UNIT 8/9: GEOLOGIC TIME AND VIRGINIA GEOLOGY ES 9, 6c, 7a

ES. 9 Historical Geology The student will investigate and understand that many aspects of the history and evolution of Earth and life can be inferred by studying rocks and fossils. Key concepts include a) traces and remains of ancient, often extinct, life are preserved by various means in many sedimentary rocks; b) superposition, cross-cutting relationships, index fossils, and radioactive decay are methods of dating bodies of rocks; c) absolute and relative dating have different applications but can be used together to determine the age of rocks and structures; and d) rocks and fossils from many different geologic periods and epochs are found in Virginia.

Unit 8 Part 1: Fossil Formation and Types Essential Questions: How are fossils formed and how might an index fossil be used in studying the Earth’s history? Essential Understandings: The history of Earth and the ages of rocks can be investigated and understood by studying rocks and fossils. Evidence of ancient, often extinct life is preserved in many sedimentary rocks. A fossil is the remains, impression, or other evidence preserved in rock of the former existence of life. Fossil evidence indicates that life forms have changed and become more complex over geologic time. Some ways in which fossils can be preserved are molds, casts, and original bone or shell.

What is a Fossil? The history of the Earth and ages of rocks can be investigated and understood by studying rocks and fossils found within sedimentary rocks. A fossil is the remains, impression or other evidence preserved in rock of the former existence of life.

Fossils Fossil evidence indicates that life forms have changed and become more complex over geologic time. Some ways in which fossils can be preserved are Molds Casts Original remains Petrified Trace fossil

Types of Fossils Organisms buried in sediment may decay or dissolve away leaving a cavity or mold. If the space is subsequently filled with sediment, an external cast can be made. Molds and casts are three dimensional and preserve the surface contours of the organism.

Types of Fossils Original Remains: All or part of an organism becomes encased in amber, ice, or tar preserving the organism

Types of Fossils Petrification A fossil in which minerals fill the cells of an organism (ex. wood, bone, etc.). This process is called petrifaction = “turned into rock”. Trace Fossils Preserved evidence of the activities of ancient organisms (footprints, burrows, etc.)

Index Fossil A fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found Index fossils can also be used to correlate rock layers Abundant, geographically widespread, lived for a short period of time

Unit 8 Part 2: Relative & Absolute Dating Essential Questions: How are absolute and relative dating techniques used to determine the age of rock strata and fossils? Essential Understandings: Relative time places events in a sequence without assigning any numerical ages. Fossils, superposition, and cross-cutting relations are used to determine the relative ages of rocks. Absolute time places a numerical age on an event. Radioactive decay is used to determine the absolute age of rocks. The age of Earth is about 4.6 billion years.

Finding Out How Old Rocks Are We are going to determine the age of rocks in two ways: 1) Relative Time = puts events in the sequence of their occurrence, but does not identify the actual date. EExample: Mrs. Cureton was born after The Return of the Jedi was released into theaters but before The Cosby Show debuted

Law or Original Horizontality To understand this we must first learn some laws associated with correlating rock age. 1) Sedimentary rocks are usually deposited in horizontal layers W L QAQA K A P V Z

Law of Superposition 2) In a sequence of undisturbed sedimentary rock layers, the oldest rocks will be at the bottom & the youngest at the top Youngest Oldest

Youngest

Oldest Youngest

Oldest Youngest Ocean Basin Deposits form Rock

Law of Cross-Cutting Relationships 3) Igneous intrusions or faults are younger than the rocks it intrudes or cuts across  Intrusions occur when magma rises up from deep within the earth and burns into the rock.  Cross-cutting Features:  Sill – an igneous pluton that is horizontal to the bedding plane of the sedimentary rocks it intrudes.  Dike - an igneous pluton that is vertical to or cuts across the bedding plane of the sedimentary rocks it intrudes.  Stock and Batholith- huge units of rising magma that cuts into rock.

Laccolith- a bulge of magma between sedimentary rock layers that is fed by a dike.

Examples of plutons showing intrusive relationships.  Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B. Sedimentary Rock D is younger than Erosion Surface C.  Diagram (2): Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B.  Diagram (3): Stock B is younger than Sedimentary Rock A. Dike C is the youngest.

(1) Normal fault (2) Reverse fault Examples of faults to illustrate cross-cutting relationships. (1) Unit A is the oldest, followed by B and C. Fault D is the youngest. (2) Unit A is the oldest, followed by B and C. Fault D is younger than unit F & E. F Fault – A break in rock layers.

