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Inch and centimeters
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2 digit Addition/Subtraction QuizShowing how to find the difference between two lengths Math3rd 9 weeks MathQuiz (Second Grade)Lesson 1: Continental Drift Theory and the Evidences that support the Theory Continental drift describes one of the earliest ways geologists thought continents moved over time. Today, the theory of continental drift has been replaced by the science of plate tectonics. The theory of continental drift is most associated with the scientist Alfred Wegener. In the early 20th century, Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes plowing through oceans and into each other. He called this movement continental drift. Pangaea Wegener was convinced that all of Earth’s continents were once part of an enormous, single landmass called Pangaea. Wegener, trained as an astronomer, used biology, botany, and geology describe Pangaea and continental drift. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers. Wegener also studied plant fossils from the frigid Arctic Archipelago of Svalbard, Norway. These plants were not the hardy specimens adapted to survive in the Arctic climate. These fossils were of tropical plants, which are adapted to a much warmer, more humid environment. The presence of these fossils suggests Svalbard once had a tropical climate. Finally, Wegener studied the stratigraphy of different rocks and mountain ranges. The east coast of South America and the west coast of Africa seem to fit together like pieces of a jigsaw puzzle, and Wegener discovered their rock layers “fit” just as clearly. South America and Africa were not the only continents with similar geology. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their positions as the continent we recognize today. Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago. Tectonic Activity Scientists did not accept Wegener’s theory of continental drift. One of the elements lacking in the theory was the mechanism for how it works—why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. (It doesn't.) Today, we know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. The continents are still moving today. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. In the process of seafloor spreading, molten rock rises from within the Earth and adds new seafloor (oceanic crust) to the edges of the old. Seafloor spreading is most dynamic along giant underwater mountain ranges known as mid-ocean ridges. As the seafloor grows wider, the continents on opposite sides of the ridge move away from each other. The North American and Eurasian tectonic plates, for example, are separated by the Mid-Atlantic Ridge. The two continents are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the Great Rift Valley system. What is now a single continent will emerge as two—one on the African plate and the other on the smaller Somali plate. The new Somali continent will be mostly oceanic, with the Horn of Africa and Madagascar its largest landmasses. The processes of seafloor spreading, rift valley formation, and subduction (where heavier tectonic plates sink beneath lighter ones) were not well-established until the 1960s. These processes were the main geologic forces behind what Wegener recognized as continental drift.Measurement cm and inchNumber Twelve Number Twelve Is Everywhere! We use the number twelve to help us do things every day. It helps us to count things, measure length, and tell time. Did you ever wonder why a year has twelve months? A ruler has twelve inches? A clock has twelve hours? How We Learned to Count. Long ago, people needed an easy way to count things. How many goats did they have? How many bricks did they need? They didn't have pencils, but they always had their fingers. How could their fingers help them count things? Your hand has four fingers and one thumb. Fingers bend into three parts. All of your finger parts add up to-twelve! To learn how people counted, use the thumb of your other hand. Count the parts of each finger. One, two, three, four, five, six. Using their fingers, people could count their farm animals. They might count six chickens, three sheep, and three goats. They had twelve animals! When their animals had babies, it was easy to count them, too. Using both hands, people could count to twenty-four. Twenty-four is two twelves! A Ruler Is Twelve Thumbs Long Fingers, and the number twelve, were also used to find length. An inch was the width of an adult man's thumb. A foot was twelve thumbs long. Soon, people cut twelve marks for thumb widths on a stick. It was easy to carry the stick. That stick was the first kind of twelve-inch ruler! Twelve Hours in a Day People used the number twelve to divide their days into parts, or hours, that they could count. In daylight, they watched the shadow of a stick or a sundial to know what hour it was. Twelve things are also called a dozen. We buy a dozen pencils in a box. We buy a dozen doughnuts or a dozen eggs. The number twelve helps us to do a dozen things every day!