Physical Volcanology Lecture 1. USGS Photo. May 12, 1991 Mount Pinatubo, Philippines. Instructor: Joe Dufek

Transcription

1 Physical Volcanology Lecture 1 USGS Photo. May 12, 1991 Mount Pinatubo, Philippines Instructor: Joe Dufek dufek@gatech.edu

2 Physical Volcanology: - Normally meets MW, but we will have some earlier Friday lectures to prepare for the St. Helens trip. -Class grade based on 4 components: - 20% Homeworks - 30% Exams (Two, equally weighted - no formal final exam) - 30% Final Project (more on this in a moment) - 20% Participation and Presentation. This includes biweekly volcano update presentations, pre-st. Helens presentation, and final project presentation.

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5 Pre- Mount Saint Helens Briefings: 15 minute review of a topic related to St. Helens eruption. Group presentations of 2-3 people (presentations on Sept. 16,19). 1. Chronology and stratigraphy of 1980 St. Helens eruption. 2. Blast and Pyroclastic Flows of St. Helens. 3. Seismic Activity of pre and post 1980 eruption 4. Columbia River Flood Basalt Emplacement Individuals will then give approximately 5 min. presentations on their research objectives.

6 Volcanology - the study of volcanoes Volcanoes are the surface expression of melting processes in the interior of planets. On Earth these melts, are silicate magmas, but other compositions are possible. For instance cryovolcanism refers to the melts of ice or clathrates that erupt, such as on Enceladus, bottom left.

7 Physical Volcanology - physics of volcanic processes 1. Understand the underlying priciples of heat and mass transfer in the earth and during eruptions. 2. Understand the classification and implications of eruptive style. 3. Appreciate the history and development of thought on this subject.

8 From a planetary perspective, magma transport to the surface is important because: 1. Rapidly transports heat. 2. Rapidly transports volatiles (water vapor, carbon dioxide, sulfer dioxide etc.) building and altering the atmosphere. 3. Builds continents, reorganizes the distribution of minerals. 4. Causes catastrophic events at the surface.

9 The scope of this class:

10 The molten rock origin for what are now called igneous rocks was a topic of active debate until the late 18th century. Two primary groups argued the origin of these rocks neptunists and plutanists. Columnar Basalt, Iceland

11 Much of this debate centered around the nature of basalts: Basalt: wt. % SiO 2, mostly pyroxene and plagioclase (fine grained)

12 Neptunism: Most rock was formed and deposited in early, pervasive oceans. Argued basalts and granites crystallized/precipitated from aqueous solution. Dendritic salt crystals growing in solution Abraham Werner

13 Plutonists argued for a new class of igneous rocks that differed from sedimentary rocks and oringated as melt from great heat in the interior of the Earth. Theory of the Earth ( James Hutton)

14 Three classical eruptions (and different human responses): 1. Thera (Santorini) 2. Vesuvius (79 AD) 3. Krakatoa

15 Thera Eruption - Santorini 5 km

16 Minoan Eruption ~60 km 3 of material Ash dispersed throughout the eastern Mediterranean (some ash found in the Nile delta). Eruption column reached an estimated height of 40 km Deposits ~50 m thick on island of Santorini Buried the city of Akrotiri - although the city appears to have been evacuated before the eruption - no bodies. Likely impacted Minoan civilization - although the extent of its impact is controversial. The source of various legends??

17 Vesuvius - 79 AD Brullov, The last day of Pompeii (1830)

18 Vesuvius - 79 AD Preceded by tremors in the decade proceeding eruption. Thousands of fatalities, including Pliny the Elder. Pliny the Younger (nephew) provided an eyewitness account to the historian Tacitus. We still refer to columns of ash and pumice that rise several kilometers in the air as plinian columns.

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20 Reconstructed Timeline Pyroclastic Density Current Pinatubo, National Geographic Plinian Column Mt. Spurr, Photo R. McGimsey

21 Krakatau AD Engraving of Krakatau before the eruption National Geographic Royal Society - May 27, 1883

22 Krakatau - Aug. 27, 1883 After several months of minor eruptions, the climatic eruption occurred on Aug. 27, Explosion audible in Australia and Diego Garcia in the Indian Ocean - over 3000 km distant from the source. Barographs throughout the world recorded the pressure wave. The eruption and resulting tsunami was responsible for ~30000 fatalities. Pyroclastic density currents traveled over 80 km from the source - over the surface of the ocean.

23 To understand the physics of these eruptions, we will first start by considering the source of magma in the mantle, and then move upward considering the path of magmas to the surface.

24 Some magma compositions from St. Helens. The chemical composition determines there physical properties, e.g. viscosity, heat capacity, conductivity, etc.

25 The composition of a magma is determined by the pressure, temperature and mineralogy of their source region its interaction with rock during transit, and its cooling and fractionation history. To begin considering these processes we first need to understand the transport of heat.

26 Why is the interior of the Earth hot? Initial Energy of Accretion. - conversion of potential energy to thermal energy. Radioactive Decay. - Radiogenic nuclides of uranium, thorium, potassium contribute heat as they decay to their daughter products.

27 Earth accreted from small planetesimals Tom Quinn s Lab at University of Washington

28 Accretionary Timeline ~10 4 yrs to produce ~10 km diameter planetesimals. ~ years to produce planetary embryos the size of the moon or Mars. ~ million years to produce planets.

29 One likely example of a larger - late stage impactor - Moon forming impact.

30 The initial accretion and coalescence (where denser materials go toward the core of the planet) result in the release of potential energy. The gravitational force between two bodies is: F = Gm 1m 2 r 2 G is the gravitational constant, G= 6.67 x m 3 kg -1 s -2

31 Integrating this force over distance gives the potential energy. U = r Gm 1 m 2 r ' 2 dr U = Gm 1m 2 r The units here are Newton meters or Joules (J).

32 Earth s Structure has evolved through time. Labrosse et al. Nature 2007 A magma ocean likely existed following the early stages of accretion.

33 Common radioactive nuclides in the Earth (based on estimates of mean mantle concentrations). Isotope H (W/kg) t 1/2 (yr) Concentration (kg/kg) 238 U 9.46 x x x U 5.69 x x x 10-9 U 9.81 x x Th 2.64 x x x K 2.92 x x x 10-9 K 2.48 x x 10-5

34 A typical single alpha decay has kinetic energy of approx J Beta decays have on order KE of J, although both are variable.

35 Due to the decay of parent nuclides, the concentration of a radiogenic isotope decreases with time. 2.0 C = C exp t ln(2) now t 1/2 C/C now 1.0 Past t/t 1/2

36 While the average concentration of radiogenic nuclides in the silicate Earth is useful for bulk calculations, radiogenic nuclides are not evenly distributed in the mantle and crust. - The common radiogenic nuclides are incompatible in crystalline silicates. - During melting and solidification the concentration of these nuclides will increase in the melt. - Due to this partitioning, radiogenic nuclides have a tendency to congregate in buoyant melt - and end-up being concentrated in the near surface crust.

37 Model of average crust and associated radiogenic heat production. Note the lack of heat production and mantle convection resulted in inaccurate calculations of the age of the Earth by Lord Kelvin (see website for supplemental reading). Rudnick and Gao, 2003

38 For better or worse, nuclear energy and weapons would be unfeasible if not concentrated significantly by Earth processes.