The History

of the Earth

The Lithosphere•The lithosphere is the hard outermost shell of our planet on which we live.

• In the mantle below it is molten (liquid) rock called magma.

Volcanos Evidence of molten rock below the lithosphere

is lava flowing at the surface from volcanos.

Plate Tectonics The lithosphere

floats above the molten rock and it is not one solid sheet but as evidenced by volcanic and seismic activity it is broken into plates.

Boundaries• The place where 2 plates meet are called

boundaries.• There are 3 types of boundaries:

Divergent Boundaries• At divergent boundaries the plates are moving away

from one another.• This is happening at the bottom of the oceans

creating a deep ocean trench.• Also referred to as sea-floor spreading.

Convergent Boundaries There are 2 type of convergent boundaries;

these are typically areas of mountain building and uplift.

Transform Boundaries• Transform boundaries happen where 2

plates are sliding past one another in opposite directions.

• The most classic example of this is the San Andreas Fault line:

The Ring of Fire• The boundaries of the Pacific Plate are

collectively known as the Ring of Fire due to the high amount of volcanic and seismic activity.

• The Hawaiian Islands have been built up from lava flowing from what is called a hot spot which is a sort of weak spot in the lithosphere.

Igneous Rock• Cooled and hardened lava is called igneous rock.• Depending on the minerals (type of molecules)

contained in the lava and the environment in which it cooled there is a wide variety of different formations of igneous rock such as:

• Lava that cools rapidly (as in in cold water) doesn’t have time to form crystals and becomes a type of black glass which is called obsidian.

• Lava that contains a lot of gases trapped in it becomes very lightweight rock called pumice.

ErosionRock is broken down into smaller and smaller pieces through many types of weathering. When those pieces are moved from one place to another it is called erosion. There are 2 general categories:

PHYSICAL

CHEMICAL

Physical Weathering The main agents of physical (aka mechanical)

are:– Water– Wind– Glaciers– Wedging

Erosion by Water• As a river flows from the mountains to the sea, the

river forms a variety of features including valleys, waterfalls, flood plains, meanders, and oxbow lakes.

• As the river flows down the mountain it picks up sediment, very tiny pieces of chipped off rock.

Erosion by Waves• The energy in waves comes from wind that blows

across the water’s surface. • As the wave approaches the shore it makes

contact with the bottom shaping and sculpting it with its full force, tumbling (smoothing) rocks and shells caught in its grip.

Erosion by Glaciers• Glaciers are giant rivers of ice that move ever so

slowly across the landscape grinding and polishing and sometimes even picking up rocks and sediment along the way.

• When they melt/recede they can leave gouges in the earth and even giant boulders behind.

Erosion by Wind• Wind is the weakest agent of erosion and really

only affects places where there are no plants to hold down the soil (dunes, deserts).

• Wind moves surface materials around in a process called deflation and it can polish rocks in a process called abrasion (think sand blasting).

Wedging• As cracks in rocks form, rain can seep down in the

cracks and then if the rainwater freezes, it expands, wedging the crack apart wider and wider. This is called frost wedging.

• Plant roots can also act as a wedge to pry apart rocks; we call this root wedging.

Sediment Deposition• The small pieces of material that come from rocks

or plant & animal remains are called sediment.• After sediment is picked up by various agents of

erosion (wind, water, glaciers, waves) it must come back down…and this is part of the process of is called deposition.

Sedimentary Rock• When layers of sediment accumulate/build up

and they stick together over time they become sedimentary rocks.

• Ex: Mudstone, Sandstone, Shale, Conglomerate• This is the type of rock in which one would find

fossils.

Metamorphic Rock• When igneous and sedimentary rock are exposed to

heat and/or pressure chemical changes happen in the rock that transform them into different rocks.

• This type of rock is especially hard and durable.• Examples:

– Marble (parent rock is limestone which is sedimentary)– Gneiss (parent rock is granite which is igneous)– Slate (parent rock is shale which is sedimentary)

What is a Rock?• Rocks are non-living substances made up of minerals.• Minerals are naturally occurring, homogeneous inorganic

solid substances having a definite chemical composition and characteristic crystalline structure, color, and hardness.

THE ROCK CYCLE

Fossils• The preserved remains or traces of an organism

that lived in the past are called fossils.• Scientists that study fossils are called

paleontologists.• The fossil record provides invaluable information

about the history of life and past environments on Earth, including the climate.

• In many cases the fossil remains are all remains of some extinct species.

• It has been estimated that over 99.9% of all species that ever lived are now extinct (no longer existing).

Types of FossilsFossils are ONLY found in sedimentary rock and there are several different types of fossils such as:

➢ Petrification➢ Casts➢ Molds➢ Carbon films➢ Trace fossils➢ Preserved remains

Petrification• Petrification means “turned into stone.”• This is a process in which all the parts of the organism are

replaced with minerals.

Casts & Molds• A mold is a hollow area in the shape of an organism.• When that mold fills in with sediment and that sediment

hardens into the shape of the organism, that “copy” is called a cast.

Trace FossilsTrace fossils provide evidence of the activities of ancient organisms such as:

➢ Footprints which tell paleontologists how they moved, if they lived alone, etc.

➢ Poop! – aka coprolite (fancy scientific term for fossilized poop)

➢ Specialized habitats such as nests, burrows or dens.

• Carbon films are thin sheets of carbon only that are left behind on rock when the other components of the organism decay away.

