Remember: Camels Often Sit Down Carefully. Perhaps Their Joints Creak. Possibly Early Oiling Might Prevent Early Rheumatism.
CSET Requirement 2.1A: Diagram the major divisions of the geologic time scale as a basis for understanding changes in the Earth's processes.
THE GEOLOGIC TIME SCALE
It's best if you diagram this on your own, listing time range, biological characteristics and major events.
This scale is marked by events of noticeable change, such as catastrophic geological impacts (extinctions), and biological change (Dinosaur farts anyone?). ( You might notice inconsistencies in dating time periods, eras and eons. Please be careful where you obtain your data, mine is based on the International Commission on Stratigraphy. Just open a couple of textbooks and you will see what I'm talking about.)
PRE-CAMBRIAN (SUPER EON) (4.6 billion years ago to 542 million years ago)
Contains the Hadean, Archean and Proterozoic Eon and is the time between the birth of the planet and the appearance of complex forms of life (bacteria). More than 85% of the Earth's estimated 4.6 billion years fall into this time span (mid-October in our year analogy).
Hadean Eon or hell-like environment (hades). Imagine oceans of liquid rock, water vapor, boiling sulfur and impact craters everywhere. Any newly formed rock was quickly melted, covered, or blasted away. The air is thick with carbon dioxide, nitrogen and sulfur. This is the time believed that an asteroid the size of Mars struck Earth and formed the Moon. Terrestrial rocks of this era have not been found, only lunar rock samples and meteorites are this old.
Archean Eon (ancient) begins about 3.8 billion years ago. The chronological boundary here is marked by significant cooling down of Earth's surface. Water vapor in the air has cooled and condensed to form oceans. Carbon Dioxide is changed into limestone and at the bottom of the newly formed ocean. The air is rich with Nitrogen and lacks "free" oxygen. At the end of this Eon, tectonic movement is similar to today's Earth where "islands" huge land masses have began to move. During the Archean Eon, single celled organisms are present in the form of cyanobacteria (important in releasing free oxygen into the atmosphere).
Proterozoic Eon (early life) begins 2.5 billion years ago and finishes 542 million years ago. This time segment exists before complex life evolves. Important events in this eon is the occurence of an oxygenated atmosphere, glacial formations (Snowball Earth), and the abundance of soft bodied multi-cellular organisms (eukaryotes). The time boundary between the Proterozoic and Phanerozoic eons is marked by the appearance of animal fossils (trilobites).
Phanerozoic Eon contains eras that we need to learn about. Previous Eons contained separations in eras which I will not cover but you should be aware of those divisions. The Phanerozoic Eon covers our present time and means "visible life." This eon contains abundance in life, emergence of terrestrial and complex plants, emergence and variance of terrestrial animals,marked by extinctions and the convergence of land masses into Pangea and its subsequent separation into current locations.
Paleozoic Era (542 to 251 mya) means "Age of Ancient Life" and has significant geological, biological and climatic change and evolution
and ends with a mass extinction know as the Permian-Triassic extinction. Gondwana is in process of formation during this era (510
-180 mya) leading to Pangea formation.
Cambrian Period (542 - 488.3 mya). Prior to this time, organisms were small and unicellular (prokaryotes). During this period the
Cambrian explosion occurred which resulted in a variety of multi-cellular organisms (eukaryotes). It should be carefully noted that
eukaryotes already existed prior to the Cambrian period, but it was during this time that eukaryotes multiplied exponentially. Land was
still barren leaving most living organisms present at sea. Arthropods developed during this period and still make up around 80% of
living animal life today. Trilobites flourished but were widely killed during the Cambrian-Ordovician extinction event (there are two other
extinctions prior to this one) which ended the period.
Ordovician Period (488.3 - 443.7 mya) Trilobites continued to thrive during this period, biological life also flourished such that
arthropods are now joined by mollusks and the first fish appear which mark the arrival of vertebrates with evidence of the appearance
of jawed organisms. This period is also where tectonic movement resulted in Gondwana heading south and the formation of the
Appalachian mountains (480 mya). The Ordovician-Silurian Extinct Event (second largest and first in record of five major extinctions)
characterized by one of the coldest times in Earth's geological history.This extinction is described as a series of events where glaciation
(colder temperatures and locking up of ocean waters) and interglaciation (warmer climatic temperature and release of ocean waters)
caused sea levels to repeatedly rise and fall severely affecting ecological niches and biological diversity.
Silurian Period (443.7 - 416 mya) Most significant during this period is the evolution and appearance jawed and bony fish; corals and
other invertebrates are dominant; transition of animals from sea to land; warm shallow seas cover much of North America; first land
plants form and first insects appear. Gondwana now covers a good chunk of the southern hemisphere and extends up to the equator
(not uniformly of course). If you noticed, when comparing time spans, this period is the shortest so far. Tiny vascular plants grew along
shorelines. Centipedes and scorpions were among the first animals to make their transition from sea unto land. The end of the period is
marked by The Lau Event Extinction, a minor one in magnitude with major impact on fauna.
