Lesson 9: Stratigraphy and Geologic Time Flashcards
What is the Principle of Superposition?
the tendency for rock layers to be chronologically stacked
the oldest rocks are at the bottom, and the layers become increasingly younger towards the top
What are exceptions to the Principle of Superposition?
igneous rocks that form from volcanic activity may vertically cut through horizontally arranged layers of rocks, and mountain building events may tilt, fold, and even flip rock layers
What is stratigraphy?
the science of using the arrangement and composition of rock layers to interpret geological history
What is a formation?
a large uninterrupted sequence of rock that is made of multiple layers that all share similar properties (such as mineral composition and average sediment grain size) and that formed under similar conditions
when a sequence of rock changes from one formation to another, it indicates that a large scale change occurred in the environment where the rocks were being deposited
What is radiometric dating?
used to age rocks in absolute terms
What is an isotope?
a varient of a chemical element that has an unusual number of neutrons
some isotopes are unstable and will undergo radioactive decay, whereby energy is released and a new atom (or atoms) with a different composition of particles results
these resulting atoms with different particle compositions are called the decay products
at what time a single isotopic atom will undergo radioactive decay is unpredictable, but a large collection of isotopes will radioactivity decay at a mathematically predictable rate
What happens to isotopes and decay products as a rock ages?
when a new rock forms, it has a ratio of isotopes and decay products that matches that of the environment
as the rock ages, the isotopes decay and the ratio of isotopes to decay products decreases
using a special machine called a mass spectrometer, it is possible to measure the isotope ratio of a rock, and this ratio can tell you how long ago the rock formed
Why can’t sedimentary rock undergo radioactive decay?
sedimentary rocks are never really “new”, that is, they are made of sediments that had already formed and that were already potentially undergoing radioactive decay
How can we tell how old a fossil and its rock layer is?
by combining radiometric dating and the principle of superposition
if sedimentary rocks that contain fossils are found between two horizontally deposited layers of igneous rocks, then dating the igneous rocks above the sedimentary layer will tell us what age the fossils must be older than, and dating the igneous rocks below the sedimentary layer will tell us what age the fossils must be younger than
so, we can confidently bracket the age of the fossils
What is the geologic time scale?
is a standardized series of chronological divisions that parses the Earth’s history into discrete named units
the largest units in the time scale are Eons, followed by Eras, Periods, and Epochs
What was the Hadean Eon?
4.6 to 4 billion years ago
is named for Hades (Greek god of the underworld)
by the beginning of this eon, the rest of the universe was already over nine billion years old
at roughly 4.5 billion years ago, the young earth collided with a smaller planetoid
this collision ejected a large mass of debris, which was held in orbit by the earth’s gravity and eventually formed the moon
by the end of the Hadean, the earth had cooled and large oceans covered much of its surface
complex organic molecules are thought to have formed in these early oceans and possibly the earliest true life forms
the oldest rocks on earth have been dated at only about 4.4 billion years old, though rocks discovered on the moon are older
What was the Archean Eon?
4 to 2.5 billion years ago
the oldest known fossils come from the Archean Eon
these fossils are of simple single-celled organisms
more advanced forms later evolved in the Archean, including cyanobacteria
the cyanobacteria were photosynthetic and eventually produced large amounts of oxygen gas, which became concentrated in the earth’s atmosphere
some cyanobacteria formed structures called stromatolites, which are some of the best records of early life
stromatolites look like lumpy stones, but when you cut them in half you can see the layers that were created as the cyanobacteria secreted sticky films that trapped particles of sediment
What was the Proterozoic Eon?
2.5 billion to 541 million years ago
at approximately 1.7 billion years ago, the first multicellular organisms evolved
because single-celled and early multicellular life had no bones or other hard parts and was usually microscopic, the fossil record of this early life is poor
within the Proterozoic, the time span from 630 to 542 million years ago is known as the Ediacaran Period
during the Ediacaran, large forms of life with some harder parts evolved, including the first animal life
What was the Phanerozoic Eon?
531 million years ago to now
is subdivided into three era, which are themselves subdivided into numerous periods
it is during the Phanerozoic that animal life rapidly evolved into a multitude of diverse forms, including dinosaurs
What was the Paleozoic Era?
541 and 252 million years ago
at the start of the Paleozoic, animal life was restricted to primitive invertebrates living in the oceans, but, by its close, great forests covered the land and teamed with reptiles, amphibians. and insects
What was the Cambrian Period?
541 to 485 million years ago
the beginning of the Cambrian marks such a dramatic diversification of aquatic animal life that is often referred to as The Cambrian Explosion
sponges, mollusks, worms, and many kinds of arthropods (including trilobites) evolved
a close early relative of the vertebrates, called Pikaia, didn’t have vertebrae, but had several other features found in vertebrates
What was the Ordovician Period?
485 to 443 million years ago
global sea levels were high
life in the oceans continued to diversify, with fish increasingly becoming the dominant large aquatic animals
What was the Silurian Period?
443 to 419 million years
until this point, fish had not yet evolved jaws
with the evolution of jaws came the evolution of large predatory fish
primitive plant life began to flourish on land
What was the Devonian Period?
419 to 359 million years ago
the first forests appeared on land
huge jawed fishes, like Dunklosteus, evolved in the seas, and the first true sharks appeared
lobe-finned “fishapods”, like Tiktaalik, ventured onto land and give rise to the tetrapods
What was the Carboniferous Period?
