Earth’s History

Introduction

– As we already learned the earth is always changing

– The history of the earth is told by the Earth’s Geological events

– It is up to the scientist to figure out the puzzle of these geological

events.

Early Earth

– There is evidence that states that the atmosphere and the oceans of the early earth began to form from gasses coming from volcanic eruptions in the earth’s interior.

– There were large amounts of

• Water vapor

• Carbon dioxide

• Nitrogen

• And other gases

What is Outgassing?- That came out of the earth from

a process called outgassing• The vapor was the source

of water for the atmosphere and the oceans

– The early earth was also covered by clouds that formed precipitation over millions of years that cooled down the hot surface of the earth and also formed oceans.

– What is the proof to this?• Scientists have studies

sedimentary marine rocks that prove this concept of an early ocean. ( 4 billions years ago)

???? Question?????– Today’s atmosphere is

made up of 21% oxygen but the gases of the volcanic eruptions do not contain oxygen

• SO WHERE DID WE GET THE OXYGEN FROM?– Scientists believe that the

early oceans contained green plants. These green plants as we know get their food from photosynthesis. » Photosynthesis

produces extra oxygen that went into the atmosphere over millions of years.

Sequence of Geological Events• Knowing the sequence of events that

took place during the formation of the earth’s crust helps us develop a history of the earth to better understand the earth

• Relative age is concerned with the sequence of events that occurred in an area

– As shown in the appearance of the rock layers

– Relative age is not really concerned with the age of the rocks

– This method uses – sedimentary rock layers– igneous extrusions and intrusions– faults – folds – continuity – similarities of rock – fossil evidence – and volcanic time markers

• As clues to determine what was the probable sequence of events

Absolute Age The actual age of a rock or a fossil is called absolute

age• The best way and more precise way

of figuring out how old a rock or a fossil is uses a technique called radioactive dating.

– EVERY RADIOACTIVE ELEMENT DECAYS

– How does this work? • Every radioactive “parent” element

release radiation until it breaks down (decays) into a stable daughter element.

• Sometimes this changing process goes through steps

• We also know how long it would take for a radioactive isotope to decay.

• We use a process of half life. Since we know how long the half life ( time takes ½ the isotope to decay) is we can then figure out how old a rock is based upon what stage of decay it is in.

• The law of superposition – This states the bottom

layer of a group of horizontal layers of rocks is the oldest. » This is used to

determine the sequence of when the sedimentary rocks were formed.

» This works as long as the group was not overturned not had an older rock put over it

• Igneous Intrusions and Extrusions – Igneous Intrusions

• formed when magma is injected into older rock layers in the crust

• younger than rock they are found in

• look for contact metamorphic rock in layer above and below the intrusion

– Igneous Extrusions • rocks that formed from lava on

the surface of the earth• younger than rock layers below• look for contact metamorphic

rock on the bottom only– Why do we look at the metamorphic

rock near these two rock formations?• The reason is contact

metamorphism – (the rock forming due to the contact of lava) will be younger then the rocks that it goes through

Folds, Faults, & Joints • Folds

– bends in the rock layers– occur after the rock layers

formed• Faults

– cracks in rock layers where some movement has taken place

– Faults produce offset layers.• Joints

– Are immovable cracks • These three rock layer

characteristics occur due to changes in the temperature and pressure

• These three rock characteristics are also younger than the rocks in which they appear.

• Since the rocks that fold faults or joints are there before these things occur

Internal Characteristics• Fragments that occur in

a rock as we can understand are older than the rock itself (since the rock was formed through these fragments)

• Cracks and veins in a rock are younger then the rock in which they are found

• Veins – are mineral deposits that fill up a crack in a rock

• Sedimentary rocks are older than the sedimentary layers and the cement that keep them together.

Unconformity

– Is the break in geological history of a rock series

– They are found between two ages of rock levels

– Usually these gaps are formed from erosion and non deposition

– It looks like a buried Erosional surface

– Since the rock stopped building up erosion occurred on the top layer and then a new age group of rocks were formed.

– We use different correlating techniques to find out how old a rock really is

– You have to be careful though to use facts and not inferences when figuring out this information • Correlation is the act of

matching rocks of similar age in different places – This is best done when

rocks are not covered in dirt and plants

– This can be so easy by identifying a certain mineral type, color and then searching for it in different places

• this is using the remains of animals that are known to have lived and died in a certain time period

• If we find one of these fossils then we can look for rock layers that look the same and figure out how old it is

• We also use Index fossils to decipher rock records

• Organism that classified as index fossils are found only in certain rocks

– How does a fossil fit the index profile – Easily recognizable– Abundant – Have been found in a wide

geographic area – Existed on earth for a brief period of

time

Volcanic Time Marker– When a volcano

erupts it leaves a layer of volcanic ash is deposited over the area

– This may be a time marker if we find a layer of ash in the layers of the rocks and the time of the volcanic eruption is known

– This time marker will be very helpful determined the ages of rocks below and above it

Problems with Correlation

– It seems based upon what we have just learned that figuring out a geological time line is very simple.

– However, it can lead to misinformation • With very careful study

of 2 like rock formation they might still be different ages.

• Also , in the same rock formation their might be rocks of different ages

– Rock Record • As we just learned that

scientist use index fossils (guide fossils ) to tell how old a rock is.

• Scale of geological time – scientists have

organized the geological time scale» eons » eras » periods » epochs

– each is a more specific amount of time then the other ones

Units of geological time periods

Phanerozoic eon Proterozoic eon

Cenozoic

Archean eon

Mesozoic Paleozoic

cretaceous

Jurassic

Triassic

– epochs are more based on fossil evidence

– However, most of the geological time is void of fossil records

Geologic Time Scale

• Scientists suggest the following is the geological time scale

In order of creation

Precambrian

Paleozoic

Mesozoic

Cenozoic

Precambrian/ Pre Paleozoic

– Makes up 85% of the earth’s history

– Very little fossil evidence from this time period

– This is because the organisms that were around had soft bodies , simple, and small ( ex; algae bacteria)

Paleozoic

– Much shorter time period

– Has an abundant of fossils

– The first vertebrates, land plants and animals developed in this era

Mesozoic

– Was even shorter – Fossils of dinosaurs

and the earliest birds and mammals formed • Included three periods

» Triassic

» Jurassic

» Cretaceous

Cenozoic

– Most recent era – Includes the fossils

of many modern plants and mammals, even the appearance of humans

– Plate motions and mountain buildings may also be identified in this time sequence by using the reference table