Unit C – The Changing Earth Pages 292 - 401. The history of our planet is one of change. There is evidence not only that Earths surface is changing but.

Unit C – The Changing EarthPages 292 - 401

The history of our planet is one of change. There is evidence not only that Earth’s surface is changing but that this change has, in turn, dramatically impacted the climate and life forms on Earth over time. In this unit, students examine scientific evidence for natural causes of climate change, for changing life forms and for continual changes to the Earth’s surface.

Unit Overview• Chapter 1 – The Abyss of Time

–Structure of the Earth–Plate tectonics–Rock cycle and the fossil record–Carbon dating

Unit Overview• Chapter 2 – A Tropical Alberta

–Fossilization–Formation of fossil fuels–Earthquakes and plate tectonics–Mass extinctions

Unit Overview• Chapter 3 – Changing Climates

–Rise of the mammals–Ice Age–Earth’s fluctuating climate

Chapter 1 – The Abyss of Time (pages 294 – 327)

1.1 – The Long Beginning (pages 296-301)

• The Earth is constructed of layers:–arranged according to density–densest material sinks to core–lightest material floats at surface

• The density of the Earth increases as you move towards the core.

• The core is made of iron and nickel

• Layers of the Earth:–crust/ lithosphere–asthenosphere–mesosphere–liquid outer core–solid inner core

Layers of the Earth

• Crust/ Lithosphere:–includes solid oceanic crust and

continental crust that floats on the asthenosphere

–outermost rigid layer of rock–125 km thick

• Mantle: –80% of Earth’s volume–solid layer –2550 km thick

• Mesosphere:–rigid in behavior–Lower layer of mantle

• Asthenosphere: –upper layer of mantle; 175km thick–“plastic” in behavior; can flow up

through crust

• Outer core:– liquid– made of iron and nickel–2260 km thick

• Inner core:–solid–made of iron and nickel–radius of 1220 km

Earth’s Layers

Density Description Thickness

Atmosphere least dense

most dense

- gaseous 300 km

-solid- most rigid layer

mantle

- least rigid or most plastic layer of mantle

- more rigid than uppermost mantle layer

core

outercore

innercore

• Deepest wells only scratch Earth’s surface

• Earthquakes help in developing theories of Earth’s structure

• Theorized that nuclear decay at core provides heat energy that drives flow of matter in mantle

• Hot materials become less dense and rise away from the core, cooling materials become more dense and sink back down

• Process of convection causes the crust to crack, tear and move

• Crust exists as “crustal plates” floating on asthenosphere

• Plates move a few centimeters per year• Movement has resulted in oceans and

mountains

Plate Tectonics• Plate tectonics is the theory that the

lithosphere consists of crustal plates that slowly move across the mantle and interact at their boundaries.

• Movement of crustal plates is due to convection currents in the mantle.

• The Earth has 15 major crustal plates

Convection in the Mantle Animation

• Sea floor spreading is due to plates separating at mid-ocean ridges.

• Youngest rock at spreading center, older rock as they move away

• Plate tectonics is confirmed by deep-sea drilling core samples

•magnetic properties of ancient rock shows magnetic fields in rock point in opposite directions

•Earth’s magnetic poles have reversed many times in Earth’s history

• When two oceanic crustal plates are moving apart the end opposite of spreading is pushed under neighbouring continental plate.

• Oceanic plate melts as it is forced down into the mantle.

• Why is the oceanic crust pushed under the continental crust and not vice versa?

• When two crustal plates have equal densities, one plate can’t slide under the other.

• Both plates weld together, pushing up huge rock wrinkles to form mountain ranges.

• At the site of the weld granite is formed.• Granite outcrops remain long after mountains

have eroded away• Outcrop: a part of a rock formation that

appears above the surface of the surrounding land

1.1 Summary• The Earth has settles into distinct layers

dependant on density: core, mantle, crust

• Nuclear reactions in core drive convection currents that push and pull the plates that make up the crust

1.2 – Early Life (pages 302-305)

• Sedimentary rock is formed from compressed layers of pre-existing rock or organic matter

• Properties of sediments and fossils preserved in each strata (layered band) provide evidence of past environments

• Sediments of Cameron Falls were deposited approx 1.5 billion years ago

• A fossil is the evidence or remains of ancient life preserved in Earth’s crust

• Oldest evidence of life dates back to 3.8 billion years

• Cyanobacteria are oldest known life–Microscopic photosynthetic single-celled

bacteria

• Early Earth was very hostile:–Frequent volcanic eruptions–Poisonous gases (methane, hydrogen

sulfide)–Oceans were above 100°C–Very little oxygen gas

• Archaea thrived in these conditions

• 1.5 billion years ago, Alberta was a tropical coastal area

• Presence of stromatolites in Alberta indicates that cyanobacteria lived in shallow waters along the coast of ancient Alberta

