The geologic time scale is a system we use to measure and describe the history of Earth. The time scale is used to: Measure how long Earth has existed Describe how Earth has developed and changed Recognize major events that occurred on Earth 1
Earth is approximately 4.54 billion years old. We divide Earth s life into large time units called eons. Earth s life is divided into 4 eons: 1. Hadean Eon 2. Archean Eon 3. Proterozoic Eon 4. Phanerozoic Eon 2
The Hadean, Archean and Proterozoic Eon are often placed under a supereon known as Precambrian. Scientists believe that no multicellular life existed on Earth during the Precambrian supereon. 3
An eon is divided into smaller units of time called eras. The Archean, Proterozoic and Phanerozoic eons are divided into eras. The Hadean Eon is not. 4
The most studied eras are those of the Phanerozoic Eon. The Phanerozoic Eon is divided into 3 eras: 1. Paleozoic Era 2. Mesozoic Era 3. Cenozoic Era 5
An era is further subdivided into periods. There are 12 important periods. All the periods are subdivisions of eras in the Phanerozoic Eon. A period is further subdivided into epochs. We will not study epochs in this unit. 6
There are some discrepancies in how Earth s history is divided. For example: The Cenozoic Era used to be divided into 2 periods: Tertiary and Quaternary. Now, it is divided into 3 periods: Paleogene, Neogene and Quaternary. Some texts still recognize only two periods. In the US, scientists often subdivide the Carboniferous Period into two parts - the Pennsylvanian and Mississippian Period. 7
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Scientists determine the number and length of time periods based on important events in Earth s history. The early eons are defined by the development of physical features of Earth - it s land, air and water. The Phanerozoic Eon and its eras and periods are often defined by the emergence of specific life forms and mass extinctions. Why did scientists divide Earth s history this way? 9
The Earth you know today took billions of years to form. In the very beginning, Earth was a hot, boiling sea of molten rock. Over time, Earth cooled and its land, atmosphere and waters formed. Then life emerged on Earth. 10
We will study four major stages to Earth s formation: 1. Hadean Earth: The birth of Earth 2. Archean Earth: The cooling of Earth 3. Proterozoic Earth: The beginning of life on Earth 4. Phanerozoic Earth: Multicellular life on Earth 11
Earth formed approximately 4.6 billion years ago at the same time as the rest of our solar system. Gravity caused gas and dust to clump together to form rocky objects the size of asteroids. These objects clumped together to form small planets. Small planets collided with each other to form the planets of our solar system, including Earth. 12
Approximately 4.5 billion years ago, a small planet collided with Earth. Some of the planet was absorbed by Earth. Debris from the rest of the planet sprayed out into space. Earth s gravity caused the debris to orbit around the planet. The debris clumped together to form our moon. 13
From 4.6 to 4 billion years, Earth was a hot and inhabitable place. Earth was covered in molten rock (magma) with a thin atmosphere, completely devoid of oxygen. 14
From 4 billion to 3.8 billion years ago, Earth was intensely bombarded with asteroids and comets. Scientists refer to this time as the Late Heavy Bombardment. Comets are water-rich objects. It s believed that Earth s oceans formed from the water provided by the comets that peppered Earth at this time. 15
The first life forms appeared around 3.8 billion years ago. It s believed these organisms were anaerobic microbes that could live in the absence of oxygen. They used carbon dioxide to extract energy. Anaerobic microbes dominated Earth for millions of years. Many scientist believe archaebacteria, such as the microbes shown above, are most similar to the first microbes on Earth. 16
By 3 billion years ago, Earth s surface completely solidified. The first continents formed although most were covered by a shallow sea of water. Over time, the continents grew in size due to volcanism and tectonic plate movement. By 250 million years ago, the continents coalesced into one large landmass known as Pangaea. 17
Approximately 2.5 billion years ago, cyanobacteria appear. Cyanobacteria, also known as blue-green algae, introduced oxygen into Earth s atmosphere. They released oxygen through the process of photosynthesis. Cyanobacteria caused oxygen to accumulate in the atmosphere. We call this the Great Oxygenation Event. Cyanobacteria (Blue-Green Algae) 18
The Great Oxygenation Event caused major climate change on Earth. In fact, it s believed Earth was completely covered in ice for millions of years. Scientists refer to Earth completely covered in ice as Snowball Earth. Scientists believe Earth was covered in ice until 650 million years ago. 19
Multicellular life emerged 550 million years ago, marking the beginning of the Phanerozoic Eon. Multicellular life exploded rapidly (relatively speaking). It took Earth more than 4 billion years to form, but only a half billion years for multicellular life to emerge and diversify on Earth. 20 Some of the first multicellular organisms were invertebrates, likes these Trilobites depicted above.
