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 s surface - important source of the Earth s atmosphere. 2. Plate tectonics - largely responsible for the long-term climate stability that has allowed life to evolve. 3. Global magnetism - shield the Earth s atmosphere from energetic particles of solar wind from the Sun which would have gradually stripped it away into space. Geological record = rocks from earlier periods of the Earth's history. Fossil record = remains of ancient organisms preserved in rocks. The fossil record is part of the geological record.
Reconstructing the history of Earth and life Types of rocks Igneous molten lava that solidifies (basalt, granite) Metamorphic structurally or chemically transformed by high pressure or heat not enough to melt it. Sedimentary gradual compression of sediments at the bottom of seas and swamps (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 upper layers 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 comparing sedimentary strata from many sites around the world (looking for layers with similar fossils) Fossils of dinosaurs appear in layers older than primates fossils - > dinosaurs lived before primates Example: A fossil found in a layer of strata below one layer dated at 1 million years must be older than 1 million years. More than 500 million years Rock analysis: Mineralogical identify the minerals (temperature and pressure conditions) Chemical elemental and molecular composition (what the rock is made of) Isotopic ratio of different isotopes of a particular element (when the rock was formed)
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 which large 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 rock is 1.25 billion years old. Accurate verified on Egyptian artifacts, tree ring data, detailed study of the Sun and other 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 are subdivided into periods. The divisions in geological scale are determined not by duration, but by specific 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 formed from local planetesimals of rock and metal and some from farther out in the solar system containing ice and rocks bound with water and gases The gas became trapped within Earth (carbonated beverage in a pressurized bottle) The main source of the Earth s oceans and atmosphere was outgassing by volcanoes. The composition of the early atmosphere was dominated by CO and had no molecular oxygen. Today atmosphere 98% N 2,<0.1% CO 2, 21% O 2. The present oxygen atmosphere is a result of photosynthesis by plants. Evidence from the analysis of zircon crystals suggests that the Earth may have been habitable as little 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 2,000 o C.
What is Earth on the inside Surface rocks have a lower density than overall density of the planet -> Earth has 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 Moon 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
Next lecture Plate tectonics and Earth`s magnetic field