Physical Geography
Hello!
I am Mr. Naetzker a social studies teacher at Klein Oak High School and the subjects I am currently teaching are World Geography and Pre-Ap World Geography.
In the next number of lessons were going to take an extensive look at the things that affect our planet: the size and shape of the Earth, the structure of the Earth, and the Earth-Sun relationship followed by the march of the seasons. We’re also going to touch on the composition of the atmosphere, controls of weather, followed by Alfred Wegener’s Continental Drift theory, followed by Plate Tectonics and plate boundaries and the things that result from the movement of tectonic plates: things like mountains, volcanoes, and earthquakes. Next we have the hydrological cycle (or water cycle) followed byDenudation which is the process of rocks being broken down by a number of processes like physical weatheringand chemical weathering. So there are a lot of things we’re going to be taking a look at. The primary source for this transcript and podcasts comes from McKnight’s Physical Geography Tenth edition: A landscape Appreciation; I will paraphrase a number of ideas from it and will also quote heavily from it in areas I believe that are needed.
Let’s briefly talk about the size and shape of the earth. You may ask the question, is the Earth large or small. This hinges on one’s frame of reference; if you’re thinking about the size of the earth in relation to the universe, then the answer is obvious that the earth is infinitely small. The diameter of our planet is 13,000 kilometers (or 7,900 miles). The moon which has an effect on our ocean tides is 385,000 kilometers (239,000 miles) away from the Earth, and the Sun which gives us our energy to survive is 150,000,000 kilometers (93,000,000 miles away). I would like to make a minor note that if we sliced the Earth in half at the Equator, the distance would be 12,756 kilometers (7,926 miles). However, if you sliced the Earth in half from Pole to Pole, the distance would be 12,714 Kilometers (7,926 miles). So the earth is not completely spherical, it is an oblate sphere. But most textbooks (including some college texts) and in this course we’ll simply treat the earth as a sphere.[1]
The Structure of the Earth
On Earth’s outer crust there are four basic spheres that overlap each other which allow life on earth: lithosphere (or Earth’s crust), biosphere (all living organisms), hydrosphere (water), and atmosphere (this one is a beast). Our knowledge of the internal structure of the earth is based off indirect evidence, in fact the deepest mine shaft extends about 2.4 miles into the earth, and the deepest we probed into the Earth is about 8miles. So a good question for you to think about is how can we know the earth is structured the way we believe it is, when we have not directly observed it? I will let you think about that while I give you what is currently believed by the scientific community: We believe the structure of the earth is composed of 4 concentric areas. Starting from the outside working in the Earth has a crust which is where the tectonic plates are. The crust underneath the ocean plates is not as thick as the crust underneath the continental plates averaging about 4 miles; whereas the crust underneath the continents averages 40 miles in thickness.This crustal layer makes up less than 1% of the total volume of the Earth, about .04% to be exact. Next is the mantlebut before we get to the mantle a Yugoslavian seismologist AndrijaMohorovicic made an observation that the mineral composition changes in a zone now named after her called the “Moho” between the crust and the mantle. The mantle extends down 2,900 kilometers (1800 miles). The mantle is about ½ of the volume of the Earth. The uppermost mantle and the crust are both generally referred to as the lithosphere. Below this area is the asthenosphere (“weak sphere”). This area is so hot that the rock losses much of its strength and becomes plastic. Below the asthenosphere it is believe that the rock becomes rigid again. Beneath this area is the outer core which is comprised mostly of molten (liquid) magma and extends 5,000 kilometers (3,100 miles). Lastly we reach the inner core which is very dense estimated to be 1450 kilometers (900 miles) in radius; the inner and outer cores are thought to compose iron/nickel or iron/silicate. They make up 15 percent of the Earth’s volume and 32% of its mass. Earth’s magnetic field is generated from the outer core; and it is interesting to note that magnetic north is different from geographic north at the North Pole. The magnetic field of the Earth changes slowly over time even tens of kilometers a year. The current degree of Magnetic North is 83ᵒN, 114ᵒW but the position can change slightly in a day. In additional to all of this, for reasons unknown at this time, the polarity of the Earth switches at irregular intervals over thousand years, that is, that Magnetic North will eventually shift to Magnetic South. So your magnetic compass instead of pointing toward the North Pole, will point to the South Pole.
No back to the question about how is it the case that we know the Earth is structured this way. John Mcphee gave a colorful imagery about Earth scientists which “are like dermatologists: they study, for the most part, the outermost two percent of the earth. They crawl around like fleas on the World’s tough hide, exploring every wrinkle and crease, and try to figure out what makes the animal move.” Even so, one of the ways scientists developed theories about the Earth’s structure is by measuring seismic waves, and manmade explosions.[2] The speed of seismic waves would change when they crossed certain boundaries. Seismologist Inge Lehmann was recording vibrations or seismic waves crated by earthquakes. It was anticipated that the seismic wave should have passed through the Earth, but in fact, the wave stopped dead in its tracks as if it hit something. As a result, he made an observation that the Earth has an Iron core.[3]
Earth-Sun Relationship
The Earth is dependent on the Sun for solar energy which without, we would cease to exist. The Earth has two basic movements: It’s every day rotationon its axis, and its annual revolution around the sun. The Earth rotates counter-clockwise (West to East) in a 24 hour period marking a full rotation. It takes the Earth 365 days to complete a revolution around the Sun. The path the Earth travels around the Sun is not a true circle but an ellipse. Because of this elliptical orbit, the Earth-Sun distance is not constant. It varies from 147,166,480 Kilometers (91,455,000 miles) at the closest point Perihelion (peri is from the Greek and means “around” and helios means “sun”) on about January 3. The furthest point is 152,171,500 Kilometers (or 94,555,000 miles) is aphelion (ap is from the Greek and means “away from”) on about July 4th.
