Volcanoes and Igneous Activity Earth Chapter 4

1
Reminder Exam this Friday Please arrive promptly
and remember to bring along a pencil (lapicero).
Multiple -choice questions on Earths Internal
Structure (page 19 - 21) Chapter 2 - Plate
Tectonics Chapter 3 - Minerals Chapter 4 -
Igneous Rocks Chapter 5 - Volcanoes
2
Mantle rock-peridotite
Primary magma basaltic
Fig 4.7
NOTE Crystallization is reverse process of
melting
3
Fig 4.23
Fractional Crystallization
4
Mineral Formulae to Remember
Rock-forming silicate minerals
Quartz - SiO2 Feldspar - CaNa silicate or K
silicate Olivine - (MgFe)2SiO4 Muscovite Mica -
KAl silicate Pyroxene - MgFe silicate Amphibole -
Ca Mg Fe silicate
Other minerals
Galena - PbS Magnetite - Fe3O4 and
Calcite - CaCO3
5
Chapter 5 Volcanoes
6
The nature of volcanic eruptions

• Characteristics of a magma determine the
  violence or explosiveness of a volcanic
  eruption
• Composition
• Temperature
• Dissolved gases
• The above three factors actually control the
  viscosity of a given magma

7
The nature of volcanic eruptions

• Viscosity is a measure of a materials resistance
  to flow
• Factors affecting viscosity
• Temperature - Hotter magmas are less viscous
• Composition - Silica (SiO2) content
• Higher silica content higher viscosity
• (e.g., felsic lava such as rhyolite)
• Lower silica content lower viscosity
• (e.g., mafic lava such as basalt)

8
The nature of volcanic eruptions

• Dissolved gases
• Gas content affects magma mobility
• Gases expand within a magma as it nears the
  Earths surface due to decreasing pressure
• The violence of an eruption is related to how
  easily gases escape from magma and the rate of
  ascent from magma chamber to the surface
• In summary
• Basaltic lavas (low viscosity 100 Pa s) mild
  eruptions
• Rhyolitic or andesitic lavas (high viscosity 1012
  Pa s) explosive eruptions
• Compare with water 10-3 Pa s

9
Materials extruded from a volcano

• Lava flows
• Basaltic lavas exhibit fluid behavior
• Types of basaltic flows
• Pahoehoe lava (resembles a twisted or ropey
  texture)
• Aa lava (rough, jagged blocky texture)
• Dissolved gases
• 1 - 6 by weight
• Mainly H2O and CO2

10
A pahoehoe lava flow (rel. gas-rich)
Figure 5.5 A
11
Aa lava flow -slow moving gas-poor
Figure 5.5 B
12
Fig 5.6 Lava tube
13
Materials ejected from a volcano

• Pyroclastic materials fire fragments
• Types of pyroclastic debris
• Ash and dust (lt 1mm dia.) - fine lava fragments
• Pumice - porous rock from frothy lava
• Cinders - pea-sized material
• Lapilli - walnut-sized material
• Particles larger than lapilli
• Blocks - hardened or cooled lava
• Bombs - ejected as hot lava

14
A volcanic bomb
Bomb is approximately 10 cm long
Figure 5.7
15
Volcanoes

• General features
• Opening at the summit of a volcano
• Crater - summit depression lt 1 km diameter
• Caldera - summit depression gt 1 km diameter
  produced by collapse following a massive eruption
• Vent surface opening connected to the magma
  chamber
• Fumarole emit only gases and smoke

16
Volcanoes

• Types of volcanoes
• Shield volcano
• Broad, slightly domed-shaped, slopes of 5
• Generally cover large areas
• Produced by mild eruptions of large volumes of
  basaltic lava over long periods
• Example Mauna Loa on Hawaii, largest mountain
  on Earth measured from sea-floor

17
Anatomy of a shield volcano
Figure 5.10
18
Fig 5.11 View of Mauna Loa shield volcano
19
Mount Cameroon, summer 2004
20
Volcanoes

• Cinder cone
• Built from ejected lava (mainly cinder-sized)
  fragments of gas-rich basaltic magma
• Steep slope angle ie 34º
• Small size volume
• Frequently occur in groups
• over 100 on Mount Cameroon

21
Cinder cone volcano
Fig 5.14
older cinder cone
2000 lava flow
22
Volcanoes

• Composite cone (or stratovolcano)
• Most are located adjacent to the Pacific Ocean
  (e.g. MtFujiyama, Mt. St. Helens)
• Large, classic cone-shaped volcano (1000s of ft.
  high and several miles wide at base)
• Composed of interbedded lava flows and
  pyroclastic debris - andesitic silicic
  composition
• Assortment of small large explosions quiet
  effusion of lava flows type (e.g., Mt. Vesuvius)

