Presentation on theme: "GEOLOGIC TIME The Age of the Earth… How old is it?"— Presentation transcript:
GEOLOGIC TIME The Age of the Earth… How old is it?
History of the World (Part I) Historical Geology tries to answer questions about the earths past: –How old is the earth? –How did great rock layers in places like The Grand Canyon form? –What are fossils, how do we find them, and where did they come from? Historical Geology also tries to unravel the mysteries of issues like: –How do geologists piece together past geologic events in the correct order? –How can we date or time stamp geologic events using radio- metric dating tools?
Volumes upon Volumes Think of Earths past as one giant, complicated history book that has already been written Geologists are trying to piece together all the volumes and chapters that have been written throughout history They do this by studying old rocks all around the world and finding out which chapters they belong in This is certainly not a very easy task The strata of these rocks found within Canyonlands National Park Utah reveal the past by speaking to a geologist layer by layer These layers are much like pages in a history textbook
The History Book of The Earth Some information from previous pages has been damaged or smudged Many pages and even chapters, especially from earlier volumes, are completely missing These missing details can make the task of piecing together earths history very difficult And yet, there is enough of the original story left behind and preserved that geologists have been able to piece together and decipher a relatively reliable story of the earths history This reconstructed history of the earth is called The Geologic Time Scale The above panoramic of Monument Valley is a testament to the vast amounts of time that geologic history covers Notice the horizontal strata supporting the towering monuments of sandstone
Is it Young or is it Old? The general accepted age of the earth has changed throughout recorded history –First estimates were that the earth was very young: In the mid 1600s, Archbishop Ussher used Old Testament chronologies and calculated the formation of the earth to have occurred in 4004 B.C. (~6000 yrs old) –In the 1700s, a French scientist calculated the age based upon the rate of cooling of the molten iron core (~80,000 yrs old) –In the 1890s, an Irish scientist calculated the age based upon how long it would take the earths oceans to become as salty as they now are (~90,000,000 yrs old)
Were getting Older!!! Only 100 years ago, the science of chemistry was completely revolutionized –Between 1898 & 1903 Marie Curie and her husband did amazing new research which, for all intensive purposes, discovered the essence of radioactivity They discovered two new elements (Polonium & Radium) They won the Nobel Prize in 1903 –Lord Rutherford shortly thereafter discovered half-life decay rates which would eventually lead to radiometric dating techniques of rocks, fossils, and meteorites (~4,600,000,000 yrs old) –The current accepted age of the earth stands at a very well rounded 4.6 billion years
Its a Freaking Disaster How do the physical features of the earths surface change over geologic time? Early theories in 1600s- 1700s argued for an idea called catastrophism –A theory that earths features have been formed by great disasters in very short time spans –These major catastrophic events no longer occur Examples: worldwide floods, earthquakes, volcanoes, & landslides shaped the earth in only hundreds of years
The Present is the Key to the Past Around 1790 James Hutton formulated the idea of uniformitarianism –The geologic process that we see going on today, have always held true throughout earths history –These processes have shaped & changed the features of the earth over incredibly long time spans (millions of years) To understand ancient rocks, we must first understand present-day processes and their results Uniformitarianism is in stark contrast to catastrophism
Old Rocks…How Old? When it comes to finding the age of something geologically, there are two ways to do it: –Absolute date – this will give you an actual definite age before the present date Years before present –Relative date – this will only determine what sequence things happened in, not how old they are The order of formation The rocks of Stonehenge stand as an ancient memorial to a civilization long gone Nobody knows exactly when it was constructed
Relative Dating…3 Rules Around 1650, a Danish scientist named Nicolaus Steno recognized and formulated three simple, yet powerful, rules for the sequences of events for geologic formation –Law of Superposition –Principle of Original Horizontality –Principle of Cross-Cutting Relationships
Relative Dating Law of Superposition In any sequence of undeformed sedimentary rocks, the oldest rock is always at the bottom and the youngest is at the top Therefore, each layer of rock represents an interval of time that is more recent than that of the underlying rocks At Arizonas Grand Canyon, its easy to imagine that the bottom layers were deposited before the upper layers At Arizonas Grand Canyon, its easy to imagine that the bottom layers were deposited before the upper layers
Relative Dating Principle of Original Horizontality – Sediment, when deposited, forms nearly horizontal layers because of gravity. Therefore, if we observe beds of sedimentary strata that are folded or tilted at a steep angle, we can assume that some deforming force took place after the sediment was deposited All sedimentary layers are deposited flat due to gravity and settling All sedimentary layers are deposited flat due to gravity and settling When we see rock layers like these in Crete that have been drastically folded, we know that they must have been moved into that position by tectonic forces after they were deposited When we see rock layers like these in Crete that have been drastically folded, we know that they must have been moved into that position by tectonic forces after they were deposited
Relative Dating Principle of Cross-Cutting Relationships Whenever a fault or an intrusive igneous rock cuts through existing sedimentary rocks, it must be younger than the structure it cuts Intrusion – Igneous rock that cut through sedimentary layers. Ex. Dikes, Batholith, etc. Fault – break in the rock layers
Types of Unconformities When we observe rock layers that have been deposited without any kind of interruption, we call those layers conformable No location on earth has a complete and continuous set of rock strata without breaks Unconformities represent a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed There are three distinct types of unconformities Disconformity Angular Unconformity Nonconformity
Unconformities contd Angular unconformities ADeposition of the original bottom sedimentary layers stops BThey are tilted CThey are eroded DThen finally they are blanketed by new horizontal strata Notice the 9 steps shown in the bottom diagram D
Winston-youd better conform Nonconformites AIgneous intrusions cut their way up into previously deposited rocks BErosion cuts down to the igneous rock CNew sedimentary layers blanket the eroded igneous rocks Note: these can also form with sediments covering older metamorphic rocks that have been exposed
Relative Dating Practice Layers 1 were deposited Layers 1 were tilted (no other layers have been tilted) Angular Unconformity A Layers 2 were deposited Disconformity B Layers 3 deposited Igneous Intrusion B (it cuts through layers 1,2,& 3 only) Disconformity C Layers 4 were deposited Volcano A erupts A B C
Practice – What is the sequence? Explain what formed 1 st, 2 nd 3 rd, etc… And explain why you know it is in that order-- that is, explain what geologic rule determines the order for each layer
History Shale – An ocean covered the area and clay was deposited forming shale (color) Folding/Tilting– Stress causes the rock to fold or tilt. Fault – Stress causes the rock to break forming a fault. Intrusion/Dike – Igneous activity forms a rock intrusion through layers. Sandstone – Sand covers the area and forms sandstone.
Unconformity – Weathering and erosion caused an unconformity where. Limestone – An ocean covers the area and calcite deposits form limestone. Conglomerate – An ocean covers the area and deposits gravel which forms conglomerate.
To Review: Earths history can be thought of as a large, multi- volume history book Geologists try to piece together this history book by looking at rocks and correlating their ages The Geologic Time Scale is the geologic history of the earth in calendar form Early theories about the age of the earth were based upon short time spans and catastrophic changes James Hutton came up with idea of uniformitarianism Currently accepted models are based upon incredibly long time frames and slow repeating processes Geologic dating can be done by absolute dating or by relative dating Nicolaus Steno came up with the three laws to help with relative dating Unconformities represent breaks in the rock record