Geologic Time and the Fossil Record

Earth is 4.6 billion years old and has changed significantly over that time, mostly through processes that we have studied.

-Astronomical events

-Plate tectonics

-Weathering and erosion

-Atmospheric gas exchange and movement

These same processes have been working since earth was a molten ball with no atmosphere and continue to work today. This theory is called uniformitarianism.

Sudden change does take place after catastrophes, but they only increase the rate at which the earth is already changing.

There is plenty of evidence to demonstrate how the earth has changed. Most of it is in the rocks. The people who study them are called geologists.

-People who study past life through such methods are called paleontologists.

There are two primary methods to decide how old a particular rock is. One looks at the layering of sedimentary rock and the other looks at the radioactive decay of isotopes in the rocks.

After rock erodes, it is deposited somewhere. Newer deposits are left above older ones, so by looking at how an area of rock is layered, one can determine their relative age.

-This principle is called superposition, and it says that younger deposits of rock lay above older deposits.

Anything found in a particular layer of rock would, logically, come from that period of time.

The continuous change of earth’s surface disrupts these layers, causing unconformities, but it is a good place to start the dating process.

A more precise method for dating is called radiometric dating.

It relies on versions of atoms called isotopes that are unstable and decay or breakdown over time (parent isotope), turning into a simpler more stable kind of atom (daughter isotope).

Different isotopes decay at different rates. However, each type of isotope consistently decays at the same rate.

By looking at the superpositioning of the sample, scientists can decide which type of isotope to use.

Then by looking at the ratio of parent isotopes to daughter isotopes in the sample, they can determine how long they have been left there to decay.

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One example is carbon-14 dating. Carbon-14 decays into Carbon-12.

After 5,730 years, half of the Carbon-14 in a sample will have turned into Carbon-12. This is called its half-life.

Carbon-14 has a relatively short half-life, some isotopes take tens of billions of years to decay.

One of the more interesting ways scientists use radiometric dating is to determine the age of fossils.

Fossils are the remains of ancient organisms, usually preserved in sedimentary rock.

Some fossils form when minerals fill-in the empty spaces inside and eventually replace the skeleton of a decaying organism.

Some form when an animal is trapped in amber, tar, or ice.

Some are trace fossils, such as footprints or nests.

Fossilization usually only occurs when an organism is buried or removed from the elements immediately after dying. Few of the countless things that have lived on earth remain as fossils.

Studying the fossil record is one of the best ways to learn about earth’s past. However, there has only been life on earth for about half of earth’s history, and there was nothing more complex than bacteria until about 540 MYA.

-There has only been life on earth for ½ of earth’s history.

-There has only been life on earth more complex than bacteria for 1/8th of earth’s history.

-Homo sapiens only been on earth for less than 1/ 3,000th of earth’s history.

-People have been recording history for less than 1/10th of that time.

The word history refers to a written record of time. Since earth is so old, we have to break it down into larger segments of time than days, months, years, decades, centuries, or millennia.

We use the geologic time scale instead.

This divides the time since earth’s formation into…

Eons

Eras

Periods

Epochs

The divisions are based on significant geologic changes to earth or changes to the organisms living on it, such as the first rocks formed on earth, the first living things, the first complex organisms, etc.

Color Quiz• The largest segment of geologic time is the…

– Red- period Blue- eon• The most accurate form of dating a rock is…

– Red- superpositioning Blue- radiometric dating• Superpositioning tells us that the oldest rocks are…

– Red- near the surface Blue- deep underground• The part of an organism that fossilizes most frequently are

the…– Red- hard parts Blue- soft parts

• Humans have only been around for a ____ segment of geologic time.– Red- short Blue- large

• The processes that shaped earth millions of years ago still work today.– Red- false Blue- true

What are tectonic plates?

The Lithosphere is broken into pieces called tectonic plates that float and move on the denser Mantle.

These plates include some oceanic crust, continental crust, and some of the mantle beneath them.

Earth’s Tectonic Plates

Restless Continents

What is continental drift?

Continental drift is the movement of the continents as a result of plate tectonics

Land masses have changed drastically over the 4.6 billion years of earth’s existence.

What we know…

245 million years ago, all of earth’s land was part of one large continent called Pangea

During the Jurassic period (~180 mya) it began to break up forming two large continents called Laurasia and Gondwana

The continents continue to move through today…

How did things change between 225 mya and 200mya?

Between 200 and 150 mya?

Between 150 and 65 mya?

Between 65 mya and today?

How do tectonic plates move?

At places between the plates, called mid-ocean ridges, a process called sea-floor spreading occurs.

