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Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: cristi@physics.queensu.ca
(Please use PHYS214 in e-mail subject)
Lecture 10.
Geology and life. Part 1
(Page 99-123)
January 28
Contents
Assignment 1. (Handing out) Due date in two weeks from now.
Textbook: Pages 99-123
• Geology and life
• Type of rocks and their analysis
• Radiometric dating
• Fossil formation
• Geological time scale
• The age of Earth
• The Hadean Earth and dawn of life
• The heavy bombardment
• What is Earth like on the inside
• Differentiation and internal heat
Acknowledgments: Images from NASA, Apollo 16 crew
Geology and life
The Earth owes its habitability primarily to a combination of its size and its distance
from the Sun, in addition to other factors.
Geology – describes processes and features that shape worlds.
Aspects of Earth’s Geology Important to Life
1. Outgassing by volcanoes on the Earth’ssurface - important source of theEarth’s atmosphere.
2. Plate tectonics - largely responsible forthe long-term climate stability that hasallowed life to evolve.
3. Global magnetism - shield the Earth’satmosphere from energetic particles ofsolar wind from the Sun which wouldhave gradually stripped it away intospace.
Geological record = rocks from earlierperiods of the Earth's history.
Fossil record = remains of ancientorganisms preserved in rocks.
The fossil record is part of the geologicalrecord.
Reconstructing the history of Earth and life
Types of rocks
Igneous – molten lava that solidifies(basalt, granite)
Metamorphic – structurally or chemicallytransformed by high pressure or heatnot enough to melt it.
Sedimentary – gradual compression ofsediments at the bottom of seas andswamps (sandstone, limestone).
Sedimentary strata
Important in the study of Earth’s history
1) because it may contain intact fossils
2) it forms in a way that tends to produce a record of time
Erosion of land -> carried by rivers -> floodplains or oceans seafloor -> weight of upperlayers compresses layers into rocks + fossils buried along with sediments
Sediments deposited at different times – look different – different rates of sedimentation,composition, grain size, type or organisms fossilized.
Sedimentary rocks are marked by distinct layers = strata.
Sedimentary strata
Detailed geological record – comparingsedimentary strata from many sitesaround the world (looking for layerswith similar fossils)
Fossils of dinosaurs appear in layersolder than primates fossils - >dinosaurs lived before primates
Example: A fossil found in a layer ofstrata below one layer dated at 1million years must be older than 1million years.
Rock analysis:
Mineralogical – identify the minerals(temperature and pressure conditions)
Chemical – elemental and molecularcomposition (what the rock is madeof)
Isotopic – ratio of different isotopes of aparticular element – (when the rockwas formed)
More than 500 million years
How do we learn the age of a rock or fossil?
Radiometric dating = measurement of proportions of various atoms and isotopes.
A radioactive isotope is an unstable nucleus that spontaneously breaks apart.
Alpha decay - a helium nucleus is ejected.
Beta decay – emits an electron & a neutron turns into a proton.
Electron capture – absorbs an electron & a proton turns into a neutron
Radiometric dating
Because of the probabilistic nature of a radio decay, we can determine the rate at whichlarge numbers of radioactive nuclei decay.
Half-life = the time for half the number of radioactive nuclei to decay
Example: rock that contains equal amounts of potassium-40 and argon-40 -> the rockis 1.25 billion years old.
Accurate – verified on Egyptian artifacts, tree ring data, detailed study of the Sun andother stars (4.5 billion years old solar system)
What does the geological record show?
What does the geological record show?
The geological record contains fewer older rocks than younger
rocks because older rocks have been destroyed by
geological processes like plate tectonics and erosion
Fossils typically contain little or no organic matter because
over time inorganic minerals have gradually replaced the
organic matter
Only a tiny fraction of living organisms leave behind any kind
of fossil remnant because the vast majority of dead
organisms decay long before any mineral replacement can
occur.
Fossils of early life are very difficult to identify in very old
rocks because early life was very simple and microscopic
in size.
The geological time scale
The geological time scale
The Earth’s geological time scale is divided into four main eons:
1) Hadean – first 500 million years on Earth (hellish conditions – Hades –underworld; early bombardment )
2) Archean (ancient life - the earliest evidence of life )
3) Proterozoic (earlier life – fossils of single-celled organisms)
4) Phanerozoic (visible life fossils visible with naked eye)
• The most recent eon – Phanerozoic – is divided in 3 major eras: Paleozoic,Mesozoic, Cenozoic (old, middle, and recent life). There eras aresubdivided into periods.