• Paleontologists that specialize in ancient plants and fish get a lot of information from these as they preserve delicate small fishbones, flowers and leaves very well.

Carbon Films

Preserved RemainsSometimes entire organisms can be preserved in special conditions where there is an absence of exposure to oxygen such as:

➢ Tar pits – thick oil➢ Ice➢ Amber – tree sap

Index FossilsIndex fossils are particularly helpful in determining the age of a rock layer in the field because they:•Are distributed widespread geographically •Existed for a short range of geologic time•Examples:

–Trilobites lived from about 500 million years ago to 250 million years ago

–Ammonites indicate an age range of about 440 million years ago to 360 million years ago

Dating TechniquesGeologists and archaeologists employ two main dating techniques to determine the age of rocks (or artifacts):➢Relative Dating➢Absolute Dating

Relative Dating• Relative dating can determine

the sequential order in which a series of events occurred, not when (the actual absolute date) they occur.

• Relative dating is done with 3 basic laws of geology in mind:➢The Law of Superposition➢The Law of Original Horizontality➢The Law of Crosscutting Relationships

Law of SuperpositionThe Law of Superposition states that in undisturbed horizontal sedimentary rocks, the bottom layer is oldest & each layer above it is increasingly younger…

The Law of Original Horizontality

The Law of Original Horizontality states that in general (though there are a very few exceptions) sediment is deposited horizontally.

Law of Crosscutting Relationships

The Law of Crosscutting Relationships states that any rock that cuts across sediment layers is younger than the youngest layer it cuts through.

UnconformityAn unconformity is a place in the rock record where an entire layer of rock has been eroded away creating a gap in the timeline.

Absolute Dating● Absolute dating uses

natural radioactive decay within the rock to establish an actual numerical date.

● Also called radiometric dating.

RadioactivityRadioactivity is the spontaneous breaking apart (decay) of atomic nuclei

• Parent – an unstable isotope• Daughter products – isotopes formed from the

decay of a parent

Radioactive Decay

Types of radioactive decay:➢ Alpha emission ➢ Beta emission ➢ Electron capture

Radiometric Dating• Half-life – the time for one-half of the

radioactive nuclei to decay (turn)• Requires a closed system• Cross-checks are used for accuracy • Complex procedure • Yields numerical dates!

Decay Curve

Carbon-14Half-life of only 5,730 years Used to date very recent events Carbon-14 produced in upper atmosphere

Incorporated into carbon dioxide Absorbed by living matter

Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history

Geologic Time Scale ❖4 billion years is a REALLY long

difficult to fathom length of time.❖Geologists have divided that

history into manageable units. ❖The timeline was originally created

using relative dates but has since been edited using radiometric techniques.

Divisions Eons are the largest, they are broken up into Eras which are further subdivided intoPeriods which are then further subdivided intoEpochs which are the smallest unit of time.

EonEons are the largest subdivision of

geologic timeFour eons most recent on top (just like

the rocks themselves):–Phanerozoic–Proterozoic–Archean–Hadean

Eras• Eons are subdivided into Eras• Eras of the Phanerozoic eon

• Cenozoic (“recent life”)

• Mesozoic (“middle life”)

• Paleozoic (“ancient life”)

Periods• Eras are

subdivided into periods.

• Fossils before the cambrian are rare so all of geologic history before the cambrian is referred to simply as the precambrian.

Epochs• Periods are further

subdivided into epochs.• We are currently living

in the holocene though scientists are now calling for a new epoch to be made called the anthropocene.

-anthro = human

Limitations of Dating● Not all rocks are

datable! i.e. sedimentary ages are rarely reliable because they are collections of sediment from rocks of differing ages.

● Therefore datable materials are often used to bracket events to determine ages.

The PrecambrianThe Precambrian is from Earth’s formation 4.6 billion years ago to about 543 million years ago and is characterized by:

– Volcanic eruptions, meteorites, intense radiation from the sun

– Early atmosphere had no oxygen– Prokaryotic organisms (no nucleus)– Cyanobacteria appeared→ producing their own food from the

sun’s energy from photosynthesis and they created the oxygen atmosphere.

– The Ozone layer forms in the upper atmosphere and absorbs radiation from the sun.

The PaleozoicThe Paleozoic spans from 541 to 252.2 million years ago and is characterized by:Rocks rich in fossils of sea creatures such as sponges, corals,

clams, squids, and trilobitesFish appear, sharks are more abundantYou can think of it as The Age of FishForests of giant ferns covered earth

Gondwanaland

The MesozoicThe Mesozoic spans from 252 to 66 million years ago and is characterized by:Dinosaurs and other reptiles, Referred to as The Age of ReptilesFirst birds appearFlowering plants appearThe “birth of the Himalayas”

What happened to the Dinosaurs?

The mesozoic ends in a mass extinction event in which ~75% of non-avian dinosaurs disappear.

The extinction event is believed to have been caused by a meteorite impact due to a rock layer from that time (called the K-Pg Boundary) that contains extremely high levels of iridium which is rare on Earth but common in meteors.

The Cenozoic

The Cenozoic spans from 66 million years ago to the present and is sometimes referred to as The Age of Mammals

– Mammals included mastodons, woolly mammoth, saber-toothed cats, camels, and giant ground sloths

– Included some periods of heavy glaciation known as ice ages– Includes the emergence of man!