Magma forces itself up to the surface and as it solidifies forming new crust. As it does so, it pushes the older crust away.

Many of the geographic features we recognize appear as a result of plates colliding opposite mid-ocean ridges.

Color Quiz• The innermost layer of the earth is called the …

– Black- inner core Orange-Mantle

• The solid surface of the earth is called the…– Black-aesthenosphere Orange-lithosphere

• Earth’s crust is divided into pieces called…– Black-tectonic plates Orange- puzzle pieces

• The movement of the continents is called…– Black-continental sway Orange-continental drift

• Continental drift is driven by sea-floor spreading taking place at …– Black- Mid-ocean ridges Orange- Mountain ridges

• During sea-floor spreading, new magma…– Black- flows under the crust Orange- forces its way up through the crust

What happens when tectonic plates converge and diverge as a result of sea-floor spreading?

There isn’t much room for plates to move around each other, so they usually slide over or under each other.

Places where plates come together are called boundaries.

There are several different types…

Convergent boundaries- when two plates collide. There are three types

1)continental-continental boundaries- collide and push up

2)continental-oceanic boundaries –denser oceanic crust slides beneath the continental crust (subduction) and rejoins the mantle

3)oceanic-oceanic boundaries-one of the plates will subduct beneath the other

Divergent Boundaries-when two plates separate, magma rises from the mantle to fill the empty space

-most common place is at mid-ocean ridges.

Transform Boundaries- When two plates slide past each other horizontally

-when they catch or bump, earthquakes occur

What causes the plates to move?

As rock heats up deep in the aesthenosphere their density decreases and they rise

Rock at the surface cools down, increases in density, and sinks at subduction zones

Activator- 11/1

-How does the picture below provide evidence for continental drift (there is more than one way)?

Evidence for Continental Drift

Geographical- continents fit together like a puzzle

Geological- rocks of the same age and type line up across continents no longer connected

Climatic- remains of glaciers show some continents not in polar regions used to be

Paleomagnetic- the alignment of magnetized particles in old rocks shows that they have moved relevant to the poles

Biological- fossils of the same species appear on currently distant continents

Geographical- continents fit together like a puzzle

Geological- rocks of the same age and type line up across continents no longer connected

Climatic- remains of glaciers show that some continents not in polar regions used to be

Paleomagnetic- the alignment of magnetized particles in old rocks shows that they have moved relevant to the poles

Biological- fossils of the same species appear on currently distant continents

How could an aquatic fossil show that the continents have moved?

Activator -11/1

Identify each of the tectonic boundaries below as divergent, convergent, or transform.

Color QuizA point where two tectonic plates meet is called a ____________Black- boundary Orange- horizon

One plate sliding under another is called _____________Black- tunneling Orange- subduction

A boundary where two plates slide passed each other is called a __________Black- convergent boundary Orange-transform boundary

When two plates collide, the one that is ____________ dives beneath the otherBlack- more dense Orange- less dense

Most divergent boundaries appear around _______________Black- mountains Orange- mid-ocean ridges

Continental drift is driven by __________________Black-sea floor spreading Orange- crust deformation

How can plate tectonics deform the earth’s surface?

The movement of tectonic plates puts stress on the earth’s crust. If it changes the rock at all, we say it has been deformed.

There are different kinds of stress…

squeezing = compression

stretching = tension

Where might you see compression? Tension? At which boundaries?

If rocks bend as a result of stress, it is called folding.

-anticline- bending upwards

-syncline- bending downwards

-monocline- bending that results in a bend that has horizontal ends

If the rock breaks as a result of stress, you have a fault.

-there are several types of faults, depending on where the block of crust slides.

Normal Fault- the hanging wall slides down relative to the foot wall

-it looks like a piece has slide down and away from the other

Reverse Fault- the hanging wall slides up and over the footwall

-it looks like a piece has slide up and into the other

Strike-slip Fault- the two pieces have slid horizontally to each other

What type of stress is causing each of these faults to occur?

Shear is a type of stress where one object is pushed in opposite directions at different points

Where do mountains come from?

Most large mountains are folded mountains

-two plates push against each other, compression grows and mountains fold up.

Some mountains are called fault-block mountains

-the tension of plates moving apart causes a series of normal faults to occur, resulting in a series of uneven peaks.

Some other mountains are volcanic.

-just beyond subduction zones, magma near the surface builds, erupts, and slowly builds a mountain of volcanic rock.

Building mountains is a form of uplift- raising pieces of crust to higher elevations

-if weight it removed from the crust in anyway, it can float a little higher on the asthenosphere and will raise up without much deformation

The crust lowering or sinking is called subsidence

-this can occur as rock cools and increases in density, as a result of massive faulting, or if material underground is removed.

