Fossils
Relative Dating
Absolute Age
Law of Superposition
Theories
100
What is a fossil?
A fossil is the evidence or remains of once living things.
100
Compare relative versus absolute age.
The relative age of a rock is its age compared to the age of other rocks. The absolute age of a rock is the number of years since the rock formed. The absolute age is more exact.
100
What is radioactive decay?
It is the breakdown of the nucleus of an unstable, or radioactive element, so that the number of protons changes, thereby causing the atoms of one element to breakdown, or decay into those of another.
100
What is the geologic time scale?
The geologic time scale is a record, in chronological order, of all the life forms and geologic events in Earth's history.
100
How old is the Earth?
Scientists hypothesize that Earth formed at the same time as the other planets and the sun, roughly 4.6 billion years ago.
200
What is a mold?
A mold is a hollow area in sediment in the shape of an organism or part of an organism.
200
How is the relative age of a rock determined? What things complicate this process?
The relative age of a rock is determined by using the Law of Superposition, which states that, in horizontal sedimentary rock layers, the oldest layer is at the bottom, and each higher layer is younger than the layers below it. Faults, intrusions/extrusions, and unconformities complicate this process.
200
What is a half-life?
A half-life is the rate of decay of a radioactive element, or the amount of time that it takes for half of a sample of radioactive atoms to decay. Because the rate of decay for each radioactive element is constant, the half-life of an element is always the same, no matter how much of the element you have.
200
True or False? Explain your answer. Humans were dominant for most of geologic time.
False. If you tried to squeeze Earth's 4.6-billion year history into a 24-hour day, modern humans don't even appear until less than a second before midnight!
200
How do scientists know the age of the Earth?
The oldest rocks found on Earth have been radioactively dated at about 4 billion years old. However, the oldest moon rocks, also radioactively dated, are 4.6 billion years old. Scientists hypothesize that the Earth and moon are the same age, because they believe that the moon is a piece from the Earth that broke off when Earth collided with a large object just as it was forming.
300
What are preserved remains?
Organisms that were preserved with little or no change. This occurs when the organism becomes trapped in tar (organisms sometimes fell in tar pits looking for water), if they were frozen (trapped in ice), or if they were preserved in amber (hardened resin, or tree sap, from evergreen trees).
300
What is an index fossil, and why is it helpful in determining relative age?
An index fossil is the fossil of an organism that is widely distributed (not just found in a limited area) and lived only a short period of time. Because they live only a short period of time but are found all over the place, they are helpful in telling the relative ages of the rocks in which they occur.
300
How do scientists use half-life to determine the absolute age of a rock?
In radioactive dating, scientists first determine the amount of a radioactive element in a rock. Then they compare that amount with the amount of the stable element into which the radioactive element decays. If they know the half-life of the element, they can use the proportion of decay to original element to determine how many half-lives, and therefore how much time, has gone by.
300
How is geologic time divided?
After Precambrian Time (a long span of time which covers 88% of Earth's history!), the basic units of geologic time are eras (long units of time) and periods (units of time into which eras are subdivided).
300
What was early Earth like?
Early Earth had a very high temperature as a result of collisions with other things in space. Denser, solid materials formed Earth's core, but the surface was molten for a while until some it's thermal energy was transferred to space. The original atmosphere included hydrogen and helium captured from the sun as it revolved.
400
What kinds of information do Paleontologists get from fossils?
Paleontologists use fossils to help them determine what past life forms were like, how they changed over time, how the Earth's surface has changed and what the past environments were like.
400
Why are fossils only found in sedimentary rock?
Fossils would be destroyed by heat and/or pressure during the formation of igneous and metamorphic rocks. Sedimentary rocks are formed when sediments build, layer upon layer, and gradually cement and harden. Fossils can easily become trapped and preserved in the layers.
400
Why is carbon-14 more useful in dating materials from plants an animals that are lived only as long as 50,000 years ago (relatively young in geologic terms)?
It has a half-life of only 5,730 years, so it can't be used to date ancient rocks and fossils; the amount of Carbon-14 left to compare would be too small to measure accurately for materials more than 50,000 years old.
400
To what era does the Jurassic Period belong, and where does it fall in the rest of the timeline? Why is it significant?
The Jurassic period is in the Mesozoic Era, which falls between the Paleozoic Era (earliest) and the Cenozoic Era (current). It is significant because it is when dinosaurs thrived.
400
What happened to Earth's first atmosphere, and how did the next one form? Was there water on Early earth?
The first atmosphere was lost when the sun released a strong burst of particles that blew it away. A second atmosphere made up of carbon dioxide and water vapor, formed when volcanic eruptions released gases from Earth's interior, and collisions with comets contributed other gases such as nitrogen. At first Earth's surface was too hot for liquid water. Eventually as Earth cooled, the water vapor condensed and it began to rain forming oceans and eroding the rocky surface. The oceans absorbed much of the carbon dioxide.
500
What are trace fossils?
A scientific theory is a well-tested concept that explains a wide range of observations. The theory of evolution, which says that there has been a gradual change in living things over time, is supported by the fossil record, because organisms appear to go from simple to more and more complex.
500
Why do unconformities cause a gap in the geologic record? What is the general rule for determining the age of faults and intrusions/extrusions?
An unconformity is a surface where new rock layers meet a much older rock surface. This happens rock layers shift and older layers erode away and are covered by much "newer" sediment. This potentially create a big gap in time where some layers were lost. Instrusions/extrusions and faults are always younger than the things they cover or cut through.
500
Why is it impossible to find the absolute age of sedimentary rocks? Why is this a problem, and how is the problem overcome?
There are many small pieces of sediment that make up a sedimentary rock, so there is no one age for the rock. This is a problem, because fossils are only found in sedimentary rock, and we can't find the absolute age of it. However, paleontologists can find the absolute age of nearby igneous intrusions/extrusions and then use the Law of Superposition to determine the age of the sedimentary rock layers.
500
During what era and period did the Age of Mammals begin?
The age of mammals, which occurred after the mass extinction that wiped out the dinosaurs, began in the Tertiary Period of the Cenozoic Era.
500
How did life on Earth develop?
Scientists cannot pinpoint when or where life began on Earth. But they have found fossils of single-celled organisms in rocks that formed 3.5 billion years ago. When those organisms which lived in water, began using energy from the sun for photosynthesis, they released oxygen, increasing the amount of oxygen in the atmosphere and gradually forming the ozone layer; the ozone layer provided protection from the harmful layers of the sun, so that organisms could begin to live on land.
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