher water content, 4-10%.
Color may be black, gray, olive green, brown, or red
Fracture is conchoidal

40. Pumice Extrusive igneous
A frothy, light-colored rock with a composition near rhyolite
It may form as crusts on more compact lava, or may occur in volcanic ejectamenta
Glassy material is filled with so many small air bubbles that pore space may be much greater than the glass volume
Rock has low specific gravity, often floating on water
Name from the Latin, pumicis, perhaps from spuma, foam

41. Pumice Photo
Pumice is a glassy, vesicular rock that is very light, often light enough to float
Upper photo from
Lower photo from
Image Source: U.S. National Park Service

42. Pumice Photomicrograph
Pumice fragments from an air fall tephra layer intercalated with Miocene Chilcotin Group basalt flows
Near Deadman River B.C.
Photo M.L. Bevier
Photo # mlb92m.jpg

43. Vitrophyre Extrusive igneous - Volcanic glass containing phenocrysts
Composition is rhyolitic
Phenocrysts include quartz, sanidine, minor plagioclase (usually clear), and lesser amounts of augite, hornblende, or biotite
The most common variety is pitchstone vitrophyre
Obsidian, perlite, or pumice vitrophyres are also possible
Name from the Latin, vitrum, glass and porphyry