Color QuizWhich force creates fold mountains?Black- shear Orange-compressionWhich force creates normal faults?Black- tension Orange-compressionWhat is a rock that folds up called?Black-anticline Orange-synclineWhat is a place where the crust has broken is called?Black- mountain Orange-faultWhat is it called when the crust lowers in elevation?Black- subsidence Orange-upliftWhich type of fault is a result of shear?Black-normal Orange-strike-slip

Earthquakes

As we know the movement of tectonic plates causes stress to the earth’s crust.

When that stress causes the rock to fold (plastic deformation) we do not get earthquakes.

When that stress causes the rock to stretch in one way or other (elastic deformation) we can get earthquakes.

Rock can only stretch so far before it breaks. When it breaks (faults), energy is released.

The energy radiating out from the fault causes the earthquake

The rock returning to a less stressed position is called elastic rebound.

Where do earthquakes occur?

Most earthquakes occur near tectonic boundaries

Type of Boundary Type of FaultTransform Strike-slip

Convergent Reverse

Divergent Normal

They can occur at the surface or far beneath it, anywhere where stress can build up.

They can occur in the middle of tectonic plates if stress can accumulate but not as frequently or with the same intensity

The area around the pacific plate is called the ring of fire, because of its intense seismic and volcanic activity.

Earthquakes are caused by waves of energy moving through the crust, called seismic waves.

There are two main types of waves, surface waves and body waves.

Body waves- travel through the earth’s interior. Since it is denser there, they move faster than surface waves.

p-waves (pressure waves or primary waves)- travel through matter causing it to go back and forth as one bit collides with the next. They are the fastest waves.

s-waves (shear waves or secondary waves)- push through solid rock only and cause it to move side to side

Surface waves- travel through the upper few kilometers of the earth’s surface

-they can move rock up and down (rayleigh waves) or side to side (love waves), but do so in a slower, more destructive manner

How are earthquakes measured?

Seismographs are instruments that detect seismic waves.

The more energy an earthquake produces the more the seismograph moves, creating an image of the earthquakes waves called a seismogram.

By comparing multiple seismograms taken at different locations, scientists can determine when an earthquake began and where it originated.

The point inside the earth where an earthquake originates is called the focus.The point on the surface of the earth directly above the focus is called the epicenter.

The strength of an earthquake is called its magnitude and is measured by observing ground motion.

-It is measured on the Richter Scale

If the scale moves up one point then ground motion has increased 10x.

3.0 = 10x 2.0

4.0 = 10x 3.0

5.0 = 10x 4.0

The Modified Mercallu Intensity Scale (MMI) measures how much an earthquake can be felt by people at a particular place.

-The closer to the epicenter you are the more intense the earthquake is to you

- It is given in roman numerals; I meaning not really felt, XII meaning total devastation to the area

The earthquake last spring in Haiti had a magnitude of 7.0.

Intensity was as high as X

The 1989 Loma Prieta earthquake was also a 7.0

But its intensity didn’t reach much higher than an VIII.

Why did the Haiti earthquake do more damage than the California quake?

Color Quiz

What motion causes earthquakes?

Black- elastic rebound orange- spring back

Which waves do the most damage?

Black- body waves orange-surface waves

Which waves are the fastest?

Black- P waves orange- S waves

Which is true about where earthquakes occur?

Black- the occur most frequently near tectonic boundaries

Orange- the occur equally everywhere on earth

The instrument used to measure an earthquake is called a _____________.

Black-seismogram orange- seismograph

The richter scale measures ______________.

Black- ground movement Orange- damage

The MMI scale measures ______________.

Black- ground movement Orange- intensity

Volcanic Eruptions

We know that volcanoes form above magma pockets and that they frequently occur near subduction zones between converging tectonic plates.

But what is a volcanic eruption?

An eruption is said to occur whenever molten magma from within/beneath the volcano breaks through the surface.

They can be explosive or nonexplosive.

Nonexplosive eruptions

-most frequent (continuous at some places)-Lava sprays, runs, or leaks from cracks in the crust-Can slowly reshape the landscape at volcanic rock is deposited on the surface

Explosive eruptions

-Much rarer

-Can destroy 100s of km in minutes

-Can blast millions of tons of debris (rock, lava), ash, and gas into the air.

-Can also cause landslides and mudslides

Nonexplosive

Explosive

Each volcano has two main parts, the magma chamber below ground and the vents (cracks in the surface through which lava is escapes).

Magma is called lava after it reaches the surface. Its composition, what chemicals it as made up of, can affect how it erupts.