Now this brings us to an interesting observation. The Earth is actually 3.3 percent closer to the Sun during the Northern Hemisphere winter than during the Northern Hemisphere Summer. So if you were thinking about this observation you would realize that even though the Earth is closer to the Sun during the Northern Hemisphere winter, it is much cooler than when the Earth is further away from the Sun during the Northern Hemisphere summer. So this leads us to an at first perplexing question, how is it the case that the Earth is farther from the Sun but its hotter, yet when its closer to the Sun its cooler? Again this a difference of 3,000,000 miles. The answer to this question is due to two factors and that is inclination and polarity which work together when the Earth rotates and revolves around the Sun. The imaginary plane around the Sun is known as the Plane of the Ecliptic which does not match the plane of the equator because Earth’s rotational axis is not perpendicular to the ecliptic plane. Rather the axis is tilted about 23.5◦ away from perpendicular and keeps this tilt throughout the year. This tilt is referred to as the inclination of Earth’s axis. Not only is Earth’s rotation inclined relative to its orbital path, no matter where Earth is in its orbit around the Sun the axis always points in the same direction relative to the stars—toward the North Star, Polaris. In other words, at any time during the year, Earth’s rotation axis is parallel to its orientation at all other times. This characteristic is called the polarity of Earth’s Axis (or parallelism). The combined effects of rotation, revolution, inclination, and polarity result in the seasonal patterns experienced on Earth. [4]
The Annual March of the Seasons
There are 4 distinguishable times of the year as the Earth completes its revolution around the Sun: June Solstice, September Equinox, December Solstice, and March Equinox. June Solstice (also known as the Summer Solstice in the Northern Hemisphere but Winter Solstice in the Southern Hemisphere), occurs on or about June 21st, the earth has reached a point where the North Pole is oriented most directly toward the Sun. On this day at noon, the rays of the Sun are striking the Tropic of Cancer. Three months later we have the September Equinox (Also known as the Autumnal Equinox in the Northern Hemisphere but the Vernal Equinox in the Southern Hemisphere) on about September 22nd. At noon the Rays of the Sun are striking the Equator and most of the Earth experiences 12 hours of light and 12 hours of darkness. December Solstice (Also known as the Winter Solstice in the N. Hemisphere, but Summer Solstice in the S. Hemipshere) occurs on December 21st. The Earth reaches the position where the North Pole is oriented most directly away from the Sun. At noon the vertical ray of the Sun now strikes the Tropic of Capricorn at 23.5◦ S; three months later we have the March Equinox (spring in N. hemisphere but fall in S. hemisphere) which occurs on March 20th. Again we have 12 hours of light and 12 hours of day on most of the Earth. [5]
Alright, we’re going to stop here and do a brief recap. Today we talked about the four spheres of the Earth: atmosphere which we’ll go into greater detail in the next lecture, biosphere which involves all living life, hydrosphere all things dealing with water, and the lithosphere which is the Earth’s crust and upper mantle. We discussed the structure of the Earth which you can think of in a diagram of four concentric circles, the lithosphere, the mantle (which has an upper and lower), the outer core (which is where Earth’s magnetic field is located and oddly that magnetic field reverses after thousands of years, and the inner core which is believed to be composed of iron/silicate or iron/nickel. We talked about the Earth-Sun relationship which without the Sun’s solar energy life could not exist. Then we covered the Annual March of the Earth around the sun. It takes 365 days for the Earth to revolve around the Sun and the Earth rotates on its axis one rotation in a 24 hours period. We also briefly talked about theaparent anomaly that the Earth is actually hotter when it is further away from the sun at Aphelion then when it is at Perihelion closer to the sun, and the reason for that is the same reason why we have the seasons which is due to the axis tilt of the Earth. Join me in the next lecture where we will begin with the Atmosphere and the composition of the Atmosphere.
Texes Essential Skills and Knowledge (TEKS)
(3)Geography. The student understands how physical processes shape patterns in the physical environment. The student is expected to:
(A)explain weather conditions and climate in relation to annual changes in Earth-Sun relationships;
[1]Hess, Darrel, and Tom L. McKnight. "Chapter 1: Introduction to Earth." McKnight's physical geography: a landscape appreciation. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2011. 9. Print.
[2]Hess, Darrel, and Tom L. McKnight. "Chapter 13: Introduction to Landform Study." McKnight's physical geography: a landscape appreciation. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2011. 353-354. Print.
[3]Edmond A. Mathez, ed. (2000). EARTH: INSIDE AND OUT. American Museum of Natural History.
[4]Hess, Darrel, and Tom L. McKnight. "Chapter 1: Introduction to Earth." McKnight's physical geography: a landscape appreciation. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2011. 16-18. Print.
[5]Hess, Darrel, and Tom L. McKnight. "Chapter 1: Introduction to Earth." McKnight's physical geography: a landscape appreciation. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2011. 17-19. Print.