Devonian Period (416 - 359.2 mya) Period characterized by the development of plants. Ferns, horsetails and seed plants appeared,
first trees and forest formation occurs. This period is significant for the increase in fish variation and is referred to as the "Age of Fish."
Gondwana is now accompanied by the slow formation of Euramerica (Laurussia) and both land masses start to meet at the equator
marking the early formation of the much larger Pangea. The period is closed by an extinction more drastic than that of the Cretaceous
(Dinosaur era). This is one of the five major extinctions of geological history whose cause is still in dispute but theory exists that an
asteroid impact severely affected shallow marine life, trilobites, ammonites and jawless fish.
Carnoniferous Period (359.2 -299 mya) Period subdivided into Mississippian and Pennsylvanian. Carboniferous means "coal
bearing" named like this because of large underground coal deposits that date to it. This coal was produced by bark trees that grew
in large quantities. Giant club mosses, ferns, giant trees and large insects were present. These plants removed large amounts of
carbon dioxide from the atmosphere and produced large amounts of oxygen (35% in comparison to today's 21%). This explains the
giant centipedes (six feet long), dragonflies (2.5 feet), huge cockroaches and scorpions; the growth of these animals is said to be
limited by the amount of oxygen available to them. Amphibians grew in size and variety. By the middle of this period an extinction
occurred as climate changed from hot and humid to cool and arid affecting tropical forest which collapse and animals that depended on
it such as amphibians. The development of amniote eggs by amphibians during this time lead to the appearance of the first "true
terrestrial vertebrate" : reptiles.
Permian Period (299 - 251 mya) Pangea has taken form and characterized by a vast arid interior. Reptiles have been able to survive
given better coping mechanisms in that environment. Their survival led to further evolution of the amniotic egg which will be significant
for later animals such of mammals. This period was brought to a conclusion by the largest extinction ever experienced on Earth where
90% of marine life and 70% of terrestrial species died out. It took ecosystems more than 10 million years to recover . Causes for the
Permian-Triassic Extinction vary from carbon dioxide accumulation in the atmosphere due to prolonged eruptions which raised
temperatures everywhere, supernova radiation, methane release from oceans, and deterioration of the ozone layer.
Mesozoic Era (251 - 65.5 mya) This era is delineated by massive extinctions that brought the end and beginning to bordering eras. Most
notable and summarized below are the development and dominance of reptiles, their extinction and the breaking up of Pangea. Meso means
"middle" and zoic "living beings." This is the "Age of Reptiles." This era is divided into three periods: Triassic, Jurassic, and Cretaceous.
Triassic Period (251 - 199.6 mya) A nearly 50 million year period where 10 million years involve recovery of ecosystems due to
previous extinction. Reptiles were most fit to survive and developed with the appearance of the first dinosaurs. The first true mammal,
flying vertebrate and cycad also appear during this time. Pangea begins to break up half way through this period forming two large
land masses: Laurasia to the North and Gondwana to the south. Another large extinction brings the Permian to a close. This event is
referred to as the late Triassic-Jurassic Extinction Event and is claimed to have been responsible for half marine invertebrate life.
Tectonic movement has been hypothesize for this event (the breaking up of Pangea).
Jurassic Period (199.6 - 145.5 mya) The true age of dinosaurs begins here and also responsible for giving the Mesozoic Era it's title:
"Age of Reptiles." As a result of the breaking up of Pangea, more coastlines appear creating a humid environment replacing what was
commonly arid into a tropical rainforest. Dinosaurs reach their peak during the Jurassic and the birds branch out from theropods.
Mammals were also present during this time. No significant extinction event marks the end of this period.
Cretaceous Period (145.5 - 65.5 mya) Dinosaurs continue to dominate on land, groups of mammals, birds and angiosperms appear
and develop. Gondwana separates into present day Australia, Antarctica, South America, Africa and India. Laurasia breaks up into
North America and Europe. The impact of a meteor or comet is widely accepted as the reason for Creataceous-Paleocene (K-T)
extinction event which ended with large animals (Dinosaurs) and organisms like plants that relied on photosynthesis were also affected
by the reduction of solar energy on Earth's surface. The lack of vegetation caused herbivores, carnivores to die and omnivores,
insectivores, and carrion eaters to survive.
Cenozoic Era (65.5 mya - present) The "New Life" era is the current and most recent geological time. It took 1 to 4 million years for
ecosystems to recover from the K-T extinction event. Mammals replaced reptiles as dominant land animals which were helped by the
dominance of angiosperms. The absence of large reptiles allowed mammals and birds to diversify with mammals occupying any available
niche. Hence the moniker "Age of Mammals."