-If it has a lot of water dissolved in it, or gases trapped inside of it, it will likely have an explosive eruption (like shaking up a soda can).

-Thicker magma tends to trap more gas and erupt more explosively.

The stuff erupting from a volcano is split into two categories, lava and pyroclastic material.

-Lava- liquid flowing from an erupting volcano

-pyroclastic material- solid material blasted into the air by an erupting volcano

Lava comes in different types, based on how it flows (viscosity).

-Pahoehoe (puh Hoy Hoy) and Block lava are thick and slow flowing (high viscosity)

-Aa (ah ah) and pillow lava are fluid and fast flowing (low viscosity)

The different types also solidify differently

AaPahoehoe

Pillow Lava

Block Lava

Pyroclastic material can be lava that solidifies while in the air or solid rock from the surface thrown into the air.

It can range in size from giant boulders to dust…

Volcanic bombs- magma that solidifies in the air

Volcanic blocks- pieces of solid rock thrown into the air

Lapilli- small pieces of magma that solidify into small pebbles in the air

Ash- magma around exploding gas bubbles solidify as tiny slivers, dust.

When pyroclastic material starts running downhill it is called a pyroclastic flow.

-These can be some of the most destructive aspects of an eruption, with lots of material moving very fast and at a very high temperature.

-When a pyroclastic flow is saturated with water it is called a lahar… it is basically like a giant wall of cement running downhill.

Color Quiz

Mount St. Helens erupting in 1980 is an example of what kind of eruption?

Red- explosive Green- nonexplosive

What are the places where magma exits a volcano called?

Red- magma chambers Green- vents

Which type of eruption occurs more frequently?

Red-explosive Green- nonexplosive

Volcanoes filled with highly viscous magma usually erupt in which kind of eruption?

Red-explosive Green- nonexplosive

What is the solid material shot out of a volcano called?

Red-lava Green- pyroclastic material

What is the run of pyroclastic material downhill called?

Red-mudslides Green-pyroclastic flow

A lahar is a type of pyroclastic flow saturated with _________.

Red- gas Green- water

After the initial danger of an eruption, how can they affect the earth?

Global climate change-

-the carbon dioxide released from volcanoes can insulate the earth and cause its temperature to rise (constant process)

-the ash shot into the atmosphere can block the sunlight from reaching the earth in the first place and cause temperatures to fall (only after an eruption)

Before man started burning fossil fuels the release of green house gases from volcanoes helped to maintain our atmosphere and temperature.

-we have increased the amount of greenhouse gases beyond their natural level.

The cooling effect of a large volcanic eruption, though, can last several years.

An eruption can also destroy an ecosystem.

After an eruption, an ecosystem basically has to start growing again from the rock up in a process called succession.

•Algae/moss/lichen- to establish soil•Small hardy plants- to fertilize soil•Larger plants•Most competitive plants (adult forest-climax community)

1985

2002

The volcanic mountains themselves are a result of eruptions. There are three types…

Shield volcanoes- wide, not steep, caused by continuous nonexplosive eruptions

Cinder Cone volcanoes- caused by the piling of pyroclastic material

Composite volcanoes- caused by the combination of slow nonexplosive and explosive eruptions. Made of a composite of solidified lava and pyroclastic material.

Eruptions often leave craters at the top of a volcano, as debris piles up in a ring around the central vent.

-craters change in size over time

When the magma chamber holding up a volcano drains, the volcano can collapse into itself leaving a depression. This depression is called a caldera.

-Yellowstone National Park is made up of three large calderas.

When lava leaks out along a fault line rather than at a single point, instead of creating a volcano it can create a plateau called a lava plateau.

The Grand Canyon is cut into a lava plateau.

Where do volcanoes form?

Volcanoes form anywhere where magma rises through the crust.

Magma rises any place where the pressure is low enough and the temperature high enough to let the rock liquefy and expand.

The continuous rise of magma at midocean ridges accounts for continental drift.

The rise of magma behind subduction zones can cause volcanoes.

Hot spots, places in the middle of tectonic plates that allow magma to rise, also allow volcanoes to form.

How can we tell when a volcano will erupt?

No one knows the exact sequence of events leading up to and causing an eruption, but there are some warning signs.

Warning signs of an eruption…

-earthquakes- as magma rises and the rock of the mountain crack, pulses of energy are sent through the ground causing earthquakes.

-gases-most volcanoes continually vent gases. If the gases change then the inside of the volcano has also changed.

-swelling-as magma and gas buildup inside the volcano, pressure increases and the mountain itself can deform.

Why is important that we learn how to predict when volcanoes will erupt?