Geologic Time - Week 13 - PDF

Telling time with the Earth The concept of Deep Time Perhaps no place on Earth better exemplifies the principles geologists use to determine the ages of rocks than Arizona’s Grand Canyon National Park http://opengeology.org/textbook/7-geologic-time/ How old is the Earth? James Usher (1581-1656...

Telling time with the Earth The concept of Deep Time Perhaps no place on Earth better exemplifies the principles geologists use to determine the ages of rocks than Arizona’s Grand Canyon National Park http://opengeology.org/textbook/7-geologic-time/ How old is the Earth? James Usher (1581-1656) - Irish Archbishop 1654: calculated lineages from the Old & New Testament Birth of the Earth: Oct 23, 4004 BC So the Earth was 6000 years old Nicolas Steno (1636-86) Italian physician Unusual triangle shaped stones were found in local rocks Called ‘tongue stones’ Though to be the tongues of dragons (!) Steno realized ‘tongue stones’ were shark tooth fossils  Realized the sharks’ teeth could have been buried in sediment with seashells on the seafloor Transformed into rocks He thought that the Earth could change over time A revolutionary thought! Steno Proposed: fossils belonged to living creatures all rocks and minerals were originally fluid newer rocks lay on top of older rocks - this became known as ‘Principle of Superposition’ James Hutton (1726-97): Scottish farmer & doctor In Scottish countryside – many features he could see in rock formations were also occurring in the present E.g. deposition of material in layers Realized that geological events that take place today (i.e. in his time) are extremely slow, and the same processes must also have occurred in the past Central idea behind Uniformitarianism Siccar Point, Scotland  1785: Hutton published book – The Theory of the Earth Physical processes we see today also happened in the past “the present is the key to the past” Informs what is known as the Principle of Uniformitarianism Rates that geological processes take today is slow Individual features in rocks/landscape take a long time to form The Earth must have a history that includes a succession of geological events The Earth existed a long time before human history http://www.sath.org.uk/edscot/www.educationscotland.gov.uk/scot tishenlightenment/jameshutton/learnmore.html  Hutton Realized: the Earth had a ‘heat engine’ in its interior Would create molten rock The exact form of the rock would depend on the rate of cooling (accidental discovery of forms formed by slow cooling of molten glass) Molten rocks could flow into older rocks Aside: 3 types of rock igneous sedimentary metamorphic Igneous: - solidification of magma - minerals & crystals created with heat - intrusive – remains below surface; cools slowly - many types of gemstones can be created - extrusive – cools quickly; lava Sedimentary - created as small rock particles on the surface of the earth are compressed & compacted Metamorphic: - heat and pressure change existing igneous & sedimentary minerals into new materials - causes change in chemistry and crystal structure Charles Lyell (1797-1875) British geologist Wrote 1st textbook on geology: Principles of Geology 1830-33 Allows for determination of relative ages of rocks Contains Principle of Uniformitarianism Physical processes that happen today also happened in the past Principle of Original Horizontality Layers accumulate horizontally and settle due to gravity Folds/tilts have occurred after sedimentary layer formed Lyell’s Principals of geology allow the relative ages of rocks to be estimated Principle of Superposition In a sequence of sedimentary layers, each layer is younger than the one below Principle of Lateral Continuity Sediments usually accumulate in continuous sheets in a region Principle of Cross Cutting Relations If one geologic feature cuts across another, the feature that has been cut is older Principle of Baked Contacts During formation of igneous intrusion – hot magma injects into cooler rock Heat from intrusion bakes (metamorphosizes) surrounding rocks Rock that has been baked is older than inclusions Principle of Inclusions An inclusion (fragment of one rock incorporated in another) is always older than the rock that contains it  Can use principles to determine the relative ages of rocks, structure and other geologic features Features can be explained by specific geologic events