3. Geol 2312 Igneous and Metamorphic Petrology
Lecture 3
Volcanic Rocks:
Lavas, Landforms, and Products
Jan. 26, 2009

4. Lecture Outline Properties of Lavas and Styles of Eruption
Physiochemical Properties
chemistry temperature volatile content viscosity
Factors leading to Explosive Eruptions
Volcanic Landforms
Central Vent Landforms
shield volcanoes stratovolcanoes volcanic complexes scoria cones maar tuff rings tuff cones domes calderas
Fissure Eruption Landforms
flood (or plateau) lavas submarine lavas dike swarms
Lava Flow Features
Flow Top Morphology
AA, pahoehoe, toe lobes
Flow Interior Structures
pillows, columnar joints, flow banding
Pyroclastic Deposits
Fall Deposits
Flow Deposits
Surge Deposits

5. Physiochemical Properties of Lavas
General Magma Type:
Ultramafic
Mafic
Intermediate
Felsic
Temperature:
1550º to 1200º
1250º to 1050º
1150º to 950º
1050º to 800º
Viscosity:
Low
High
Gas Content:
Very Low (<<1%)
Low (<1%)
Intermediate (1-3%)
High (2-5%)
Typical Composition (wt. %)
SiO2
46.5
50.0
57.7
70.5
TiO2
0.3
1.9
1.0
Al2O3
3.1
15.9
16.6
14.1
FeO
11.2
10.3
7.2
2.8
MnO
0.2
0.1
< 0.1
MgO
32.9
7.0
3.7
0.7
CaO
4.8
9.7
6.5
1.7
Na2O .1
2.9
3.4
3.6
K2O
.01
1.1
1.8
3.9
P2O5
n.a.
Total
99.0
99.3
98.3
97.8
Trace Elements (ppm) Cr
3000
200 10 2 Ni
1000
150 15 Ba 20 40
300
350 Zr 35
170

6. Controls on Viscosity (resistance to flow)
Viscosity increases with:
SiO2 concentration
decreasing temperature
increasing crystallinity of magma
decreasing volatile content (H2O, CO2, SO2, H2, HCl, Cl2, F2)

7. Effusive Eruptions Mafic magma Relatively low gas content (<1%)
Fountaining followed by flow as gas content diminishes
Creates vesicular to massive lava flows
Photos from USGS

8. Explosive Eruptions
Driven by degassing of magma as it rises up the neck of the volcanic vent
The dramatic increase of volume resulting from degassing causes the magma to be violently thrust out the neck and shattered into fine fragments – ash
Creates pyroclastic deposits
Eruption Model
Water solubility (carrying capacity) in rhyolite as function of pressure; from Yamashita (1999)

9. Central Vent Volcanic Landforms STRATOVOLCANOES
Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows, are called stratovolcanoes. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions.
Mt St. Helens (pre-1980 eruption)

10. Central Vent Volcanic Landforms SHIELD VOLCANOES
Built almost entirely of fluid mafic lava flows. Flow after flow effusively pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape.

11. Central Vent Volcanic Landforms SMALL VOLCANOES
Steep-sided cone formed by accumulation of ash, lapilli, bombs and blocks around a central vent resulting from a low volume, weak explosive eruption; also know as cinder cones
Broad accumulation dome with a large central crater resulting from eruption through water-saturated ground; steam (phreatic) explosions are common
Similar to a maar, but lacking central crater; encounter water at a shallower depth than maar and thus is not a phreatic
Steeper-sided and smaller accumulation of volcanic debris than tuff ring; similar shape to scoria cone, but layers dip inward near neck

12. Central Vent Volcanic Landforms SCALES

13. Central Vent Volcanic Landforms CALDERAS and DOMES
Lava dome structure (from Winter, Fig. 4-7)
Lava dome building in Mt. St. Helens crater

14. Fissure Eruption Landforms
Laki fissure, Iceland – Erupted 1783 creating the largest lava flow in human history

15. Fissure Eruption Landforms PLATEAU (FLOOD) BASALTS

16. Fissure Eruption Landforms DIKE SWARMS
Feeder conduits to eroded plateau basalts

17. Lava Flow Features

18. BASALTIC LAVA FLOW CONTACT
Lava Flow Features
BASALTIC LAVA FLOW CONTACT
Massive
Basalt
Amygdaloidal
Basalt

19. BASALTIC LAVA FLOW SURFACES
Lava Flow Features
BASALTIC LAVA FLOW SURFACES AA AA
Pahoehoe
HAWAII
Pahoehoe
NORTH SHORE

20. Lava Flow Features TOE LOBES Modern-day Hawaii
1.1 Ga North Shore Volcanics

21. PILLOW STRUCTURES (SUBMARINE ERUPTIONS)
Lava Flow Features
PILLOW STRUCTURES (SUBMARINE ERUPTIONS)

22. Lava Flow Features COLUMNAR JOINTING Shovel Point, MN
CRB, OR (Winter, 2001)
Gooseberry Falls, MN

23. Lava Flow Features MISCELLANEOUS Convoluted Flow Banding in Rhyolite
Amygdule Cylinders in Basalt
Plagioclase Porphyritic Texture in Basalt
Coarse Ophitic Texture in Basalt

24. Pyroclastic Deposits PRODUCTS OF EXPLOSIVE ERUPTIONS
TUFF
Winter (2001) Fig. 2-5

25. Pyroclastic Deposits FALL DEPOSITS
Mt. St. Helens
Figure Approximate aerial extent and thickness of Mt. Mazama (Crater Lake) ash fall, erupted 6950 years ago. After Young (1990), Unpubl. Ph. D. thesis, University of Lancaster. UK.
Figure Maximum aerial extent of the Bishop ash fall deposit erupted at Long Valley 700,000 years ago. After Miller et al. (1982) USGS Open-File Report

26. Pyroclastic Deposits FLOW DEPOSITS
Figure Types of pyroclastic flow deposits. After MacDonald (1972), Volcanoes. Prentice-Hall, Inc., Fisher and Schminke (1984), Pyroclastic Rocks. Springer-Verlag. Berlin. a. collapse of a vertical explosive or plinian column that falls back to earth, and continues to travel along the ground surface. b. Lateral blast, such as occurred at Mt. St. Helens in c. “Boiling-over” of a highly gas-charged magma from a vent. d. Gravitational collapse of a hot dome (Fig. 4-18d).

27. Pyroclastic Deposits COMPLETE ERUPTIVE PACKAGE - IGNIMBRITE
Figure Section through a typical ignimbrite, showing basal surge deposit, middle flow, and upper ash fall cover. Tan blocks represent pumice, and purple represents denser lithic fragments. After Sparks et al. (1973) Geology, 1, Geol. Soc. America
Graded Tuff- Episodic Eruptions/Surges
Tuff +
Heating + Pressure  Welded Tuff