Changes Over Time

Early History of Earth What was early Earth like? Most scientists think the Earth was very hot Earth’s surface may have been hit hard and often by meteorites These meteorites would have heated the surface

Early History of Earth The inside of the Earth was also under a lot of pressure It became so hot volcanoes violently gave off lava and hot gases that relieved the pressure

Early History of Earth The gases helped form the atmosphere, but it was not an atmosphere you could breathe The early atmosphere did not have oxygen

Early History of Earth About 4.5 billion years ago, Earth cooled enough for the water vapor in the atmosphere to condense This probably lead to millions of years of thunderstorms Enough water fell to create oceans Some scientist think that some time after this, between 3.9 and 3.5 billion years ago, life formed in the oceans

History in Rocks Scientist cannot be exactly sure how the Earth was formed because rocks are constantly destroyed and new rocks created The oldest rocks found on Earth are about 3.9 billion years old Rocks still provide an important source of information about many forms of life that existed on this planet

Fossils A fossil is evidence of an organism that lived long ago and that became preserved in Earth’s rocks Scientist believe that 95% of all species of plants and animals that ever lived on Earth are now extinct

Fossils Paleontologists are scientist who study ancient life They use fossils as clues to things that happened long ago

Types of Fossils There are five types of fossil Trace fossils Casts Molds Petrified/Permineralized fossils Amber-preserved or frozen fossils

Trace fossils A trace fossil is any indirect evidence left by an animal Footprints Trails Burrows Trace fossils tend to show behavior or how an organism lived

Casts When minerals in rocks fill a space left by a decayed organism, they make a replica, or cast, of the organism

Molds A mold forms when an organism is buried in sediment and then decays, leaving an empty space Ex. Leaf imprint

Petrified/Permineralized Fossils Petrified- minerals sometimes penetrate and replace the hard parts of an organism Ex. Petrified wood Permineralized- void spaces in original organism are filled by minerals

Amber-Preserved or Frozen Fossils At times, an entire organism was quickly trapped in ice or tree sap that hardened into amber Ex. Mosquitoes caught in amber (Jurassic Park)

Fossil Formation Often, fossils formed when plants and animals died in mud, sand, or clay These dead organisms would become covered with more mud, sand, or clay Over time, the area where the organism died became compressed or packed down It hardened into a type of rock called sedimentary rock

Dating Fossils Relative dating- scientist look at the different levels of rocks Rocks closest to the surface or the youngest Rocks toward the center of the Earth are the oldest

Dating Fossils Radiometric dating- scientist use this method to determine specific ages of rocks They utilize radioactive isotopes in rocks Most of the rock that can be dated this way are volcanic or igneous rocks which are closely associated with the sedimentary rocks

A Trip Through Geologic Time By looking at fossils and by figuring out the age of rocks, scientist have come up with something like a calendar of Earth’s history We call this calendar the geologic time scale

Geologic Time Scale The scale is divided into four sections called eras Each era represents a very long period of time Four Eras are: Precambrian Paleozoic Mesozoic Cenozoic

Geologic Time Scale Each era is subdivided into periods Divisions were selected on information from fossils Divisions were based on organisms that lived during that time and it will also show mass extinctions Mass extinction- an event that happens when many organisms disappear from the fossil record at the same time

Geologic Time Scale The time scale begins with the formation of Earth about 4.6 billion years ago The earliest fossils have been found in Precambrian rocks The first fossil found was of an ancient organism that resembled some modern cyanobacteria

Geologic Time Scale In the Precambrian, living things were mostly one-celled organisms By the end of the Precambrian, however, multi- celled organisms such as sponges and jellyfish were present. Precambrian makes up about 87% of Earth’s history Precambrian ended around 543 million years ago

Geologic Time Scale The Paleozoic Era begins around 543 million years ago During this era, the number of different kinds of organisms increased greatly Paleozoic era is divided into six different divisions

Geologic Time Scale Paleozoic Eras: Cambrian Ordovician Silurian Devonian Carboniferous Permian

Geologic Time Scale Cambrian, million years ago, is referred to as the “explosion” because so many new types of living things appeared Worms, sea stars, and arthropods lived in the oceans Towards the end of the Cambrian, the first vertebrates appeared, squids and fish

Geologic Time Scale Ordovician, million years ago, the fish begin to diversify Silurian, million years ago, the first land plants appear Devonian, million years ago, the first amphibians are seen Carboniferous, million years ago, the first seed plants and first reptiles are seen Permian, million years ago, the first conifers appear

Geologic Time Scale At the end of the Paleozoic era, there was the largest mass extinction that shows up in the fossil record About 90% of the species that lived in the oceans and about 70% of land species disappeared

Geologic Time Scale The Mesozoic Era, million years ago, shows many changes This era is divided into three periods: Triassic Jurassic Cretaceous

Geologic Time Scale Triassic Period- fossils show that mammals appeared on Earth during this period Jurassic Period- many scientists think that birds evolved from dinosaurs during the Jurassic Period

Geologic Time Scale Cretaceous Period- many new types of mammals appear and also flowering plants became plentiful on Earth A mass extinction occurred at the end of the Mesozoic Era- the end of the dinosaurs Scientists estimate that 2/3 of all living species became extinct

Extinction Why did this extinction happen? Some scientist propose: -Earth was struck by a meteor; the collision causing a huge, possibly poisonous dust cloud to fill the atmosphere -This cloud might have changed the climate and so fewer species survived -There is a huge crater in the waters off eastern Mexico that scientists suggest was the impact site of the meteor

How Did Earth Change During the Mesozoic Era? Scientists believe that in the early part of the Mesozoic, all the continents were joined together as one large landmass Later, the landmass broke up and the continents began to drift apart At the end of the Mesozoic the position of the continents were similar to what we know today

Earth’s Movement The continents are part of the rigid plates on Earth’s surface The plates sit on top of a molten layer of rock, which allows the plates to move around The theory of how continents move is called plate tectonics The plates are still moving at approximately 6 centimeters per year

Geologic Time Scale The current era, the Cenozoic, began about 65 million years ago During this time large mammals and humans first appeared The modern human species first appeared about 200,000 years ago