Viscosity

1. Magma (or lava if erupted to the surface) is composed of liquid, solid (mineral crystals) and gas. Its composition is largely controlled by its source. The image shown above is a pahoehoe basalt flow.

2. Magmas (lavas) are subdivided largely bysilica content. As silica (SiO2) content increases iron and magnesium content (FeO and MgO) decreases. Note that lighter elements, such as sodium (Na2O) and potassium (K2O) content follow the silica trends. Elemental composition of magmas and rocks are described in terms of oxide composition because of their common bonds with oxygen.

3. Viscosity The viscosity(resistance to flow) of a magma is controlled by its silica contentand its temperature. Why do you think viscosity units are graphed using a logarithmic scale (each unit change is a 10-fold increase)?

4. The viscosity(resistance to flow) of a magma is controlled by its silica content and its temperature. High silica content and low temperature magmas tend to have higher viscosities. The image above is an aa basaltic lava flow that has cooled and degassed.

5. Basaltic composition magma forms at four different tectonic settings. Basaltic magma is always derived from a partial melt of the asthenosphere.

6. Basaltic magma forms from a partial melt of the asthenosphere. Partial melting of the asthenosphere occurs at a depth (100-350 km) where the geothermal gradient intersects the melting temperature curve for upper mantle rock (garnet peridotite). It is important to note that the geothermal gradient is dependent upon pressure (depth), while the melting temperature curve is dependent upon pressure (depth) and composition of the substance. Basaltic magma is a dry melt (little dissolved water) and its melting temperature decreases with decreasing pressure (as the magma rises). As basaltic magma further melts it density decreases causing it to continue to rise until it reaches the surface. Note that the temperature of the rising magma is ~200° C higher than its melting temperature at the surface (more liquid). km

7. Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity (remember viscosity related to high SiO2 content and low temperature).

8. Basaltic composition magma (lava) has a relatively low viscosity and will flow great distances from its vent. It is dark colored because of its mafic mineral content(largely pyroxene and Ca-rich plagioclase).

9. Aa Flow (Think about what you would say if you had to walk on this aa flow (ah, ah). Pahoehoe Flow (Smooth word, smooth flow). Pahoehoe (ropey textured) basalt flows have a lower viscosity than aa(blocky textured) flows, which have degassed and cooled.

10. Granitic magma forms from a partial melt of continental crust, which contains dissolved water. Dissolved water content in a magma reduces its melting temperature with increasing pressure (water molecules will inhibit the silicate tetrahedra from forming bonds). Note that the melting temperature curve for a wet granitic melt increases with decreasing pressure (opposite of basaltic dry melt). Melting occurs at a depth of 35-45 km within continental crust. As granitic magma rises it solidifies (point X) as its melting temperature increases while the geothermal gradient (actual temperature) decreases. Granitic composition magmas rarely reach the surface as volcanic rhyolite flows because of the high water content and corresponding increase in melting temperature as it rises towards the surface. km

11. Granitic composition magma is produced at continental collision margins. As the continental crust thickens it begins to partially melt at depth. Igneous intrusions (plutons) form below the mountain belts. Volcanism is rare in continental collision boundaries. As collisional tectonic mountain ranges are uplifted the overlying marine sedimentary and metamorphic rocks are eroded exposing the underlying granitic plutons. The granitic rocks of New Hampshire and Vermont represent old granitic plutons that were intruded when the Appalachian Mountains formed 300 million years ago as North American continent colided with proto-European continent.

12. Granitic rock excavated from a quarry in Barre, Vermont formed as plutons beneath the Appalachian Mountains when North Africa collided with eastern North America 300 million years ago.

13. Granitic composition magma reaches to the surface in Yellowstone Park because the continental crust is being heated closer to the surface (5-10 km) by upwelling basaltic magma generated from a asthenosphere hotspot.

14. The Yellowstone Caldera (Wyoming) formed following a very large eruption ~600,000 years ago. The rhyolite flows are very viscous and internal gas pressures can be very high. Why is explosivity of a volcanic eruption related to viscosity and gas content of the magma?

15. Migration of Yellowstone hotspot volcanism over the past 16 million years.

16. Intermediate composition magma can crystallize below the surface beneath subduction zones and create large plutonic bodies composed of coarse-grained igneous rock. Compositons can range from granite to diorite. El Capitan shown on the left is part of the Sierra Nevada intrusive complex that formed over 90 million years ago when a subduction zone existed along the margin of California. The plutonic bodies comprising the Sierra Nevada are similar in origin to the plutonic bodies forming under the modern Cascades. Grano-diorite rock from the Sierra Nevada

17. Andesitic magma is produced from a partial melt of oceanic crust along subduction zones. Introduction of water forced out of the subducting plate lowers the melting temperature of the upper mantle, which rises and partially melts the overlying crust. In an ocean-continental convergent margin it may mix with partially melted continental crust, increasing the magma’s silica content (becomes more felsic). Mount St. Helens dacites are more silica-rich than Mt. Rainier andesite, likely due to continental source.

18. Mt. St. Helens is composed of intermediate composition dacitic flows. Dacite is slightly more felsic (has greater silica content) than andesite, but more mafic (higher Fe and Mg content) than rhyolite.