 D is the oldest rock layer because it gets cut across by everything.  C is the next oldest because it gets cut across by B and cuts through D  B is the next oldest because it gets cut across by a and cuts through C and D  A is the youngest because it cuts across everything.

F is younger than D, C, and B because it cuts across them, but older than A because it doesn’t cut across it. E is the youngest rock because it cuts across all others. The rocks are currently being eroded on the surface.

Which is oldest: X, T, K, or H? H Which is youngest? X

Which formed first: the S layers or the fault that goes through them? S

Folds are bends in rock that form when rock is put under stress, usually when two continents collide together. Which happened first: the rock layers or the folding of them? The rocks

Sometimes the folds get eroded away and you only see tilted rock layers. Which happened first: the rock layers A –E or the tilting of those rocks? Rocks A-E and they were originally… horizontal

Law of Included Fragments 4) The pieces of one rock found in another rock must be older than the rock in which they’re found Ex. Conglomerate The clasts in this conglomerate were already formed before the actual conglomerate formed. It was weathered, eroded, deposited and lithified before it became a conglomerate.

The dark pieces found in the lighter rock layers are older than the layer they are in.

What existed first: the little pieces of rock within rock layer S or rock layer S? The little pieces within S

Unconformities Unconformity = a place in the rock record where rock layers or parts of rock layers are missing from erosion. There are three main types 1) Angular Unconformity 2) Disconformity 3) Nonconformity

1. ANGULAR UNCONFORMITIES Angular unconformities are characterized by an erosional surface is between (cuts into) folded or dipping (tilted) strata and horizontal layers. Overlying strata (layers) are deposited basically parallel with the erosion surface. Erosional surface sualizations/es2902/es2902page01.cfm?chapter_no=visualizati on

2. DISCONFORMITIES Disconformities are characterized by an irregular erosional surface which is between flat-lying sedimentary rocks. Erosional surface

3. NONCONFORMITIES Nonconformities are characterized by an erosional surface which truncates igneous or metamorphic rocks. Erosional surface

Nonconformities happen when an igneous intrusion is exposed and weathered and eroded.

Geologic history Surface Erosion (youngest) ______F___________ ______J___________ ______B___________ ___Erosion A________ _____R____________ _____S____________ _____M____________ ______K ___________ ______P_____(oldest)

B D J T P G Z X F L E A Q Folding of Layers G,X,P,T,Q,D,E,A,L Erosion A Geologic history Surface Erosion (youngest) ____B_________ ____F_________ ____Z_________ ____Erosion A__ ____Folding____ ____G_________ ____X_________ ____P_________ ____T_________ ____Q_________ ____D_________ ____E_________ ____A_________ ____L___(oldest)

F C C F G B B Erosion Z Fault R G E E H Geologic history Surface Erosion (youngest) ____A_________ ____D_________ __Erosion Z____ ____Fault R____ ____H_________ ____C_________ ____F_________ ____G_________ ____B_________ ____E___(oldest)

Erosion P Folding Z Geologic history Surface Erosion(youngest) ____G________ ____B________ ____D________ ____E________ ____A________ _Erosion P____ __Folding Z____ ___C_________ ___F_________ ___H___(oldest)

Absolute Dating Radiometric Dating techniques use naturally-occurring radioactive isotopes Isotope -- form of an element that has additional neutrons Radioisotope -- isotope that spontaneously decays, giving off radiation Rate of Radioactive Decay Radioisotopes decay at a constant rate. Rate of decay is measured by half-life Half-life -- time it takes for one-half of the radioactive material to decay. Decay products Radioisotopes may decay to form a different isotope or a stable isotope. May be a series of radioactive decays before a stable isotope is formed. Stable isotope is called the "daughter" formed from decay of radioactive "parent"

Absolute Dating Radiometric Age Dating Radioisotopes are trapped in minerals when they crystallize. Radioisotopes decay through time, and stable isotopes are formed. Determining the ratio of parent isotope to daughter product reveals the number of half-lives that has elapsed. Common isotopes used in age dating Uranium to Lead -- half-life of U-238 is 4.5 b.y. Carbon half-life of C-14 is 5730 yrs

Unit 8 Part 3: Geologic Time Scale Essential Questions: What is the geologic time scale? Essential Understandings: Paleozoic, Mesozoic, and Cenozoic fossils are found in Virginia.