Paleogene Period (65.5 - 23.03 mya) This era is notable for the evolution of mammals from small simple forms into a large group
of diverse animals.Hot and humid conditions that existed during the late Mesozoic era gave way to cool and dry climate. Continents
continued to settle unto their current positions and India collided with Asia forming the Himalayas. As Earth cooled, tropical forests
became restricted to equatorial regions. The arrival of the first grass species which expanded and formed new ecological environments
such as savannas and prairies.
Neogene Period (23.03 - 2.58 mya) Mammals and birds continue to evolve into current species unlike other organisms which
remained relatively stable. Geologically North and South America join at the isthmus of Panama separating the Atlantic and Pacific
oceans affecting ocean currents and global climate conditions and forming the Gulf Stream. Global climate cooled during this time
leading to glaciations on the next period. Grassland animals diversify to the new environments such as savannas. Later during the
Pliocene Epoch, these grassland animals began to die as the climate undergoes a cooling trend.
Quartenary Period (2.58 mya - present) This is the most recent of the three periods under the Cenozoic Era in which glaciations
occur (ice ages) and the appearance and expansion of modern humans. Human beings begin to dictate our next period with their
impact on the natural world. Geologically, much movement is not perceived during this short time but significant shifts such as land
bridge between Europe and Great Britain, and a connection of North America with Asia existed with the Bering Strait. Glaciations (ice
ages) occur leading to the disappearance of many mammals from North America (horses and camels).
Geologist have divided geologic history into EONS, ERAS, PERIODS and EPOCHS. This scale subdivides 4.6 billion years of Earth geologic history based on key terms you need to know. Western Europeans used RELATIVE DATING at first. ABSOLUTE DATING, and RADIOACTIVE DATING were methods later used to fine-tune the time scale. The law of superposition, original horizontality, inclusion and unconformities will be discussed under "Geologic Structures." These are important in this section but most crucial are forms of dating and how scientist came up with the Geologic Time Scale. We need to link these to our understanding of Changes in the Earth's Processes or factors that affect how Earth has changed and is still changing.
4.6 billion years is an astronomical number to us humans in sense of time. Many analogies have been discussed to understand this number, but my favorite is the year, months, days, minutes and seconds comparison. It goes likes this: Say that Earth's geologic history is compressed in one year. Earth formed on January 1st. First evidence for life we have shows up around the beginning of March. Multicellular organisms show up in the middle October with humans not showing up until 11 minutes before midnight on the last day of the year (Dec 31st).
So how did scientist come up with the number 4.6 billion for the beginning of Earth's geologic history?
RELATIVE DATING: This method of dating does not tell us exactly when something occurred but only the sequence when something took place or when an event followed or preceded another. This is not and outdated method and still used, however it is not precise in the measure of time. Rate of sediment deposition was used to gauge geologic time putting estimates of Earth's beginning from about 3 million years to 1.5 billion. Ocean salinity was also use to gauge the passage of time but inconsistencies deemed this method inaccurate. The Law of Superposition sequenced geological historical events. The law simply states that in a sequence of sedimentary rocks, each layer is older than one above, and younger than one below. Relative dating did not to give an absolute age of Earth.
ABSOLUTE DATING: This is the process of assigning a precise numerical age to an organism, object or event. Here, technology is important as in many cases in the scientific field. Radiometric Dating is a procedure that measures radioactive decay of rocks and minerals to calculate age. Some atoms in minerals of rocks are unstable. Remember that atoms contain neutrons, protons and electrons. Protons and neutrons are important to understand radioactive decay since it's these two that are in the nucleus and combined give us the mass number. Protons give us the atomic number of an element. Atoms of the same element always have the same number of protons. However, the number of neutrons in the nucleus can vary; these are called isotopes. Some isotopes have unstable nuclei; this means that the forces that keep protons and neutrons together are not strong enough to maintain the integrity of the nucleus. As a result, the nuclei breaks apart or DECAYS at a constant rate. This rate of decay is measured to obtain absolute dating. Let's conclude by saying that certain elements have isotopes that are measured and that these "Parent" atoms decay and produce new "Daughter" products. In this way Carbon -14 turns to Nitrogen -14, Uranium -238 -> Lead -206 (Potassium -40 -> Argon -40). Every radioactive isotope has a 1/2 life or HALF LIFE. This is the time it takes for a half of the radioactive Parent sample to decay to it's stable Daughter. This fact is key to radiometric dating. So the Half-life it takes Carbon -14 to Nitrogen -14 is 5,730. This isotope is useful for dating organic Earth material which contain this element. However, for older measurements, isotopes such as Uranium -238 are measured which can range up to 4.6 billion years. In conclusion radiometric dating of terrestrial, meteorite and lunar rocks gave us the 4. 6 billion years.