• The divisions in geological scale are determined not by duration, but byspecific changes in the geological record.
The age of Earth
• The oldest intact rocks found on Earth date
back to about 4.0 billion years ago.
• Tiny grains of zirconium silicate (zircons)
found embedded in sedimentary rocks have
been radiometrically dated to 4.4 billion
years ago.
• Moon rocks brought by Apollo astronauts –
are older than any Earth rocks – volcanism
and other geological processes4.4 billion
years ago.
• The Earth’s crust appears to have already
differentiated from the interior
approximately 4.5 billion years ago.
• Isotopic analysis of meteorites suggests the
Earth and the rest of the solar system
formed 4.57 billion years ago.
The Hadean Earth and the dawn of life
• Models of planetary formation – Earth formedfrom local planetesimals of rock and metal andsome from farther out in the solar systemcontaining ice and rocks bound with water andgases
• The gas became trapped within Earth (carbonatedbeverage in a pressurized bottle)
• The main source of the Earth’s oceans andatmosphere was outgassing by volcanoes.
• The composition of the early atmosphere wasdominated by CO and had no molecular oxygen.
• Today atmosphere 98% N2,<0.1% CO2, 21% O2.The present oxygen atmosphere is a result ofphotosynthesis by plants.
• Evidence from the analysis of zircon crystalssuggests that the Earth may have been habitable aslittle as 100 million years after its formation.
Present-day volcanoes release
water vapors, carbon dioxide,
nitrogen, sulfur-bearing gases,
and hydrogen.
Hadean - The heavy bombardment
Hadean - The heavy bombardment
• Most craters on Moon’s highlands
formed during the period of early
bombardment (first few hundred
million years of the Earth’s history).
• Geological processes (plate tectonics,
volcanism, and erosion) have erased
the carters from Earth surface.
• The dark maria on the moon are huge
impact basins filled in by lava flows.
• Analysis of rocks returned from the
lunar maria during the Apollo program
suggest they formed between 3.9 and
3.0 billion years ago.
The heavy bombardment
All bodies in the solar system should have
suffered equally from collisions during
the period of early bombardment.
A body with relatively few craters on its
surface ->younger surface due to
geological activity or other processes.
Planetary scientists use crater counts to
estimate the ages of planetary surfaces. Moon- the far size
Io
Possible sterilizing impacts
A sterilizing impact is one during which
all life on a planet is destroyed.
During the Hadean Eon, life may have
existed but probably could not have
survived for long periods due to
sterilizing impacts.
• Probably happened 5-10 times during
Earth’s first few hundreds million
years of existence
The amount of energy released by an
impact with an asteroid more than
250-400 km – vaporize Earth’s
oceans and raise the global surface
temperature to 2,000oC.
What is Earth on the inside
Surface rocks have a lower density than overall density of the planet -> Earthhas a dense central core.
Information on Earth internal structure from seismic waves.
Earth`s interior structure:
1) CORE – highest density material; solid inner core made of Ni and Fe,surrounded by a molten outer core.
2) Mantle – rocky materials with moderate density - silicate minerals (Si, O)
3) Crust – lowest density rocks – igneous rocks
Earth differentiation and internal heat
• Geological processes on Earth’s surface - related to internal heat.
• Transfer of heat in the Earth’s mantle is dominated by convection.
• Differentiation = process by which denser materials (Fe) iron sank to the center of
the Earth while less dense materials (rock) rose to the surface (e.g. oil & water).
• Sources of heat that helped differentiate the Earth: impacts, gravitational potential
energy, radioactive decay.
• The amount of geological activity occurring on a terrestrial planet is related to its
size and the amount of internal radioactive heating.
The impact that created the moon occurred
after differentiation.
Differentiation occurred quite early.
Differentiation and internal heat
• All worlds in our solar system went differentiation.
• Since then they have been cooling with time.
Cooling rate depends on
1) the size of the world
2) The existence of an ongoing heat source
- radioactive decay, long half-time – heat for billions of years
- Tidal heating for moons of jovian planets
Moon
Next lecture
• Plate tectonics and Earth`s magnetic field
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