Inside the Earth

1. Inside the Earth Layers of Composition ONACD –Editable Curriculum designed for teachers by teachers

2. The Crust Inner Core The Mantle Outer Core Inside the Earth Inside the Earth– Apple Analogy The usual introduction to the inside of the earth is to use an apple. An apple gives you a good sense of the scale. If the earth was an apple, our deepest drills haven’t even punctured through the “skin”! We have drilled 3km and soon hope to drill 7km under the ocean.

3. The Crust The earth’s crust makes up less than 1% of the total volume of the earth. It is made of metamorphic(extreme heat and pressure), igneous(once molten) and sedimentary rocks (made by weathering / erosion and compression). The temperature can be as hot as 6000C. The crust itself is a relatively thinsolid layer and is in two categories: continental and oceanic. • ContinentalCrust (mainly Granite – density ~ 2.7 g/cm3) The continents form large land masses higher than the oceans. The average thickness of continental crust is 35km and is made of three forms: • Shields – Ancient igneous land masses that form the core of the continents (contain the oldest rocks on earth) • Mountains – “Young” folded rocks (usually from plated tectonics) Sedimentary Basins – Depressions on continents (become filled with sedimentary rocks or become seas)

4. Oceanic Crust (mainly Basalt which is denser than granite at ~3.2 g/cm3) The land under the oceans makes up the oceanic crust. The average thickness of oceanic crust is only 7km and has three layers: • The top layer (1km) Mud, Sand, and dead organisms washed to see from the continents (recall that during landslides these can form “turbidity currents”) • The middle layer (1km) Volcanically formed layer of fine grain basalt rock • The bottom layer (5km) Slow cooling magma forms course grained “Gabbro” rock

5. Key Concept: • Since oceanic crust is denser and thinner than continental crust, oceanic crust is subducted when continental and oceanic crusts collide. The continents contain the oldest rocks since they essentially “float” over the denser ocean crusts. Note: • Do you remember growing crystals in an earlier grade? Slow cooling forms large crystals. If you forgot (or didn’t): try making “rock candy” out of supersaturated sugar solutions at home.

6. The Mantle (density around 3.8 g/cm3) The mantle makes up about 80% of the total volume of the earth. It is primarily made of siliconoxide SiO2, magnesiumoxide MgO, and ironoxide FeO. Recall that the mantle also contains radioactive uranium which releases tremendous amounts of thermal energy. The temperature at the boundary of the crust varies around 9000C to over 20000C near the outer core. Even at these extreme temperatures the mantle remains mainly solid because of the great pressure exerted on it from the crust (but it is ductile enough for convection currents).

7. Based on the way earthquake waves travel through the mantle it is classified with three main layers: upper, transition zone, and lower. • Upper Mantle (~400km thick) – This is where the least dense materials rise too; it is also the least viscous which enables the strongest convection currents. • Transition Zone (~250km thick) – A complex area which little is known about. It is suspected to be a main source of basaltic magmas. • Lower Mantle (~2000km thick) – This is the most dense region of the mantle and is possibly made of several different chemical compositions.

8. The Core The earth’s core is the densest region in the planet and makes up about 16% of the earth’s volume (and about 32% of the earth’s mass!). The temperature of the core is estimated to be in the range of 5000 to 60000C. The core itself is in two layers: a liquid outer core and a solid inner core. • Outer core (~2200km thick and density around 11 g/cm3) A liquid layer made of iron, nickel, sulphur, and lighter elements. • Inner core (~1200km thick and density around 13 g/cm3) A solid core under extreme pressure made of iron, nickel and perhaps some other elements like gold and platinum. It appears to be rotating slightly faster than the earth.

9. Inside the Earth again Inside the Earth again – A little more detail Using information generated by earthquake waves, scientists are able to determine even more information about the earth’s interior than we learned with the “Apple Analogy”. SeismologyPart 1 – Body Waves Earthquakes generate two types of waves: Primary (P) waves and Secondary (S) waves.

10. Primary Waves - These are compression (or “longitudinal”) waves as they are like the pulse that can travel along a stretched spring when you quickly compress (push) one end. - Compression waves can be transmitted, refracted and reflected much like other waves you studied in previous grades.

11. Secondary Waves - These are called secondary because they travel slower than primary (P) waves. - This type of wave is a transverse wave; it is like the wave you produce when you wiggle one end of a rope. - Transverse waves do not travel through liquids.

12. Seismology Seismology is the study of earthquakes using information like P-waves and S-waves. Seismologist set up devices (seismographs) around the world to record earthquakes. When an earthquake occurs, they can determine the depth and location of the earthquake source. They can also determine the makeup of the earth’s interior. Consider the diagram to the right which traces the waves produced from an earthquake:

13. On the other side of the globe, S-waves are not recorded because they can’t travel through the liquid outer core. There is also a “shadow zone” where no P-waves are recorded because they are refracted by the liquid outer core. They are also refracted by the varying densities of the layers of earth’s interior. There are earthquakes everyday on earth and so seismologist continually receive more and more data to be able analyze the interior design of earth.

14. Look at the following image from USGS (http://earthquake.usgs.gov/eqcenter/recenteqsanim/world.php) which shows earthquake activity (460 earthquakes) in the world for one week:

15. Conclusion The majority of the above earthquakes are smaller in magnitude (under 7 magnitudes). An earthquake of magnitude 8 or larger on the Richter scale only occurs a few times a year. An earthquake occurs when a sudden release of energy occurs in the earth’s surface. This is usually the result of moving land masses or volcanic activity. The location of the source of the earthquake is called the “focus”. The sources of most earthquakes occur within 70km of the surface and are called “shallow-focus” earthquakes. Deeper focus earthquakes cause more damagethan shallow focus earthquakes. The surface of the earth directly above the focus is called the “epicenter” of the earthquake.