Lesson 9: Stratigraphy and Geologic Time

Question 1

What is the Principle of Superposition?

Answer 1

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

Question 2

What are exceptions to the Principle of Superposition?

Answer 2

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

Question 3

What is stratigraphy?

Answer 3

the science of using the arrangement and composition of rock layers to interpret geological history

Question 4

What is a formation?

Answer 4

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

Question 5

What is radiometric dating?

Answer 5

used to age rocks in absolute terms

Question 6

What is an isotope?

Answer 6

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

Question 7

What happens to isotopes and decay products as a rock ages?

Answer 7

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

Question 8

Why can’t sedimentary rock undergo radioactive decay?

Answer 8

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

Question 9

How can we tell how old a fossil and its rock layer is?

Answer 9

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

Question 10

What is the geologic time scale?

Answer 10

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

Question 11

What was the Hadean Eon?

Answer 11

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

Question 12

What was the Archean Eon?

Answer 12

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

Question 13

What was the Proterozoic Eon?

Answer 13

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

Question 14

What was the Phanerozoic Eon?

Answer 14

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

Question 15

What was the Paleozoic Era?

Answer 15

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

Question 16

What was the Cambrian Period?

Answer 16

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

Question 17

What was the Ordovician Period?

Answer 17

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

Question 18

What was the Silurian Period?

Answer 18

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

Question 19

What was the Devonian Period?

Answer 19

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

Question 20

What was the Carboniferous Period?

Answer 20

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

Question 21

What was the Permian Period?

Answer 21

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

Question 22

What was the Mesozoic Era?

Answer 22

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

Question 23

What was the Cenozoic Era?

Answer 23

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

Question 24

What was the Paleogene Period?

Answer 24

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

Question 25

What was the Neogene Period?

Answer 25

23 to 2.6 million years ago

global temperatures continued to cool

the first hominids evolved in Africa

Question 26

What was the Quaternary Period?

Answer 26

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

Question 27

What was the Triassic Period?

Answer 27

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

Question 28

What are the characteristics of Ichthyosaurs?

Answer 28

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

Question 29

What are the characteristics of Plesiosaurs?

Answer 29

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

Question 30

What are the characteristics of Pterosaurs?

Answer 30

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

Question 31

What was the Jurassic Period?

Answer 31

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

Question 32

What are the characteristics of Pterodactyloids?

Answer 32

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

Question 33

What was the Early Cretaceous Period?

Answer 33

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

Question 34

What are the characteristics of Mosasaurs?

Answer 34

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

Question 35

What was the Late Cretaceous Period?

Answer 35

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