23
Anatomy of a composite volcano
Fig 5.9
24
Mt. St. Helens prior to the 1980 eruption
25
Mt. St. Helens after the May 18, 1980 eruption
(4km3 ejected in few hours)
26
LATERAL BLAST (or pressure wave) from sudden
collapse downed trees in 400 sq km area. Blast
wave travelled gt 300 mph.
27
Profiles of volcanic landforms
Figure 5.12
28
Volcanic Phenomena

• Nuée ardente A deadly pyroclastic flow
• Fiery pyroclastic flow made of hot gases infused
  with ash and other debris
• Also known as glowing avalanches
• Move down the slopes of a volcano at speeds up to
  200 km per hour
• Lahar volcanic mudflow
• Mixture of volcanic debris and water
• Move down stream valleys and volcanic slopes,
  often with destructive results

29
A pyroclastic flow on Mt. St. HelensThese flows
are composed of basal avalanches of large hot
blocks fine-grained ash-surge clouds that
generate a large ash-plume.
Fig 5.20
30
Other volcanic landforms

• Caldera
• Steep-walled depressions at the summit
• Generally gt 1 km in diameter
• Produced by collapse of magma chamber roof
  following major explosive eruption

31
Formation of Crater Lake, Oregon
Figure 5.22
32
Fig 5.24 Aerial view of Crater Lake, Oregon
33
Pyroclastic flow deposits - ignimbritesSilicic
and intermediate magmasConsists of ash, pumice,
ignimbrite rock and other debrisMaterial ejected
at high velocities deposited rapidly to form a
thick, sheet-like deposit blanketing the
landscape.
Top of Ignimbrite deposit
Ignimbrite landscape in N. Chile
34
No caldera-forming eruption has been witnessed
historically BUT in 1912 a large eruption of Mt
Katmai, Alaska blanketed the landscape with
ignimbrite - v. remote area (now known as The
Valley of Ten Thousand Smokes). In continental
U.S.A. there are 3 calderas which potentially
could produce an enormous eruption 1/ Long
Valley caldera, eastern California 2/ Valles
caldera, New Mexico 3/ Yellowstone caldera,
Wyoming - MOST LIKELY TO ERUPT
Why is there volcanism in Wyoming?
35
Other volcanic landforms

• Fissure eruptions and lava plateaus
• Fluid basaltic lava extruded from crustal
  fractures called fissures
• Historical eruption of Laki Fissure, Iceland when
  14 km3 of basalt lava along 25 km long fissure,
  bad environmental effects. 1783 was known as
  Year Without a Summer in Europe extensive dry
  fog due to volcanic aerosol

36

Fig 5.39
Global distribution of Flood Basalts (black)
Hot-Spots (red)
37
Fig 5.37
Less voluminous basalt outpouring from plume tail
- linear volcanic chain
Initial vast outpouring of basalt lava-flood
basalts
38
Volcanology of Lesser Antilles Arc
11 main islands
Island of Saba-northernmost
Grenada-southernmost
All volcanism dominantly andesitic (60 SiO2),
with exception of Soufriere, St. Vincent
(basaltic andesite-58SiO2)
Andesites volcanoes generally involve
dome-building eruptions
39

• Most famous dome-eruption, Mt. Pelee 1902-07
• In May, 1902 after only few weeks of activity,
  large pyroclastic eruption decimated St. Pierre
  (30,000 killed)
• Why large event so soon? Influx of fresh magma
  into chamber could not withdraw conduit-filling
  material easily

40
Dominica - 7 potential eruptive centers Famous
boiling lake in Valley of Desolation (dome
complex) Most recent earthquakes (2001) focused
in South (Plat Pays complex) v.close to Roseau !!!
41
Soufriere St. Vincent Different profile (fluid
basaltic andesite lavas) Vast crater-1 mile dia.
150m high crater rim
Small, explosive eruptions in 1971 1979
42
Soufriere Hills Volcano, Montserrat Earthquake
swarms from Dec 1992 - July 1995 Nov 1995 onwards
lava emplaced in Englishs Crater dome
construction (sporadic collapse regrowth) with
many pyroclastic flows Eruption continues today
after more than 10 years !!!

• Very xtal-rich lava(60 to 95 crystals), almost
  solid
• - most viscous lava erupted at Earths surface