Exam One Flashcards
What are the three rock types?
- Sedimentary
- Metamorphic
- Igneous
How are sedimentary rocks formed?
Formed from deposits of pre-existing rock (weathering, erosion, deposition)
How are metamorphic rocks formed?
when rocks are subjected to high heat and high pressure
How are igneous rocks formed?
Melting, magma cools and then crystallizes
Which type of rock forms at the Earth’s surface?
- Sedimentary rocks
How do sedimentary rocks tell us about life and the environment when and where they formed?
- Grain Size: tells us how close/far the rock is from its origin.
- Grain Shape: tells us about the mode of transportation.
- Structures tell us about the environment it formed in (ripple marks, graded bedding, etc.)
Why do we study the history of Earth?
Teaches us about past life and environments
loose grains or crystals (broken pieces of older rocks; ex: sand); weathered pieces of rock and other materials
sediment
grains or crystals held together. (ex: sandstone); when sediments are compacted and cemented together
sedimentary rocks
What are the four steps of the sedimentary cycle?
- Weathering
- Erosion
- Transport
- Deposition
break-down of older rocks into sediment; how sediment gets created (can happen anywhere rocks are on the surface of the Earth)
weathering
when the sediment created by weathering starts moving; removed from where it formed
erosion
the movement of sediment from one place to another (sediment in motion
transport
What are the four main ways a sediment is transported?
- Wind
- Water
- Gravity
- Ice
when the sediment stops moving; what allows it to potentially become a sedimentary rock in the future
deposition
Larger sand grains stop where water (speeds up/slows down)
slows down
What kinds of sediment are produced by weathering?
Clastic sediments (gravel, sand, silt, and clay)
how sediments change during transport
sediment maturity
A mature sediment is (poorly/well) sorted and (poorly/well) rounded
well-sorted and well-rounded
What are the three things taken into account when determining a sediments maturity?
- Grain size
- Grain shape
- Composition
How can grain size tell us how far sediment has traveled?
Large= hasn’t traveled far
Small= has traveled far
How can the shape of grains tell us how far sediment has traveled?
Angular= hasn’t traveled far
Rounded= has traveled far
How can the mineral composition tell us how far sediment has traveled?
The farther a sediment has traveled, the fewer kinds of minerals are left (losing the ones that can’t survive transport; more minerals near the source)
Most common mineral on Earth’s surface:
olivine
Most common mineral in Earth’s crust:
feldspar
One of the toughest minerals at Earth’s surface:
quartz
What clues tell us that a sediment was deposited in a continental (land) environment and not in the ocean?
- Color: red signifies a dry environment, a dark color indicates it formed in the ocean
- Roundness (sediment maturity): more angular on land and rounded in ocean
- Fossils: can tell us if animal lived on land or in ocean
- Sedimentary structures (river channels, sand dunes)
What are 5 examples of continental depositional environments?
- Desert
- River
- Freshwater
- Glacier
- Lake
What does sediment color tell us about depositional environments?
- Red= sediment formed in hot or dry environment
- Dark= sediment formed in a wet environment (ex: swamps → where coal forms from dead plants)
Immature sediments are ________. (Either continental, marine, or transitional)
continental
How can we tell that a sediment was deposited in a desert?
Often is a red color and often has cross bedding.
Sand dunes are created by transport of ______. It cannot move heavy things like water can, so it moves things like sand.
wind
Which part of a river can move heavier sediments?
- Deeper, faster moving water
- Shallow, slower moving water
Deeper, faster moving water
how can we tell that a sediment was deposited in a river?
- grey/green color
- often poorly sorted
Why are beach sands usually only made up of quartz? What happened to the other minerals as the sediments traveled from rivers to the coast?
- quartz is resistant to both physical and chemical weathering
- a long journey allows weaker minerals to be broken down
Symmetrical ripples form from what? What does this tell you about its depositional environment?
- back-and-forth waves (equal slopes) - tells you the sedimentary rock was deposited at the beach
What sediment is most likely to end up far out in the ocean/offshore? Why?
Clay because it’s the smallest grain size so it can travel farther.
What rock type is found offshore?
Shale (made up of clay); carbonaceous sedimentary rocks
What type of rock forms from coral reefs?
Limestone (made from calcite shells)
What can limestone tell us about the environment of the area it was formed in?
Coral reefs need clear, warm, tropical water far from shore or major rivers.
Grain size (increases/decreases) away from the shore.
decreases
Theory that states that rocks formed from sudden global disasters, not slow sediment deposition.
Catastrophism
Did the theory of catastrophism say the Earth was very young or very old?
very young
theory that states that ancient rocks formed the same way as modern ones
uniformitarianism
Did the theory of uniformitarianism say the Earth was very young or very old?
very old
Why did early uniformitarians like Hutton interpret that the Earth was very old?
Because the sediment cycle takes a long time. He stated that the processes of erosion, deposition, sedimentation, and upthrusting were cyclical and must have been repeated many times in the Earth’s history.
What is the order of the periods in the Geologic Time Scale?
- Cambrian
- Ordovician
- Silurian
- Devonian
- Mississippian
- Pennsylvanian
- Permian
- Triassic
- Jurassic
- Cretaceous
- Paleogene
- Neogene
- Quaternary
Today’s geologists are mostly uniformitarians, but they allow for exceptions… What are two examples of exceptions?
- Komatiite- igneous rock formed nowhere on earth today
- Tektites- debris from asteroid impact
- order of events
- benefits: easy to figure out, helps test cause and effect
- downside: doesn’t give us exact time, not precise
relative dating
- numbers, often years before the present
- Benefits: can say how fast things happened and how often
- Downside: difficult, expensive, not always possible
absolute dating
the study of rock layers
stratigraphy
What does the law of superposition state about the age of rocks and the order in which they formed?
younger rocks form on top of older rocks
What does the principle of original horizontality tell us about how sedimentary rocks form?
Rock layers (sedimentary rocks) originally form horizontally bc of gravity; all tilting occurs later once the sediment solidifies and becomes rock
igneous intrusions and faults cut through older rocks, therefore are younger.
Rule of Cross Cutting Relationships
Cross-cutting relationships: If rock A is cross-cut by rock B, which one is younger? Which one is older?
Rock A: older
Rock B: younger
What is an unconformity and why does it form?
- missing rock layers due to erosion
- why: older rocks were weathered and eroded before new rocks were deposited on top.
Why are fossils used to compare the ages of rock layers?
Different fossil species lived at different times; layers with the same fossil species are about the same age.
matching rock layers from one place to another
correlation
What are index fossils?
Good fossils for matching rock ages
3 Features that make a fossil an index fossil:
- Abundant and found all over the world
- Common/easy to find
- Existed for a short time/changed rapidly
Two atoms with the same number of protons are the same
chemical element
Different forms of the same element, which have the same number of protons but a different number of neutrons
isotope
What does it mean for an isotope to be radioactive?
Their nuclei are unstable (because of the number of neutrons) and break down at a constant rate over time
How can the rate of decay of radioactive isotopes help us date rocks?
- Scientists can find out how old the rock is depending on the radioactive isotopes half-life.
- Crystals only form with parent isotopes.
- Daughter isotope that forms later is trapped in the crystal.
In radioactive decay, a parent isotope changes into the daughter isotope of (the same/a different) element. Therefore, there is always a change in the number of (neutrons/protons)
- a different
- protons
the time it takes half the parent isotope in a sample to decay into daughter isotope.
half-life
Sedimentary rocks are dated by (absolute age/relative age) compared to igneous rocks.
relative age