Summary of Life on Earth: The first life forms on Earth were small and simple. They were anaerobic microbes. For billions of years, these organisms dominated Earth. The emergence of cyanobacteria led to the death of the first organisms on Earth. Oxygen was poisonous to anaerobic microbes. However, oxygen in the atmosphere was necessary for the emergence of multicellular life. Multicellular life exploded 550 mya. Once multicellular life emerged, life quickly (relatively speaking) became more complex and diverse. 21
Some of the first multicellular organisms to appear on Earth were invertebrates such as corals, jellyfish, sponges and worms. These early organisms most likely resembled today s extant coral, jellyfish, sea sponges and marine flatworms. 22
The first vertebrates appeared 486 mya. These animals were jawless fish. The first land plants appeared 434 mya. By 400 mya, fish with teeth appeared. Some of the first toothed fish were sharks. These early organisms most likely resembled today s extant lampreys, liverwort and mackerel sharks. 23
By 360 mya, Earth s land was dominated by ferns. These ferns are extinct today. The first amphibians and insects also appeared 360 mya. The first reptiles appeared approximately 300 mya. These early organisms most likely resembled today s extant ferns, true bugs and salamanders. 24
Conifers (cone-producing plants) appeared 250 mya. The earliest dinosaurs appeared around 225 mya, during the first period of the Mesozoic Era, the Triassic Period. 25
The first mammals appeared 160 mya. They were small and primitive. Birds first appeared 150 mya. Flowering plants appeared 130 mya. These early organisms most likely resembled today s extant platypus, coastal birds and flowering plants known as Amborella. 26
The first primates appeared 60 mya. The first humans appeared 2 mya. 27
Over the past 550 million years, many different kinds of organisms emerged. Organisms became larger, more diverse and more complex. However, Earth s history is not only marked by the appearance of new organisms. Earth s history is also marked by the disappearance of organisms. 28
Extinction is the complete loss or death of a species. A mass extinction is an event when an abnormally large number of species die out simultaneously or within a short time frame. Extinction often occurs because of: A natural disaster Climate change Competition (with a new species) Human activities (ex. Hunting, deforestation) 29
There are 5 mass extinctions that occurred during the Phanerozoic Eon: 1. Ordovician-Silurian Mass Extinction 2. Late Devonian Mass Extinction 3. Permian Mass Extinction 4. Triassic-Jurassic Mass Extinction 5. Cretaceous-Tertiary Mass Extinction 30
Ordovician-Silurian Mass Extinction Occurred 443 mya Most species lived in the sea so it makes sense that this extinction event led to the death of nearly 85% of species in the ocean. It s believed that climate change, specifically an ice age, caused this mass extinction. The ice age affected the chemistry of the ocean and sea levels. 31
Late Devonian Mass Extinction Occurred 359 mya 75% of all species on Earth died out during this mass extinction. Life in shallow seas were worst affected. Might have been caused by an asteroid impact, climate change and/or new kinds of plants that emerged. The fossilized coral above was one of many species that went extinct during the Late Devonian Mass Extinction. 32
Permian Mass Extinction The Great Dying Occurred 248 mya (at end of Permian Period) 96% of species on Earth died out over a period of millions of years Might have been caused by asteroid impact, volcanism and/or sea level changes Trilobites (their fossils shown above) were one of the species that went extinct during the Permian Mass Extinction. 33
Triassic-Jurassic Mass Extinction Occurred 200 mya 50% of species died out. Marine reptiles, large amphibians, coral reef building organisms and mollusks were worst affected. Plants were not affected. Might have been caused by asteroid impact, volcanism and/or climate change. Its cause is widely speculated. 34
Cretaceous-Tertiary Mass Extinction Occurred 65 mya Caused death of the dinosaurs, ammonites and many flowering plants Most likely caused by asteroid or comet impact near the Yucatan Peninsula 35
Fossils help us study what life was like in the past. A fossil is the preserved remains of a once living organism. A fossil can be the entire organism, part of the organism or an impression of the organism. 36
Fossils are most often found in sedimentary rock. Sometimes fossils are found in other materials, such as tar, amber or ice. Fossils are NEVER found in igneous or metamorphic rock. 37
A fossil forms over thousands of years. Here s how: 1. When an organism dies, its body falls to the ground. If in a body of water, its body sinks to floor. 2. Sediment, such as sand, silt or clay covers the dead organism. The sediment forms layers, which turn to rock. 38
3. The dead organism is buried in rock. The organism, or at least the soft parts of the organism usually decays, leaving behind an impression or fossil. 4. Over time, the rock erodes or wears away. This exposes the fossil, which can be observed and studied by scientists. Sometimes scientists dig up rock to look for fossils. 39