Geologic Time Scale – a timeline of Earth’s 4.6 billion year old history divided into periods of time by major events or changes on Earth What do we call these major events or changes? Geologic Time Scale

Catastrophic Events – events that cause major destruction or change life completely Examples: Mass extinctions Ice Ages Meteors Major volcanic eruptions Catastrophic Events

1. Hadean 2. Archean 3. Proterozoic 4. Phanerozoic 1-3 make up Precambrian Time

AMOUNT OF TIME: 90% of Earth’s history Lasted nearly 4 billion years of Earth’s 4.6 billion year total MAIN EVENTS: Volcanic ash & dust ->clouds formed ->rain Single-celled micro-organisms in ocean towards the end NO animals! NO plants!

Explosion of Life! Contains 3 Eras: Paleozoic Era – “Age of Amphibians” Mesozoic Era – “Age of Dinosaurs” Cenozoic Era – “Age of Mammals”

544 mya – 248 mya All life in ocean until end fish developed Reptiles, Insects, Ferns developed –> life moving onto land towards end Ends with a Mass Extinction possibly from a meteor strike – kills 90% of all ocean species!

248 mya – 65 mya Dinosaurs Rule! Small mammals, birds, and flowering plants Ends with Mass Extinction due to meteor strike off Mexico Dust cloud blocked sunlight, killed plant life and affected food chain

65 mya – Present Large warm-blooded mammals, modern birds Animals developed migration techniques Quaternary Period = the past 2 million yrs Ice ages 1 st modern human fossils = 100,000 yrs old

Unit 9 Part 1: Virginia Provinces Essential Questions: Where are most fossils found in Virginia? Essential Understandings: Each province has unique physical characteristics resulting from its geologic past. The five physiographic provinces of Virginia are Coastal Plain, Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateau.

Unit 9 Part 2: Geology and Formation Essential Questions: How can you compare and contrast the geology of the physiographic provinces of Virginia? How can plate tectonics be evidenced by features found in Virginia? Essential Understandings: Virginia has a billion-year-long tectonic and geologic history. Virginia has five physiographic provinces produced by past episodes of tectonic activity and continuous geologic activity.

Coastal Plain Description Essential Understanding: The Coastal Plain is a flat area composed of young, unconsolidated sediments underlain by older crystalline basement rocks. Geology These layers of sediment were produced by erosion of the Appalachian Mountains and Piedmont and then deposited on the Coastal Plain when sea levels were higher in the past.

Piedmont Description Essential Understanding: The Piedmont is an area of rolling hills underlain by mostly ancient igneous and metamorphic rocks. Geology The igneous rocks are the roots of volcanoes formed during an ancient episode of subduction that occurred before the formation of the Appalachian Mountains

Blue Ridge Description Essential Understanding: The Blue Ridge is a high ridge separating the Piedmont from the Valley and Ridge Province. Geology The billion-year-old igneous and metamorphic rocks of the Blue Ridge are the oldest in the state.

Valley and Ridge Description Essential Understandings: The Valley and Ridge province is an area with long parallel ridges and valleys underlain by ancient folded and faulted sedimentary rocks. Geology The folding and faulting of the sedimentary rocks occurred during a collision between Africa and North America. The collision, which occurred in the late Paleozoic era, produced the Appalachian Mountains.

Appalachian Plateau Description Essential Understandings: The Appalachian Plateau has rugged, irregular topography and is underlain by ancient, flat- lying sedimentary rocks. Extends into other states, like Kentucky and Tennessee. Only a small part of the Appalachian Plateau is in Virginia Geology The area is actually a series of plateaus separated by faults and erosional down-cut valleys. Most of Virginia’s coal resources are found in the plateau province.

Virginia Fossils In Virginia, fossils are found mainly in the Coastal Plain, Valley and Ridge, and Appalachian Plateau provinces. Most Virginia fossils are of marine organisms. This indicates that large areas of the state have been periodically covered by seawater. Chesapectan Jeffersonius (pictured below) is the Virginia State Fossil

Unit 9 Part 3: Virginia Resources Essential Questions: What are the significant energy resources found in Virginia? Essential Understandings: Virginia has many natural resources. In Virginia, major rock and mineral resources include coal for energy, gravel and crushed stone for road construction, silica for electronics, zirconium and titanium for advanced metallurgy, and limestone for making concrete.

Coal in Appalachian Plateau Coal is an organic sedimentary rock used as a source of fuel Used in coal-fired power plants to generate electricity Major resource of Virginia

Resources in Virginia Other resources of Virginia include: gravel and crushed stone for road construction silica for electronics zirconium and titanium for advanced metallurgy and limestone for making concrete.