https://www.ck12.org/c/earth-science/determining-relative-ages/lesson/Determining- Relative-Ages-MS-ES/?collectionCreatorID=3&conceptCollectionHandle=earth-science- %3A%3A-determining-relative-ages&collectionHandle=earth-science Pause To video Men of Rock - Deep time : 0min – 40 min https://www.youtube.com/watch?v=FYfuI2uZLmg William Smith (1769-1839) British engineer, geologist The digging up of oil & gas after the Industrial Revolution exposed a lot of ‘bedrock’ Smith noticed that there were fossils in layers of sedimentary rocks Some fossil species present in 1 (or a few) layers, but not upper layers This means some species have become extinct Can determine the relative ages of strata by looking at fossils  “William Smith noticed a curious turnover in fossils between two rock layers near Bath, England. The lower layer was rich in plants; the upper layer was rich in seashells. The turnover Smith noticed is now recognized as the boundary between the Carboniferous and Permian Periods.” https://earthobservatory.nasa.gov /features/WilliamSmith Strata in East Anglia Layers of strata can be interpreted with Lyell’s Principles of Geology to estimate relative ages of various layers Can also use fossil succession Strata in Skye, Scotland  Portugal at Telheiro Beach Siccar Point (Scotland) Unconformities Sometimes there can be a large temporal gap between layers Discontinuities can occur because of: non-deposition; erosion; folding This makes geological records incomplete No one location on the Earth provides a complete timeline of the Ecuador Earth’s history http://www.geologyin.com/2015/10/types-of-unconformities.html https://openpress.usask.ca/physicalgeology/chapter/19-2-relative-dating-methods-2/  Fossil succession is a way to correlate the ages of strata from different locations http://opengeology.org/textbook/7-geologic-time/ A Geological Map of England and Wales and Part of Scotland, first published in 1815 by William Smith. On a scale of 5 miles per inch, the map measured 6 feet by 8 feet 6 inches. http://www.nhm.ac.uk/discover/first- geological-map-of-britain.html The Geologic Column By correlating strata around the Earth, can make a composite geologic column that represents the entire Earth’s history  Millions of years ago Timescales in Earth’s history Visible life 1st life Bacteria, archaea (1st cells) No rock record Timescale for Life Archaen Eon: simple bacteria & archaea Late Proterozoic Eon: complex shell-less invertebrates Precambrian-Cambrian boundary: appearance of invertebrates with shells Cambrian Period in Paleozoic Era: sudden diversification of life in a short interval – “Cambrian Explosion” Paleozoic Era – more complex life Eons: Pause Watch video A Brief History of Geologic Time https://www.youtube.com/watch?v=rWp5ZpJAIAE The age of the Earth: Lord Kelvin (1824-1907) 1863: - Assumed Earth formed as molten object - Calculated the amount of time it would take to cool down - 1st estimates: 100-200 million years old - Later revised estimates: 20 million years old - Problematic with long timescales required for evolution, based on work of Darwin (1809-1882) Kelvin made some incorrect assumptions: The Earth was not an infinite source of energy The energy to run geological cycles came from the Earth’s internal heat [CORRECT] The heat is not replenished [INCORRECT] The Earth can be approximated as a undifferentiated sphere (constant composition, density throughout) [OK] Marie Curie (1867-1934) 1900: discovers radioactivity 1903: wins Nobel Prize for Physics for discovering radioactivity 1911: wins Nobel Prize in chemistry for creating a means of measuring radioactivity Radioactivity: the splitting of a larger nucleus into smaller ones Different types of particles/ energy released Radioactivity is the spontaneous decay of large atomic nuclei into smaller ones 3 types of radioactive radiation: Alpha rays – helium nuclei (2 protons, 2 neutrons) Beta rays – electrons Gamma rays – part of electromagnetic spectrum Various types of radioactivity all give energy to and heat surroundings : mechanism for heating the Earth that Kelvin did not know about  Kelvin never believed that radioactivity could have a significant impact on heating the interior of the Earth – never recanted his calculations (20 million years) Modern age estimate for the Earth: 4.55 billion years Mantle & crust: heat from radioactive decay of elements Can find radioactive elements in igneous and metamorphic rocks Core: primordial heat left from Earth’s formation Pause Watch Deep time video 40-57min We will continue discussing radioactivity next lecture Video gives good introduction