We can determine what organisms lived more recently compared to organisms that lived long ago depending on the rock layer fossils are found in. Fossils in superficial or shallow layers of rock are younger than fossils in deeper layers of rock. Therefore, we can assume the organisms that formed these fossil lived more recently than organisms that formed fossils found in deeper layers. Fossils found in the same layer of rock often indicate organisms that lived at the same time. 40
Scientists can relatively determine how old fossils are. Fossils found in deeper rock layers are older than fossils found in shallower rock layers. Scientists can also absolutely determine how old fossils by measuring certain substances in the fossils and/or rocks the fossils are found in. Youngest Oldest 41
Fossils can help scientists determine what the environment was like in the past. For example: If fossils of marine organisms are found in a location that is now dry land, that area must have been covered by the ocean at some point in the past. If fossils of land plants are found in a location that is now covered by water, that area was dry land at some point in the past. Fern (land plant) Ammonoid (marine organism) Trilobite (marine organism) Ammonoids and trilobites are marine organisms. This region must have been covered by ocean water if fossils of these organisms were found here. 42
There are different types of fossils: Mold Fossils Cast Fossils Trace Fossils True Fossils Petrified Fossil 43
A mold fossil is an impression of a once-living organism. A mold fossil forms when an organism dies and its hard parts are buried in mud, clay or other material that turns to rock. Over time, the hard parts dissolve, leaving behind an impression or mold of the once-living organism. 44
A cast fossil is a filled in mold fossil. A cast fossil forms when an organism dies. Its hard parts form a mold fossil in rock and then the mold is filled in with some sort of mineral. This leaves behind a cast of the original organism. 45
A trace fossil or imprint is an indirect fossil. It is a fossil of biologic activity, not a fossil of a body structure. A trace fossil can be an impression (such as a footprint or feeding mark), a nest or burrow made by a once-living organism. 46
A true fossil is the actual organism or part of the organism. True fossils form when the organism does not decay and its body parts are preserved. True fossils often form when an organisms gets trapped in ice, tar or amber. The original features of the organism, like its color and shape, remain in tact. Bones, teeth and eggs are true fossils. These are the actual body parts of a once-living organism. True fossils are the best fossils because they give the most information. 47
A petrified fossil is different from other fossils. A petrified fossil forms when the organic matter of a dead organism or part of a dead organism is replaced by minerals and turned into stone. 48
An index fossil is a fossil that helps scientists date unknown or less studied fossils. Index fossils have two characteristics that make them helpful in dating other fossils: Index fossils are widely distributed, found in many different regions of the world. Index fossils are limited in time span. They are only found during a relatively short time frame because the once-living organism only lived during a short time frame. 49
When an unknown fossil is found in the same layer as an index fossil, scientists can infer that the unknown fossil is from the same time period as the index fossil. How do index fossils help us date other fossils? 50
If an unknown fossil is found in a layer of rock with two index fossils from different but overlapping time periods, the unknown fossil must have lived during the overlapping time period. The scaphite ( ) lived 150-65 mya. The turritella ( ) lived 65 mya to present day. Since the oyster ( ) is found in a layer with fossils of both organisms, it must have lived during an overlapping time period. Therefore, the oyster must have lived 65 mya. 51
Index fossils are also helpful in determining missing fossils or layers of rock in different locations. Sometimes rock layers and fossils within them erode. Sometimes fossils do not form. Index fossils help match layers of rocks in different locations when this happens. Location A Rock layer eroded Location B A layer of rock must have erode at the red line at location B. You can tell this occurred because there is a layer of missing index fossils between the top and bottom layer at this location. 52
Common Index Fossils Organisms Time Period it Lived Illustration Atlantic Calico Scallop Quaternary Period to recent (1.8 mya-present) Turritella (Sea Snails) Scaphites Ammonites (Perisphinctes) Brachiopods (Mucrospirifer) Graptolites Trilobites (Paradoxides) Tertiary Period to recent (65 mya-present) Cretaceous Period (150-65 mya) Mesozoic Era (245-65 mya) Devonian Period (410-360 mya) Cambrian to Carboniferous Period (510-320 mya) Paleozoic Era (most common in Cambrian Period) (540-245 mya) 53
