Geologic Time and Origins of the Earth

Thinking about time

• Geologic time vs. “normal” time

• What is the date today?

• What is the day?

• What is the time?

• Why do we care?

Geologic time• Thinking in terms of “millions of years”

• Try it in seconds, just for fun:– 1,000,000 seconds

• How many minutes is this?

– 1,000,000 seconds / 60 seconds per minute– This = 16,667 minutes

• How many hours is this?

– 16,667 / 60 minutes per hour = 278 hours• How many days?

– 278 hours / 24 hours per day = 11.6 days

• So, 1 million seconds ~ 12 days

How does 1 million compare to 1 billion?

• Again, in seconds just for fun:– 1,000,000,000 seconds

• How many minutes is this?

– 1,000,000,000 seconds / 60 seconds per minute– This = 16,666,667 minutes

• How many hours is this?

– 16,666,667 / 60 minutes per hour = 277,778 hours• How many days?

– 277,778 hours / 24 hours per day = 11,574 days

Keep going…

• 11,574 days--how many weeks is this?

• 11,574 / 7 days per week = 1,653 weeks– Great, now how many years is this?

• 1,653 weeks / 52 weeks per year

• This equals 32 years

• So, one billion seconds = 32 years

• And one million seconds = 11 days

• Now instead of seconds, lets think in YEARS

Geologic time

• Video clip: Carl Sagan and the Cosmic Calendar (5-7 minutes)– From the “Cosmos” Public Television Series, 1981– Episode 1, “The Shores of the Cosmic Ocean”– About 10 minutes before the end– Available at the UTEP Library for viewing

Geologic Time

• The big divisions are:

• Proterozoic time before 544 m.y.a.

• Paleozoic time from 544 - 245 m.y.a.

• Mesozoic time between 245 - 66 m.y.a.

• Cenozoic time between 66 - 2.5 m.y.a.

• Even MORE divisions

Geologic time scale

Earth Origins

• AFTER “Big Bang”

• Earth Origins are described by the Nebular Hypothesis

• RECAP: Scientific Terms– Hypothesis = provisional explanation for

observations; subject to continued testing & modification

– Theory = An explanation for some natural phenomenon that has a large body of supporting evidence; testable & repeatable

Nebular Hypothesis 1

• Solar system did not exist 5 billion years ago

• A giant cloud of dust and gas occupied this area of space – Many times larger than the present solar system – A lot of space between individual atoms of gas,

metal and ions

• Why do we think this?– We observe large gas clouds in space, which

contain basic building blocks of our solar system

Nebular Hypothesis 2

• Something caused this cloud to condense

• Maybe a nearby star exploded, sending its mass out into space

• This mass collided with our dust cloud– Forcing atoms and dust grains to collide (accretion)

• Each atom & grain has its own gravity

• The bigger the grain, the bigger its gravity

• The bigger the gravity, the bigger the chance for accretion

• (“snowballing” effect like positive feedback)

N.H. 2, continued

• Nearby supernova caused cloud to condense

• Accretion began

• Why do we think this?– We have seen supernovae– We have seen the remainder of star mass

ejected into space– We can mathematically model what happens

when this mass collides with gas clouds

Nebular Hypothesis 3

• Cloud continues to condense• Center of cloud attracts the most matter• Cloud flattens into a disk and begins to rotate• Incredible amount of heat generated in the center

– So much heat that atoms are fused together, and the sun “turns on”

• Why do we think this? – Explains the similar rotation of all planets– Explains how the planets are all orbiting in one plane

(DISK)– Explains location of sun– Explains motion that we see in other gas clouds

Nebular Hypothesis 4

• A new sun in the center of a disk of dust

• More volatile elements move away from the sun– H, He, Methane, Ammonia

• Accretion continues in the disk – Planets form by collision

• Why do we think this?– Explains why inner planets are rocky & outer planets

are gaseous– Explains old craters on planetary surfaces

Nebular Hypothesis 5

• Accretion continued until most of the matter accreted into planets

• Accretion occurred in stages, with the “Late Heavy Bombardment” happening ~3 b.y.a.

• Why do we think this? – Explains relative emptiness between planets– Explains different apparent ages of craters on

moon, Mars– Explains old craters on planetary surfaces

Nebular Hypothesis

• Video Clip: Carl Sagan’s Cosmos Series– Episode 8, “Travels in Space & Time”– Available for viewing at the UTEP library– About 15 minutes before the end of the episode

Why is the N.H. a hypothesis and not a theory?

• Observations are indirect

• We were not there to see it

• What we can see is very slow

• So…– we do not try to produce a solar nebula model that

exactly describes our Solar System and the planetary orbits and their masses

– rather we look for a model that describes the characteristic properties of the planets at their observed locations in the Solar System

Making the Earth 1

• Accretion acts over an extended area (the disk) and for a extended period of time

• Solid grains condense out of the nebula’s gas– This is a chemistry process

• Grains accrete into larger bodies (planetesimals)– This is a dynamic, collisional process

• Planetesimals collide to produce protoplanets• Protoplanets accrete more material and become

genuine planets

Making the Earth 2

• Earth cools a bit:– Elements, minerals condense/crystallize

• Iron, nickel, then rocky material

• Layered Earth forms:– Stays molten due to heat of radioactive elements

and L.H.B.

• Iron and nickel sink to center, rocky material floats outward (density sort).

• This is called differentiation

Layered Earth