• Stromatolites are layered structures built by cyanobacteria

• Growing and dying cyanobacteria slowly deposited layer upon layer of calcium carbonate (limestone), leaving large mounds

• Fossilized stromatolites are called “trace fossils” because they are the remains of the cyanobacteria and not the organism itself

• Stromatolites are Alberta’s oldest fossils

• Cyanobacteria use chlorophyll to make glucose from the Sun’s energy, water and carbon dioxide

• Oxygen is a by-product of photosynthesis• Cyanobacteria played a key role in changing

the Earth’s atmosphere

• The creation of an oxygen-rich atmosphere is one of the most significant events in geological time

• Impact on the evolution of future life• Impact on Earth’s geology

• Oxygen readily reacts with metals to form compounds

• Banded iron; alternating bands of red and grey–Red is iron (III) oxide; Fe2O3(s)

–Grey is silica and other minerals

• Dissolved iron ions carried to ocean react with free oxygen

• If oxygen is present, iron (III) oxide is formed

3O2(g) + 4Fe(aq) 2Fe2O3(s)• Iron (III) oxide is insoluble and sinks to the

bottom of the ocean• Iron (III) oxide acts as a chemical indicator of

oxygen in Earth’s early atmosphere

Explanation of banded iron formation

• Late in the Precambrian Era, life barely survived:–10 million year ice age; termed “snowball

Earth”–Small pockets of liquid by thermal vents–Freeze and thaw at the end of ice age may

have lead to Cambrian explosion – huge increase in biodiversity and complexity of life

Snowball Earth Clip

1.2 Summary• The first producers, cyanobacteria,

transformed the Earth’s atmosphere• Cyanobacteria left behind stromatolites,

Alberta’s oldest fossils• Banded iron shows an increase in free oxygen• The stage was set for dramatic biodiversity

following snowball Earth

Challenge Question (page 305, #8)Shown is a 700 million-year-old sedimentary rock in Namibia. The big embedded rocks are the type that glaciers pick up and carry with them as they flow.a) Define the climate theory supported by these glacial

deposits.b) Just above the geologists hands are sedimentary layers

of limestone known only to be deposited in warm seas. What do these limestone deposits indicate?

1.3 – Strange Rocks (pages 306-313)

• Nicolas Steno wondered how fossilized shark teeth could end up embedded in stone high above sea level.

• What processes might have been involved?

• Law of superposition: higher strata in a sequence of rock layers are younger than lower strata

• The law of superposition:–Proposed by Nicolas Steno–Gives geologists a way to keep track of order

in which rock layer forms (relative dating)–Relative dating in the process of placing

rocks and geological structures in the correct chronological order

–Pattern of rocks in a strata is called the stratigraphic sequence

• Intrusion:–A body of a rock that forms from the

invasion of magma into a pre-existing rock formation

– Intrusion is younger than surrounding rock since it was formed by molten rock forcing its way through pre-existing rock

–Exception to the law of superposition

The Formation of Sedimentary Rock

• Limitation of relative dating is that it does not reveal the absolute age of events or fossils

• Absolute age: the number of years that have elapsed since an event occurred

Question: Determine the relative age of the lava and the road.

Question: Explain why you cannot precisely determine the absolute age of the road or the lava on the road

• Law of superposition was used by early geologists to rank strata and the fossils contained in them in chronological order

• 100 years after Steno, William Smith observed reoccurring fossils at multiple survey sites

• Smith argued that the rocks containing the same fossils must correspond closely in age

• These distinct fossils are like an index• Index fossil: a fossil used to determine the

relative age of a layer in a stratigraphic sequence or to match stratigraphic sequences from different locations

• Index fossils allowed Smith to publish the first geographical map of England

• What makes an index fossil useful?–Appears only briefly in geological time–Has a wide geographical distribution–Easy to recognize

• During 19th century (1800s) geologists used index fossils to assemble a generalized relative time scale for all of Earth

• Called the Geological Time Scale• First time for a unified history of the Earth

Geological Time Scale:• Divided into 4 major eras

–Precambrian–Paleozoic–Mesozoic–Cenozoic

• Major eras are broken down into periods

• Some periods are broken into epochs

Era Period Epoch

1.3 Summary• Fossils are the remains of once living things• Law of superposition – rock layer is

younger than those below it• Discovery of index fossils lead to the

Geological Time Scale• Eras Periods Epochs

1.4 – Getting a Handle on Time ( pages 314 – 318)

• Catastrophism is a theory that cites major violent disasters as the main forces that shape Earth

• Believed that these processes were of different type or intensity than observed today