Relative dating is the science of determining the relative age of rock layers and/or fossils within those rock layers based on the comparative positions of those rock layers. Relative dating does not determine the actual age of a rock layer. It determines the relative age of one rock layer in comparison to other rock layers. In other words, it determines which layer of rock is oldest, older, youngest, etc. 54
There are four important rules that help scientists date layers of rock: 1. Law of Superposition 2. Principle of Original Horizontality 3. Cross-Cutting Relationships 4. Intrusive Relationships 55
According to the Law of Superposition, sedimentary rock layers are laid down on top of each other. The bottom is the oldest and the top is the youngest. In order for the rock layer to form, a layer of rock must have existed below it. Therefore, lower or deeper layers are always older than higher or shallower layers. Youngest layer Oldest layer 56
The Principle of Original Horizontality states that sediment is deposited horizontally or in flat rows. Sediment is deposited on the surface of Earth. Over time, sediment accumulates and compacts to form sedimentary rock. Sediment is deposited this way due to gravity. 57
Faults exist in some layers of rock and can create fractures. The fractures can cause layers of rock to fold or slide. Cross-cutting relationships is a principle that states that a fault or fracture through rock layers must be younger than the layers through which it cuts through. A fault cannot cut through rock if the rock did not previously exist. 58
Sometimes magma from within Earth slowly pushes up through rock in Earth s crust. The magma cools and forms igneous rock. We call this an igneous intrusion. Intrusive relationships states that an igneous intrusion is younger than the layers of rock through which it intrudes or cuts through. 59
Sometimes geologic processes, such as erosion, can alter the size and shape of rock layers. Erosion can cause all or part of a rock layer to wear away. This can happen when a fault pushes rock layers or when moving water erodes away layers of rock. 60
What are the relative ages of the different layers of rock? Number them 1 through 5, with 1 being the oldest and 5 being the youngest. 61
What are the relative ages of the different layers of rock? Number them 1 through 5, with 1 being the oldest and 5 being the youngest. 3 5 2 4 1 62
Scientists have the ability to determine the age of rocks and fossils with a method called radioactive or radiometric dating. When this process involves carbon, we call this method radiocarbon dating. 63
Carbon is an important compound in the environment. It combines with other elements to form substances important to the environment and substances that make up living things. Carbon Dioxide Fossil Fuels Living Things Organic Matter 64
A very small percentage of carbon is radioactive. This carbon is called carbon-14. Radioactive carbon is unstable. Over time, it breaks down or decays to become stable, releasing radiation or excess energy. 65
A living thing takes in carbon from the environment, including radioactive carbon. It uses and incorporates the carbon in its body. When the organism dies, it stops taking in carbon. The carbon in its body remains until it decomposes. If it becomes a fossil, the carbon remains in the fossil. Radioactive carbon is also found in rocks. When the rock forms, some of the carbon is radioactive. 66
Carbon-14 breaks decays at a specific rate. We call this rate the half-life. A half-life is the amount of time it takes half of a radioactive sample to decay to a stable product. 67
Carbon-14 has a half-life of 5,730 years. This means that 100 gram of carbon-14 will take 5,730 to decay to 50 grams of carbon-14. The other 50 gram becomes a stable decay product (specifically, Nitrogen). 5,730 years 100 grams of carbon-14 50 grams of carbon-14 & 50 grams of Nitrogen 68
After a second half-life, half of the 50 grams of Carbon-14 becomes stable. Therefore, 25 grams of the original sample is Carbon-14 and the remaining 75 grams is Nitrogen. 5,730 years 50 grams of carbon-14 (& 50 grams Nitrogen) 25 grams of carbon-14 & 75 grams of stable Nitrogen 69
Scientists study the ratio of carbon-14 to its decay product in fossils to determine how old a fossil is. Studying this ratio helps scientists calculate how many half lives the fossil or rock has been through. 70
This half-life of carbon-14 is a long time but just right for radioactive dating fossils that are less than 50,000 years old. Scientists can use other radioactive elements found in living things and rocks to date objects that are older than 50,000 years old. Why do scientists use carbon-14 to date rocks and fossils? 71
Images obtained from commons.wikimedia.org courtesy: MarioProtIV, Timwether, MathewJParker, Heinrich Harder, Toby Hudson, Papa Lima Whiskey, Tiit Hunt, Nobu Tamura, Steve Parker, Sanjay Acharya, ScottRobertAnselmo, Mdf, Stefan Kraft, Scott Zona, Lillyundfreya, MatthiasKabel, MathKnight, Steev Selby, Didier Descouens, Wilson44691, Ballista, Michael S. Engel, Daniel Schwen, Eli Duke, Shyamal, Hannes Grobe, Pollinator, R.M.Hantemirov, Smith609, Silvan Leinss, CSIRO Artist s rendition of early Earth images obtained from NASA Tree core sample photos obtained from USGS.gov Other images obtained from the Public Domain Clipart by Stephanie Elkowitz 72