359 to 299 million years ago
amphibians were widespread in the abundant swamps, and reptiles, the first amniotes, evolved
much of the coal that is mined today formed from the rotting plants of Carboniferous swamps
What was the Permian Period?
299 to 252 million years ago
the continents collided together and formed a single super continent called Pangaea
reptiles evolved into three main lineages: the anapsids (which would go on to evolve into turtles), the synapsids (which go on to evolve into mammals), and the diapsids (which would go on to evolve into lizards, snakes, crocodilians, and dinosaurs)
many of the terrestrial rocks from this period of time represent dry, desert environments
the single greatest mass extinction in our planet’s history occurred at end of the Permian, with one of the most notable losses being the trilobites
What was the Mesozoic Era?
252 to 66 million years ago
often referred to as the Age of Dinosaurs
it is during this time that dinosaurs evolved and become the dominant form of large terrestrial animal life
many kinds of marine reptiles evolve, including the ichthyosaurs, plesiosaurs, and mosasaurs
the first true turtles, crocodiles, lizards, snakes, mammals, and bird evolved at this time as well
the first flowering plants evolved towards the end of the Mesozoic Era
What was the Cenozoic Era?
66 million years ago to now
is often referred to as the Age of Mammals
although mammals had been around since the Triassic, the extinction of the dinosaurs (except for birds) allowed mammals to evolve larger forms and to fill many new ecological roles
grasses only became abundant around this time
What was the Paleogene Period?
66 to 23 million years ago
global temperatures began to cool
mammals diversified into a variety of new forms, including primates, bats, and whales
birds also diversified
What was the Neogene Period?
23 to 2.6 million years ago
global temperatures continued to cool
the first hominids evolved in Africa
What was the Quaternary Period?
2.6 million years ago to now
the earth experienced several large glaciation events, or “ice ages”
the first anatomically modern humans evolved
human civilization spread
What was the Triassic Period?
252 to 201 million years ago
during the first ten million years of the Triassic, life gradually recovered from the mass extinction that occurred at the end of the Permian
the first mammals and dinosaurs evolved during the later portion of the Triassic, and so did the first pterosaurs (the first vertebrates to fly)
the supercontinent Pangaea began to break apart
many of the dinosaurs from this period of time look fairly similar to each other
What are the characteristics of Ichthyosaurs?
the name literally means “fish lizard”, but ichthyosaurs are not lizards, and they are certainly aren’t fish
they are a group of reptiles that took on a fish-like lifestyle and evolved to a very fishy body form
the ancestors of ichthyosaurs were fully terrestrial reptiles, but, just like the ancestors of modern whales, dolphins, seals, and sea turtles, the group found success by making an evolutionary return to water
to adapt to an aquatic life, ichthyosaurs evolved paddle-like front and hind limbs, a finned tail, and even a shark-like dorsal fin
they never evolved gills and needed to come to the surface in order to breathe air
What are the characteristics of Plesiosaurs?
had large chests and torsos, broad paddle-shaped limbs, and relatively short tails
in front of their shoulders, plesiosaurs varied tremendously
some had short necks and huge jaws, others had elongated serpentine necks with small heads
What are the characteristics of Pterosaurs?
commonly called “pterodactyls”
close relatives of dinosaurs who branched off from the reptilian family tree at roughly the same time in the Triassic that dinosaurs did
they were the first vertebrates to fly
have membranous wings supported by a single extremely elongated finger
What was the Jurassic Period?
201 to 145 million years ago
dinosaurs diversified, this was the peak of sauropod diversity, and they were the dominant terrestrial herbivores
small and medium sized ornithopods, like Allosaurus, were the dominant terrestrial carnivores
the stegosaurs are almost completely restricted to the Jurassic, and the first ankylosaurs, ornithopods, and ceratopsians appear at this time, although they are not particularly abundant or diverse
the first birds evolved
What are the characteristics of Pterodactyloids?
differed from rhamphorhynchoids in the morphology of their tails, which were short, and the carpels in their wrists, which were elongated and made a greater contribution to the length of the wing
many had large head crests, which were presumably display structures
there were many species of small pterodactyloids, some smaller than a robin, but some species had wingspans of over ten meters, making them the largest animals to ever fly
What was the Early Cretaceous Period?
146 to 100 million years ago
dinosaurs continue to diversify and the first flowering plants evolved
new therapods, like spinosaurids and carcharodontosaurids evolve, coelurosaurian theropods became more diverse, and iguanodonts become larger and more adundant
What are the characteristics of Mosasaurs?
were relative of modern monitor lizards and snakes
like ichthyosaurs and plesiosaurs, mosasaurs had tail fins and limbs modified into paddles, but the bodies and tails of mosasaurs were more elongate
many mosasaurs are the right size to have preyed on small and medium sized fish, but some were true sea monsters with huge jaws and bodies over eighteen meters long
these aquatic giants seem adapted for deep-sea big-game hunting, and they likely ate large fish and other marine reptiles
What was the Late Cretaceous Period?
100 to 65 million years ago
often considered the apex of non-avian dinosaur diversity, many of the most famous dinosaurs come from this period of time
the coelurosaurian theropods are abundant and diverse in the northern hemisphere, and include the tyrannosaurs, ornithomimids, therizinosaurs, oviraptorosaurs, dromaeosaurids, troodontids, and many more interesting clades
a large meteor collided with the earth, and this event along with its catastrophic consequences is though to have brought about a mass extinction, which killed all non-avian dinosaurs