• Fit into the understanding of the day• Common belief was that the Earth was

several thousand years old• Belief that “present” day changes would

have been too gradual to result in the many geological formations

• James Hutton considered “father of modern geology”

• Formulated the theory of uniformitarianism• Uniformitarianism: the principle that the

geological processes in action today have always fundamentally operated in the same way throughout Earth’s history

• Hutton noticed vertical columns beneath horizontal strata

• Layers of unconformity where there was no apparent pattern

• Hypothesized vertical columns used to be horizontal but were tilted and followed by periods of erosions and finally more sediments

The Rock Cycle• Hutton suggests that the Earth operates on a

self-sustaining system driven by subterranean fire

• Hutton’s thinking started the modern understanding of how rocks form

• Types of rock:–Sedimentary–Igneous–Metamorphic

• Sedimentary – consists of eroded fragments of other rock types–Layers of sediments are compressed–Formed at the surface of the Earth under

low temperatures–Examples: sandstone, banded iron

• Igneous rock: forms when molten magma intrudes into the crust or extrudes onto the surface–Formed deep in the crust or mantle under

extreme heat–Entire mantle consists of igneous rock–Examples: granite, basalt

• Metamorphic rock: forms when sedimentary or igneous rock is transformed at molecular level by intense heat and pressure–Formed at the sites of collision between

crustal plates–Examples: slate, marble, quartzite

• The rock cycle: the continual change of rocks from one type to another.

• Driven by energy at Earth’s core.

• Charles Lyell used the scientific process to support and strengthen Hutton’s theory of uniformitarianism

• Argued the processes responsible for present day formations have always operated in same manner

• Helped build Geological Time Scale• Great influence on Charles Darwin

1.4 Summary• Studying Earth’s history is difficult

because it occurs on such a large scale• Billions of years recorded in thousand

of layers• Given enough time processes have

changed Earth many times

1.5 – Pinpointing Time (pages 319 – 324)

• Geologists in the late 19th century generally agreed that the Earth was millions of years old, but there were no accurate methods to measure the absolute age of Earth

• Marie Curie discovers radioactivity during the early 20th century

• Discovery of radioactivity leads to a new and accurate method for measuring the absolute age of rocks.

• Radioactivity: the emission of energy from the nuclei of unstable atoms as they change to become more stable atoms

• Ernest Rutherford discovered that the energy emitted from radioactive materials was in the form of high-speed particles.

• Intensity of radiation is measured by detecting the number of particles emitted per second

• Rutherford discovered the property of radioactive decay

• Every 55.6 s the radioactivity of radon-220 decreased by half

Time elapsed Radioactivity0 time 100%

55.6 s 50%

55.6 s (x2) 25%

55.6 s (x3) 12.5%

• The constant time increment for half of a radioactive sample to decay is called its “half- life”

• Half-life constant is specific to isotope

• Size of radioactive sample does not affect half-life

• Radioactive decay graphs have an exponential curve

• The % of remaining radioactive material never reaches zero, but gets infinitely close!

• Atoms are radioactive because they are unstable

• Radon-220 spontaneously changed into polonium-216 by losing 2 p+ and 2 no

• Original unstable atom is called parent isotope

• Stable product is called daughter isotope

• The rate of radioactive decay is not affected by heat, cold or pressure

• All radioactive elements decayed like clockwork – the half-life always elapsed at constant intervals

• Because radioactive decay of an element occurs at a fixed rate (half-life) the decay process can be used to measure the time passed since a rock or fossil formed

• The invention of the mass spectrometer has allows scientists to detect the elements and their isotopes that are present in a sample of rock

• The percentage of each isotope present in a sample can be determined

To determine the age of a sample:1) Determine the parent and daughter

isotopes by using the table2) Determine % of each 3) Use decay curve to determine the number

of half-lives that have elapsed4) Look up the amount of time for each half-

life for that element. Multiply it by the number of half-lives that have elapsed

• Tiny crystals called zircons are used to date the rock that they are found in

• Zircons contain uranium and are very durable, making them ideal for radioactive dating

• The uranium clock is set at zero when the crystal forms and begins to decay from that point onward

Dating Organic Remains• Carbon-14 is a rare isotope that is created

high in the atmosphere when nitrogen-14 is bombarded by cosmic radiation

• The carbon-14 is incorporated into plants through the process of photosynthesis

• The carbon-14 atoms then make their way into the food chain

• When an animal dies the carbon-14 clock is set at zero because dead animals don’t ingest carbon.

• If the animals remains are preserved, the date can be determined by measuring the amount of carbon-14 remaining

1.5 Summary• The absolute age of rocks and fossils can

be determined through radioactive dating• The invention of the mass spectrometer

has lead to more accurate predictions of age

• Dates can now be assigned to rock layers that contain radioisotopes

Ocean seismic technology animation