The Rock and Fossil Record Geologic Time The Rock and Fossil Record

Geologic Time Scale time divisions Geologists use different _______ ________ to keep track of Earth’s history. These divisions are based on: Type of ____________ present during the time period Type of ____________ changes occurring at the time. Geologic Time Scale – a record of Earth’s history, beginning 4.6 bya, that shows events, time units, and ages. life forms geologic

Geologic Time Scale There are three types of subdivisions of geologic time: ______ – major subdivisions of time based on differences in life-forms. __________ – subdivisions of eras based on types of life and geologic events (mountain building or plate movements). _________ – subdivisions of periods based on types of life and geologic events. (Epochs are found in the Cenozoic Era alone because the fossil records and rock layers are more complete for this time.) ERA Period Epoch

Era Period Epoch Phanerozoic Eon Cenozoic Quaternary Recent Pleistocene Neogene Pliocene Miocene Paleogene Oligocene Eocene Paleocene Mesozoic Cretaceous Jurassic Triassic Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Precambrian Time

Earths Story and Those Who First Listened. uniformitarianism Scientists use ____________________ to interpret the past and predict the future. James Hutton 1795 – “Theory of Earth” The _______ is the key to the _____. Things that happened now must have happened in the past. present past

Uniformitarianism versus Catastrophism Uniformitarianism – equal to slow change Catastrophism – equals quick sudden changes. Which is right? A little of both.

Relative Dating - Superposition Here’s a stack of newspapers collecting dust in the living room. Everyday as the paper is read it is put in the pile. Someone comes to you and asks about an article from last week. Where do you look?

Relative Dating? Which came first? Which layer is older? Geologists use relative dating to determine the ____________ of objects. Principle of Superposition – younger layers lie above older layers in an undisturbed sequence of rocks. A relative age C B A

The Principle of Superposition In an undisturbed layer of rock the oldest rock is on the ___________ and the youngest is on the _______. BOTTOM TOP

The Principle of Superposition Exceptions to the Principle include tectonic forces acting on the rocks to _______, tilt, fault, or __________ rock layers. MOVE ERODE

Original Horizontality: sedimentary layers are initially deposited in horizontal layers The forces of reality beat up on the earth and break it into small pieces, which then wash down rivers to the ocean and settle out on the seafloor. In horizontal layers.

Relative Dating Used by geologist to determine the order of events and the relative age of rocks by their ___________ in the stack. POSITION

Relative Dating This image shows a stack of rocks with 3 different types of rock and one fault. Which is the oldest?

Relative Dating Here’s the sequence: A, B, fault, C C B B A A

Unconformities Rock layers are a record of past geologic events. Most are ____________ because they are missing layers. If sediment stops being deposited and _________ goes to work removing material then you will have a gap in the record. These gaps are called _________________. INCOMPLETE EROSION UNCONFORMITIES

Angular Unconformities

Angular Unconformities

Angular Unconformities

Disconformity No obvious evidence of missing rock layers except for an __________ surface caused by erosion. UNEVEN

Nonconformity When sedimentary rock layers form above ____________ or intrusive ___________ rock layers. The metamorphic or igneous rock is uplifted and eroded. Sedimentary rocks are then deposited on top. METAMORPHIC IGNEOUS

Nonconformity Arrow points to contact of Cambrian Tapeats Sandstone on Precambrian Vishnu Schist.

Cross-cutting relationships: the thing being cut is ________ than the thing doing the cutting OLDER You can’t cut it if it isn’t there. Igneous intrusions: Dikes, Sills, Batholiths, Laccoliths

Cross-cutting Relationships

Fossils

Fossils Definition: Fossil – the remains, imprints, or traces of once-living organisms, usually preserved in rock, that tell us when, where, and how those organisms lived.

Fossils Fossils help scientists determine when and where plants and animals lived. Also, how they lived.

Fossil Formation Necessary Conditions: Quick Burial Hard Parts The body of a dead organism must be protected from scavengers and microorganisms. ________________ (sediments from floods, landslides, etc.) ________________ (bones, shells, or teeth) _________________________ Fossils typically do not withstand the heat and pressure involved in forming igneous and metamorphic rocks. Limestone, Shale, Sandstone are examples Quick Burial Hard Parts Sedimentary Rocks NEARLY ALL FOSSILS ARE FOUND IN SEDIMENTARY ROCK

Fossil in Virginia Appalachian Plateau Valley & Ridge Coastal Plain Why not the Blue Ridge and Piedmont?

Fossil in Virginia Era Period Epoch Cenozoic Quaternary Recent Pleistocene Tertiary Pliocene Miocene Oligocene Eocene Paleocene Mesozoic Cretaceous Jurassic Triassic Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Precambrian Time Paleozoic, Mesozoic, and Cenozoic fossils are found in Virginia 544 mya – present Paleozoic Mesozoic Cenozoic

Petrified Remains Some or all of the original materials in the remains have been replaced by minerals.

Carbonaceous Films The tissues of most organisms are made of compounds that contain carbon. Sometimes that’s all that is left. Pressure and heat force gases and liquids from the body and leave an outline of the body.

Carbonaceous Films Swamps and deltas where large amounts of plant matter are areas in which these fossils are found.

Molds and Casts Seashells or other hard parts fall into soft sediment such as mud and get buried. Compaction and sedimentation turn the sediments to rock. Pores in the rock let water and air reach the object and allow it to decay. The resulting void is called a mold. Other sediments may fill the mold and solidify into a cast.

Original Remains Sometimes the original remains are preserved. Insects trapped in amber (crystallized resin) Ice men, mammoths Tar Pits

Trace Fossils Fossilized tracks or other evidence of animal activity. Dino-tracks Coprolites

Index fossils Species inhabit Earth for a period of time before evolving into new species or dying out. Paleontologists use animals that exist for a short time period as index fossils. Index fossil – species that existed on Earth for a relatively ______ period of time, were _________, were ___________ geographically. short numerous wide-spread

Index fossils Index fossils are used to determine the age of rock layers. 200 Life Span (mya) A 500 - 200 300 B 400 - 200 400 C 400 - 300 500

Fossils and Ancient Environments Fossils also indicate the ______________of an area long ago. Remember Antarctic rocks contain fossils of tropical plants environment

Fossils and Ancient Environments Most VA fossils are of __________________ Brachiopod fossils indicate a shallow sea environment. Brach’s are found in the mountains of Virginia What can you conclude from this? marine organisms

Absolute Dating Absolute time places a _____________ age on an event. numerical Absolute time places a _____________ age on an event. _________________ is used to determine the absolute age of rocks. Radioactive decay

Radiometric Dating Radiometric Dating – determining the age of a material by comparing the ratio of the ___________ (undecayed) material to the ___________ (decayed) material. Example: A rock has an isotope that takes 10,000 years to decay into half of the parent material. The half-live is 10,000 years. Scientist examine the rock and find exactly half of the material is “parent” material and half is “daughter” material. Then the age of the rock has to be _______________ years. parent daughter 10,000

C