GEO1003 Igneous and Metamorphic Petrology

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Spring 2009
GEO1003 Igneous and Metamorphic Petrology
Almost all Earth is igneous or metamorphic rocks

• Introduction to the Course
• Very Brief History of Igneous Petrology

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• Web Page http//skilling.geology.pitt.edu/GEO1003
  /syllabus.htm
• Labs start next week with James Gardiner
  (jbg10_at_pitt.edu)
• See handout today for textbooks, class and exam
• schedules, grading etc
• 3 exams (60 total) and lab (assignments and
  quizzes, 40)
• Exams are not comprehensive
• Class average posted on web after grading
• Office Hours 4-6 Mondays-Wednesdays

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History of Igneous Petrology Themes

• Up to 18th century history of igneous petrology
  is history of volcanology
• What is internal heat source?
• How deep is heat source?
• Do basalts (and granites) crystallize from water
  or molten rock?
• Intimately related to biblically-based ideas of
  age of Earth and Deluge until 18thC
• In 20th C, plate tectonic theory, recognition of
  heat from radioactive decay,
• melting experiments on rocks and new analytical
  techniques revolutionized understanding
• of origin of igneous rocks

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Ancient Ideas Introduction

• Ideas about volcanoes only
• Roman, Greek and several central American
  civilizations existed in areas of active
  volcanism
• Descriptions (often fanciful) and interpretations
  (more fanciful often) of Vesuvius, Etna
• and Iceland are most common
• Greek-Roman-Judaeo-Christian idea of hell is hot
  and volcanic and is presumably derived
• from ideas about Mediterranean volcanoes, like
  Etna
• Greek god of hot underworld was Hephaestus and
  the Roman equivalent was Vulcano (who
• had his forge on Vulcano in the Aeolian Islands
  (north of Sicily), Italy
• Many ideas based on underground winds or
  exhalations circulating under Earth
• No thought to igneous rocks that formed at depth

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View of other Aeolian Islands from Vulcano
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AD79 Eruption of Vesuvius

• Best known description of igneous phenomena in
  classical literature
• Most famous description of AD79 eruption is by
  Pliny the Younger who wrote about
• the eruption, his escape from it, and the death
  of his polymath uncle Pliny the Elder
• (below) (by asphyxiation)
• Pliny the Younger described the eruption column,
  pyroclasts (pumice, ash)
• Pliny the Elder most famous for his 37 volume
  Natural
• History

He began to work long before daybreak.He read
nothing without making extracts he used even to
say that there was no book so bad as not to
contain something of value. In the country it
was only the time when he was actually in his
bath that was exempted from study. When
travelling, as though freed from every other
care, he devoted himself to study alone. In
short, he deemed all time wasted that was not
employed in study."
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Ancient Ideas

• Aristotle was an iconic figure in the ancient and
  Medieval world and his ideas about
• volcanoes were commonly accepted
• His exhalation theory suggested that
  underground winds caused earthquakes and
• eventually broke out of the surface shattering
  the surrounding rocks so quickly that
• they caught fire
• Seneca the Younger also favoured a windy
  explanation, but also suggested that the
• winds ignited combustible fuels like sulfur and
  coal in the subsurface by friction
• Wasnt until the Renaissance that we see any
• improvement on these early ideas,

Seneca
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Athansius Kircher (1602-1680)

• Catholic priest in Germany
• Wrote one of first (if not the first) book solely
  about volcanoes
• Fire-Vomiting Mountains with Their Remarkables
• His ideas were still largely an amalgam of
  classical ideas
• He coined the delightful word ignivomous for
  volcanic
• eruptions and rocks
• Suggested that heat sources were deep-seated,
• see first cross-section of Earth drawn by him on
  right

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Sketch of Vesuvius by Kircher, showing its
internal fire
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Sir William Hamilton

• Envoy of Britain to court of Naples in 1760s
• Hobby was study of Vesuvius, but he also visited
  Etna,
• Vulcano and Stromboli
• Recognized authority on volcanoes in 18th century
• He described lava flows (active and old) on
  Vesuvius
• in detail
• He collected and analyzed samples of lava and
  fumarolic salts
• Hamilton believed volcanism arose from combustion
  of various subsurface fuels

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18th Century Neptunism versus Plutonism

• Really have to wait until late 18th century
  before any serious scientific thought
• on volcanic and plutonic rocks
• In 16th-17th century an argument developed that
  many rocks like basalt and granite
• had been precipitated from a primordial ocean.
  This was challenged in the 18th century
• During 18th century there was a vigorous debate
  between those who saw all
• rocks as sedimented out of, or precipitated from,
  water (Neptunists) and those
• who suggested that some hard rocks like granite
  and basalt had once been molten
• (Plutonists)
• James Hutton was a leading plutonist
• Battle raged between supporters
• of German Abraham Werner
• (Neptunists) and supporters of
• Hutton

Hutton
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James Hutton (1726-1797)

• Founder of modern geology
• Leading proponent of plutonism and old (gt6000
  years) age for Earth in his famous
• book Theory of the Earth. No vestige of a
  beginning, no prospect of an end
• Was first person to actually study igneous rocks
  in the field in any detail. Let us open
• the book of nature and read her records
• He observed that many basalts he studied had been
  forced (intruded) into sedimentary
• rocks, and suggested that this could only happen
  if they were once molten
• Prior to Hutton all granites were thought to be
  oldest rocks on Earth, and most suggested
• they were crystallized from water
• He suggested that granite was a mass of
  subterranean lava as he observed dikes and
• veins of granite intruded into the surrounding
  rocks (just like basalt)
• Hutton argued that granite could not be
  precipitated in layers from an ocean as it
  intruded
• older rocks

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Hutton sketched intrusive contacts like this at
base of a sill in Edinburgh
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Internal Heat Source of Earth Radioactive Decay

• In 19th C, chemists such as Humphrey Davy
  suggested chemical reactions may be heat
• source in Earth
• Since 1920s know that internal heat source of
  Earth is not combustion, friction or
• chemical reactions but energy released during
  radioactive decay
• Most heat released from decay of U and Th
  isotopes in core
• Some internal heat from original accretion of
  Earth
• Some internal heat also generated during
  differentiation (separation of interior of a
• a planet or moon into layers of increasing
  density with depth)
• Some tidal heating from pull of Sun and Moon (but
  minor)

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Plate Tectonics

• Like all branches of Earth Science, plate
  tectonic theories revolutionized our
  understanding
• of igneous rocks
• We know that the geographical distribution of
  volcanic and plutonic activity on Earth can only
• be explained using plate tectonics
• We now know that certain igneous rock types can
  only be formed at certain types of plate
• boundaries
• We can now analyze an isolated igneous rock
  sample and infer its tectonic setting