I. In this last section of the course, we'll be concerned with the lithosphere, which is the rock and mineral sphere of our planet (that is, most of it!). A. The branch of physical geography concerned with the lithosphere is called "geomorphology." B. Remember how biogeography was the branch of physical geography concerned with the biosphere and how it was shared with biology? Well, geomorphology is shared with geology: Some geomorphologists are geographers and some are geologists. Sometimes our tidy disciplinary structures don't quite coïncide with what people are actually doing! C. Anyhow, in this section, we will be concerned with: 1. The structure and composition of the planet itself. 2. Processes by which the earth's surface is built up (tectonism). 3. Processes by which the earth's surface is worn smooth, as by rivers, glaciers, waves, and winds (gradation). II. Structure of Planet Earth. A. Earth is structured in layers. There are two different ways of classifying these layers (there's that classification thing again). B. The first of these entails classification by the physical state of matter and general chemical composition. I'll start at the center of the earth and work out to the surface in this discussion. 1. The innermost part of the planet is believed to exist in a solid state: This is called the "solid inner core." a. It extends out some 1,250 km out from the center of the earth, or just under 20 percent of the way out from the center to the surface (the earth's radius is roughly 6,330 km: Lecture 2). b. It is made up of iron (Fe) with some nickel (Ni), very dense and heavy elements. c. This can be surmised from the following argument: i. The average density of the most abundant materials making up asteroids in our solar system is about 5.5 g/cubic centimeter (iron is the most common solid in outer space, particularly in the inner solar system, where Earth is located). ii. The average density of Earth's crustal materials, however, is only 3.3 g/cubic meter. iii. Assuming Earth formed from the materials most common in the inner solar system, the heavy stuff has to be present somewhere in the earth. iv. Earth, like the other inner planets, went through a molten phase early in its history, which allowed gravity to stratify substances by density. So, iron and nickel would settle at the "bottom" of the gravity well: the center of the young planet. v. The solidity of the inner core is surmised from the performance of iron and nickel in the laboratory and in massive computer models at pressures similar to those at the core of the planet: Even though it's incredibly hot in the earth's interior (current estimates are that it's about 5,500 K, which is close to the temperature of the surface of the sun!), the pressure is so great that the melting point of iron contaminated by nickel is elevated far beyond those high temperatures (6,500 K), leaving the nickel-iron as a solid. 2. The next layer out is the liquid outer core. a. This, too, is dominated by nickel-iron. b. This is known from the earth's strong magnetic field, which can be produced by currents in liquid iron. c. It extends out to some 3,450 km from the center of the earth, which means it's about 2,200 km thick. d. It is known to exist in a liquid state because of the behavior of earthquake waves, particularly shear body waves or secondary waves. i. Liquid cannot respond to shear forces (lateral or side by side forces), so it can't transmit shear waves. ii. As a result, there is a seismic shadow on the side of the earth antipodal to an earthquake's epicenter: Seismic stations there will not receive secondary waves, though they will get primary (compressional) waves from a given earthquake. Primary waves, too, generate a shadow centered roughly 120° from the focus due to being refracted or bent inward as they slow down crossing into the liquid inner core. iii. It's not quite as direct as the size of the seismic shadow equalling the size of the liquid outer core, because seismic waves speed up as they travel through denser solids and refract or bend when they pass through different materials, and this curves their path back out to the surface. Factoring that in, we can still compute the size of the outer liquid core's outer boundary from the size of the shear wave seismic shadow on the opposite side of the earth. Shear wave seismic shadow:
Primary wave seismic shadow due to velocity change:
3. The earth's mantle is the third layer out. a. It covers everything from the liquid outer core to the bottom of the earth's crust. b. It extends, then, from about 3,450 km out from the center all the way to within anywhere between 5 and 100 km of the surface. It's not quite 2,900 km thick. c. The mantle is rather variable, but it is dominated by actual rock, not just iron alloys. i. This is largely silicate in basic structure (built around the silica and oxygen combination, SiO2). ii. These rocks are, however, highly enriched in iron and magnesium, so they are called "ultramafic" rocks ("ma" from magnesium and "f" from the chemical symbol for iron, Fe). iii. So, these rocks are extremely heavy and dense compared with typical surface rocks, and they are usually very dark as well because of the iron and magnesium in them. iv. The mantle varies in its state of matter, from a soft and nearly liquid condition near its inner boundary with the liquid outer core and again near the top, a few kilomters under the earth's crust. In other areas, it may show nearly brittle solidity. The softest parts of the mantle can flow in response to heat, kind of like "silly putty"?
4. The fourth and final layer out is the crust. a. This is very, very thin, ranging from maybe 5 km under the oceans to as thick as 100 km under certain mountainous areas of the continents (usually, it's about 40 km thick under the continents). b. The Mohorovicic Discontinuity (often called, simply, "the Moho" by its friends) marks the transition from the top of the mantle to the bottom of the crust: It's an area where there is a sharp increase in the velocity of seismic waves as they pass into an area of different density and rigidity. Andrija Mohorovicic first noticed this effect back in 1909, and it has since been tied in theory with the mantle-crust transition. c. Rocks in the crust are solid to the state of rigid and brittle. d. They are also highly variable, including rocks of molten origin, rocks of sedimentary deposition origin, and rocks that have undergone all sorts of structural and chemical alterations. e. The crust itself can be divided into one or two sublayers, depending on where you are: i. One kind of layer is found everywhere, under the oceans and way under the contintents. a. This layer is dominated by relatively heavy, dark, dense rocks of "mafic" composition. Most of these mafic rocks are of volcanic origin and are called "basalts." b. This dense, heavy mafic layer is sometimes called the "sima" (dominated by silicate rocks of mafic composition) and sometimes the "mafic layer" and sometimes the "basaltic" layer. c. It tends to be relatively thin, usually from about 5-12 km in thickness. ii. A second layer is normally found only on the continents. a. It is made up of light rocks primarily composed of silicates enriched in the lighter elements, such as aluminum (Al), potassium (K), and sodium (Na). b. This layer is sometimes called the "sial" (dominated by silicate rocks with lighter elements mixed in, such as aluminum) or the "silicic layer" (overwhelmingly silicic) or the "felsic layer" (for potassium-rich versions of feldspar). These rocks commonly occur in the earth's crust as "granite," so this layer is sometimes called the granitic layer. c. This layer is considerably thicker than the basaltic lower layer, around 40 km.
