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Unit 4: Earth and Life History
Transcript of Unit 4: Earth and Life History
Chapter 7: Lesson 1
Relative Ages of Rocks
Geologists can determine the relative ages of rocks by studying the order of rock layers, fossils, and geologic processes that are occurring today.
Superposition & Fossil Record
Layers of rocks are called strata or stratum. Danish physician, Nicolas Steno, present four principles that helped geologists study strata and interpret the rocks' history.
Absolute Ages of Rocks
How would you go about finding the age of the Earth? In the past, scientists have tried measuring rates of erosion to see how long it would take mountains to erode. They tried calculating the time it would take Earth to cool from molten mass to its present temperature. These attempts were unsuccessful at predicting the Earth's age. Scientists have finally discovered a natural "clock" that allows for great accuracy. This clock allows for the age of Earth, meteorites, and the Moon to all be properly dated.
The Rock Cycle
The rock cycle is s series of process that make and change rocks through heating, melting, cooling, uplift, weathering, burial, and increasing pressure.
Beginning of Geology
Before the late 1700's, most scientists believed that rocks formed by evaporation of minerals dissolved in sea water. By observing rock features and the movement of sediments in streams, James Hutton first realized there were two process happening on Earth. One process built rocks and one process tore them down.
Hutton had an idea that "the present is the key to the past." His principle of uniformitarianism states that the processes that are at work today are the same processes that have been at work in Earth's past.
This allows scientists to observe the processes that are active on Earth today, and use them to interpret what happened in the past.
Change is a slow process...
nobody has ever seen a mountain suddenly appear
nobody has ever seen a river valley suddenly widen
Hutton knew this and he reasoned that if erosion occurred for a long time, it could greatly change Earth's surface.
Igneous rocks are produced when magma solidifies. There are different kinds of igneous rocks and they are identified by the size of the crystals that they contain.
Large crystals form when magma cools very slowly underground.
Small crystals form when magma cools rapidly above ground.
Metamorphic rock is any rock that is put under extreme pressure or heat short of melting.
Sedimentary rocks form from pieces of preexisting rocks. There are four processes that form sedimentary rocks: weathering, transportation, deposition, and lithification
1st Step: physical or chemical breakdown of rocks into smaller pieces
Physical: doesn't change the mineral composition
frost wedging: water freezing cause cracking
Chemical: changes the mineral composition
usually involves rainwater mixing with other chemicals
The 2nd step of rock formation is transportation. It occurs when sediments move downhill to lower areas where they come to rest. Sediments vary is size and shape - these different rock sizes are called clasts. Sediments move for various reasons:
The 3rd step in the formation of sedimentary rocks is deposition. It occurs when sediment being transported by water, wind, or a glacier slows down or stops. This usually happens in lower areas on the landscape and is called depositional environments. You can often times see the layering in the rock (this is common in sedimentary rocks).
The final step in the formation of sedimentary rock is lithification. As older sediment layers become buried beneath layers of younger sediments, the weight of the younger sediments, the weight of the younger sediments squeezes the older sediments together, compaction. Liquids filled with minerals then seep into the pore spaces between sediments and turn it into cements. This compaction and cementation changes the sediments into rock. The sediments have become lithified.
Principle of Superposition
In a stack of undisturbed sedimentary rock layers, the layers on the bottom were deposited before the layer on top. Fossils in the rock also follow this same principle.
Oldest at the bottom.
Youngest at the top.
Principle of Original Horizontality
Rock layers are originally deposited in horizontal, or nearly horizontal, layers. This principle supports rock strata that are now at an angle must have been tilted after the rocks formed.
Principle of Original Lateral Continuity
Sedimentary rocks form layers that cover large areas. Imagine a trench being cut in the ground to bury a wire. Now the ground is divided, with the once-continuous grass on either side. Rock strata are affected the same way.
Principle of Cross-cutting Relationships
A rock layer or feature that cuts across other rock layers is younger than the layers being cut. Imagine magma getting injected into and across a sequence of strata and then cooling into granite.
Atoms & Isotopes
Atoms are the building blocks of all matter. Atoms are made of protons, neutrons, and electrons. Protons and neutrons make up the dense nucleus of an atom. Electrons are found circling the nucleus.
Carbon is one of more than 100 known elements that exist on Earth. An element is defined by the number of protons in its nucleus. Carbon, atomic #6, has 6 protons. The number of neutrons and electrons can change from time to time. The atomic mass should be the sum of the protons and neutrons in an atom. When the neutrons in an atom change an isotope occurs.
Isotopes may be stable or unstable. Carbon has two stable isotopes and one unstable isotope. Radioactive decay occurs when an unstable atomic nucleus changes into another nucleus by emitting particles and energy, resulting in a stable isotope/element. A nucleus that undergoes radioactive decay is said to be radioactive. Radioactive decay is important because this is the natural clock that has helped scientists find the ages of Earth's rocks. The original atom is the parent isotope and the new stable isotope is the daughter isotope. Some isotopes take more than one radioactive decay to find a stable isotope. Uranium-238 goes through 14 steps to become stable.
Ernest Rutherford and Frederick Soddy in 1902, discovered that parent isotopes decay into daughter isotopes at a constant rate. This is called rate of decay. To measure radioactive decay scientists look at the half-life of the isotope. The half-life is the amount of time it takes for half of the original material to decay. This has allowed geologists to determine the absolute age of apiece of the Earth's crust by calculating the absolute age of the minerals that compose it. Different isotopes have different half-life times.
Determining Earth's Age
Radiometric dating is the procedure that scientists use to calculate absolute ages of rock and minerals. By measuring the amount of parent material and comparing it to the daughter material in a rock, the number of half-life the material has been through can be counted. Igneous rocks are the most common rocks used for radiometric dating.
Absolute Age of Earth
In 1989, scientists made an important discovery. They identified Earth's oldest rocks, found primarily on the continental shields. The continental shield areas are exposed rock that dates back to the Precambrian time - billions of years ago. Scientists found zircon crystals Canada and Australia that dated back between 4.0-4.4 billions years.
Scientists use the same procedures for the Moon and meteorites. Confirming the entire solar system formed at the same time!