19. Continuous reaction series occur when Ca atoms continually exchange with Na atoms when the melt and solid phases are not separated You can have any proportionality of Na/Ca plagioclase (feldspar) minerals. The composition of the plagioclase mineral in a crystallizing magma changed continuously (even though the crystal structure remains unchanged. The zoned feldspar crystal (shown on left) is composed of Ca-rich feldspar in the center and Na-rich towards the outer rim. Zones in between are intermediate compostion (50/50 Ca:Na).

20. early formed olivine crystals (shown as brightly colored crystals under polarized light) later formed pyroxene crystals (shown as gray color under polarized light). Discontinuous reaction series occur when early formed crystals form entirely new minerals through reaction with the melt. For example, olivine will crystallize first (highest melting/freezing temperature) leaving a magma that is slightly more silica-rich and possessing a greater proportion of other elements (Ca) that were not present in the olivine structure. Olivine will react with the magma to produce and entirely new mineral pyroxene. The discontinuous reaction series will occur as long as the melt and crystal phases can react.

21. Differentiation of magma can occur from fractional crystallization of magma, where the solid phase is separated from the melt phase. The solid phase will have a composition that is relatively more mafic than the residual more silica-rich melt phase. The reverse reaction process occurs when rock is subjected to partial melting, where the melt phase is separated from the residual solid phase.

22. Garnet-peridotite upper mantle (asthenosphter). Basalt/gabbro composition magma Andesite/diorite composition magma Granite/rhyolite composition magma As earlier formed minerals are removed from the magma by fractional crystallization, a greater proportion of the denser elements (i.e., FeO & MgO) are removed leaving a residual melt that is more enriched in SiO2 and lighter elements. Minerals and rocks that form later will have a greater proportion of lighter elements (i.e., SiO2, Na2O and K2O).

23. Continuous Reaction Series Discontinuous Reaction Series Because minerals crystallize at specific temperatures certain minerals will be compatible and form together in igneous rocks(e.g., olivine, pyroxene, and Ca-rich plagioclase). The crystallization temperature is highest for olivine and becomes progressively lower until quartz forms last from the residual SiO2 melt.

24. A version of this diagram is always on my midterm! You should understand how to use it to determine igneous rock classification and mineral content Igneous rocks are classified based on texture and composition. Fine-grained (aphanitic) and porphyritic igneous rocks form at the surface of the earth in volcanic settings. Coarse-grained (phaneritic) igneous rocks form underground in intrusive complexes. To determine mineral composition of a given igneous, project a line vertically downward and read the percentage numbers along the Y-axis of the chart. For example, rhyolite/granite (beneath the white dashed lines) is composed of 20% quartz, 20% potassium feldspar, 40% Na-plagioclase, <10% biotite and <15% amphibole.

25. Gabbro Diorite Granite Coarse grained igneous rocks crystallize slowly underground. Composition of the rock depends upon the source of the magma and its cooling history.

26. Grano-diorite is intermediate composition between granite and diorite. Note the quartz, plagioclase and biotite crystals.

27. A pink granite is dominated by potassium feldspar (pink crystals), quartz (gray glassy appearance), plagioclase (porcelain white mineral) and biotite (black sheets).

28. Basalt is a fine-grained igneous rock that is erupted along diverse tectonic plate settings. Its black color and hardness is distinct. Do not try this unless you are trying to collect on your life insurance policy or a big proponent of Darwin’s natural selection theory!

29. A close-up image of basalt. The Ca-rich plagioclase crystals appear gray. Small olivine phenocrysts appear green. Which minerals formed first?

30. Andesite is a porphyritic rock that forms at subduction zones. The large phenocrysts are plagioclase and the small phenocrysts are amphibole. The gray ground mass is composed of biotite, potassium feldspar and plagioclase. Which mineral scrystallized first, which crystallized last?

31. Rhyolite forms from very viscous silica-rich lava. It is the fine-grained equivalent of granite. Eruptions are typically very explosive because of the high silica content coupled with high gas content.

32. Obsidianforms from the residual melt of a fractionated felsic magma body and is composed almost exclusively of silica. It is an amorphous glass in that it does not have a crystalline structure.The dark coloration is due to the presence of small amounts of magnetite. Note the characteristic conchoidal (rounded) fracture diagnostic of silica.

33. Porous Textured Igneous Rocks Pumice (felsic composition) or scoria (intermediate or mafic composition) form when gas bubbles are trapped in rapidly cooling pyroclasts (air fall). Gas bubbles can also be trapped in solidifying lava flows such as this vesicular basalt shown on the left. Note the presence of green olivine phenocrysts.

34. A large pyroclastic eruption of Mount Pinatubo in the Philippines (1992). The ash and other volcanic derived clasts can become welded together to form fine-grained tuff or coarse-grained volcanic breccia.

35. Volcanic ash (tephra) derived from the Mount Mazama (Crater Lake, Oregon) eruption 6800 years ago.

36. Volcanic tuff is comprised of welded ash and fine-grained volcanic lithic and pumaceous fragments. This photo was taken of the Bishop Ash in eastern California.

37. Volcanic breccia forms from a welded, mixture of large, angular volcanic clasts within a matrix of fine ash. This photo was takenon Lipari Island, Italy by Raymond Coveney.