THE EARTH’S LAYERS Continued

TO REVIEW!! The crust is less than 1% of Earth by mass. The mantle (inner and outer) represents about 68% of Earth by mass. The core (inner and outer) represents about 31% of Earth by mass. The mantle has two layers: a solid-ish layer and a more molten, liquid-ish layer. The core has two layers: the outer core is liquid. The inner core is solid. Both sections of core are comprised of iron and nickel. The inner core is the densest layer of earth.

MORE ON THE CORE The density of Earth’s surface layers is much less than the overall density of the planet. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the remaining 15%. If Earth’s core were not metal, the planet would not have a magnetic field. Metals such as iron and nickel are magnetic, but rock, which makes up the mantle and crust, is not. Metallic meteorites are thought to be representative of the core. The 85-15% ratio of nickel and iron is seen mirrored in meteorite fragments. Scientists know that the outer core is liquid and the inner core is solid because of S- waves and P-waves. Also because of Convection currents. (the heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core.)

MORE ON THE MANTLE: CONVECTION AND CONDUCTION Heat flows in two different ways within the earth: Convection and Conduction. Conduction : Heat is transferred through rapid collisions of atoms, which can only happen if the material is solid. Heat flows from warmer to cooler places until all are the same temperature. The mantle is hot mostly because of heat conducted from the core. (Think of Heat/Electricity conductors) Convection : If a material is able to move, even if it moves very slowly, convection currents can form. Convection in the mantle is the same as convection in a pot of water on a stove: the currents start forming near the core where the material heats up. As the core heats the bottom layer of mantle material, particles move more rapidly, decreasing its density and causing it to rise. The rising material begins the convection current.

MORE ABOUT THE CRUST: DID YOU KNOW THE CRUST HAS LAYERS? There is the lithosphere, asthenosphere, and mesosphere Lithosphere : composed of both crust and a portion of the upper mantle that behaves more brittle/rigid/solid. Asthenosphere : composed partially of molten upper mantle material that can “flow” and is kinda liquid Mesosphere : composed of mantle under both the Lithosphere and Asthenosphere, but above the outer core. (Think of it this way: L east deep, A verage depth, then M ostly deep.) The Difference between mesosphere and asthenosphere is likely due to density and rigidity differences and other physical properties, not really due to any difference in chemical composition.

THE CRUST IS WEIRDER AND MORE COMPLICATED THAN WE THOUGHT If you think about it, this makes sense: the crust is thinner at, say, a desert landscape than at Mt. Everest. Also, though, the crust is thinner by the ocean. Thus the crust can be divided into two very different types: each with their own distinctive properties, chemical and physical. Oceanic Crust and Continental Crust. CrustThicknessDensityCompositionRock Types Oceanic5-12 km (3-8 mi) 3.0 g/cm3MaficBasalt and gabbro ContinentalAvg. 35 km (22 mi) 2.7 g/cm3FelsicAll Types

OCEANIC AND CONTINENTAL CRUST Oceanic crust: composed of mafic magma that erupts on the seafloor to create basalt lava, which flows or cools deeper down to create the intrusive igneous rock gabbro. All of these are different kinds of rocks, with different physical and chemical properties, and different densities. The gabbro is considered deformed because it is created because of intense faulting at the eruption site of a volcano, be it under the crust or above it. Sediments, primarily muds and the shells of tiny sea creatures, coat the seafloor. Sediment is the thickest near the shore where it comes off the continents in rivers and on wind currents. Continental crust: composed of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust. Because granite is thick and has relatively low density, continental crust will rise higher on the mantle than oceanic crust, which sinks into the mantle. It forms basins which, when filled with water, form the planet’s oceans.

SO…LITHOSPHERE When we think of the crust, we basically think of the Lithosphere. And, if you think about it, it makes a lot more sense that the Atmosphere is included as a layer of the earth. Atmosphere, Lithosphere, Asthenosphere, Mesosphere, core. The Lithosphere is the outermost, mechanical layer of the earth, which behaves as a brittle, rigid solid. It’s about 100 km. thick. The definition of lithosphere is based on how the earth materials behave, which is why it includes the part of the mantle, which is brittle. Since it is rigid and brittle and the inner layers of the earth are not, stress actions cause the lithosphere to crack and break. Otherwise known as an earthquake.