5. So, that takes care of the first system for classifying the layers of the planet, the one that classifies the layers by states of matter (liquid, solid, and ductile or soft solid) and general chemical composition (nickel-iron, ultramafic rock, mafic rock, and felsic rock). This system gives rise to the solid inner core, the liquid outer core, the mantle, and the crust, with the crust subdivided into sima and sial. C. The second classification of the layers within the planet is strictly by states of matter only: solid, liquid, or ductile solid. It is mechanical, then, rather than compositional. 1. This classification was developed specifically for the needs of plate tectonic theory (about which beaucoup later), for understanding the drift of the continents in deep geological time. a. Since this one concerns the surface plates and the processes driving them around, I'll start with the surface and work down this time. b. In this system, the word "lithosphere" means the hardest, most rigid layer of the outermost planet, the layer subject to failure and division into plates (yes, this can be confusing, because the word "lithosphere" also has a more generic usage, describing the entire rock and mineral part of the planet). i. It extends down to about where compressional heating of rocks and minerals reaches roughly 1,300° C. Past the immediate surface of the earth, temperatures of rock start climbing because of the compression of all the material lying above it. Rocks cooler than 1,300° C in general behave in a rigid manner, while those warmer begin to weaken and behave in a soft, plastic manner (capable of flowing around). ii. The 1,300° C isotherm lies a few kilometers below the bottom of most of the ocean floor (except where ocean floor piles up at certain plate boundaries, where it can thicken to 100-150 km or so) and up to 250-300 km under the deep mountain roots and the shield areas at the heart of the continents. iii. As such, the lithosphere includes all of the crust in the other classification system, plus the most rigid part of the upper mantle. This is the zone subject to breakage into new, rigid plates. c. In this mechanical system, the word "æsthenosphere" (loosely rendered, the "wimpy sphere" or the "weak sphere") refers to the softest, most plastic part of the upper mantle, an area where rock weakens because it's awfully close to its melting point (and localized pockets DO in fact melt, providing magma for volcanoes and such). i. Interestingly enough, there is seismological evidence for the probable existence of the æsthenosphere. Seismic shear waves generally slow down rather remarkably as they enter this zone of not-quite-liquid (remember shear waves can't pass through liquids, so it's not surprising that they would slow down as they pass through rocks just this side of liquid). This "low velocity zone" is taken as empirical support for the existence of the æsthenosphere. ii. The æsthenosphere extends down to roughly 700 km, at which point the rocks are under enough pressure to overcompensate for the great temperature and resume a more rigid mechanical condition. iii. The lithospheric plates move about on top of the æsthenosphere, in response to currents of flow in the æsthenosphere that may reflect conduction of heat up from the core. More on all that later. d. The next layer down is the "mesosphere." i. Yep, you're right, you DID hear that word before. In a maddening disconnect between geology and climatology, between one end of physical geography and another, someone picked and popularized a word that already has a well- established meaning in studies of weather and climate. The mesophere you met already is the third layer up in the atmosphere; here it's the third layer down in the planet! Isn't that exasperating!? ii. Anyhow, the mesophere now refers to the most rigid part of the lower mantle, below about 700 km or so. This is where pressure compensates for (compressional) temperature increases to return mantle rocks to a more rigid, solid condition. e. The fourth layer down is exactly the same one we bumped into before: the liquid outer core, extending down from about 2,900 km. Here, we've passed beyond the lighter silicate rocks and into the realm of nearly pure iron (iron with some nickel). So, the temperatures are high enough to transcend the melting point of iron even at these pressures, and we get the liquid outer core. f. The fifth and last layer down is the solid inner core we met before, below about 5,100 km and extending right to the center of the earth. Once again the pressures are so great as to force the melting point of even pure metal above the actual temperatures down there, and so we believe the inner core is solid nickel-iron. g. Here are the two systems side by side:
Well, that's enough of the layered structure of the planet. There are two different systems for classifying these gravity-induced layers: States of matter plus general chemical composition and the strictly mechanical states of matter system. The first classification system really brings out the rôle of gravity in layering our once-molten planet: The heaviest, densest (and locally most abundant) stuff, nickel-iron, settled out at the bottom of the planet's gravity well, its center. This gave us the core. All rock materials were displaced towards the surface, giving us the mantle and crust. These are also seen to reflect gravity layering: The mantle is made up of the heavier ultramafic rock, while the basalt layer of the crust is made up of mafic rock, and the granitic layer of the crust (on top of the continents) is made up of felsic rocks, the lightest of them all. The second classification is specific to the needs of understanding plate tectonics, so it focusses on mechanical properties and behavior: strictly the states of matter (solid, plastic, and liquid). This helps us conceptualize the rigid, brittle lithospheric plates sloshing around on the weaker æsthenosphere below and gives us room to think about mechanisms driving the currents in the æsthenosphere (such as convection of heat from the core, ultimately). Next time, I'll go into the composition of the crust: elements, minerals, and rocks, and how rocks transform from one type to another.
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First placed on web: 11/11/00 Last revised: 07/03/07