Presentation on theme: "Earth’s Structure View of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?"— Presentation transcript:
1 Earth’s StructureView of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?
2 Origin of the Earth Meteors and Asteroids bombarded the Earth Gravitational compressionOur early Earth grew from a barrage of extraterrestrial impacts, increasing in mass over time. In the early stages of planetary accretion, the Earth was much less compact that it is today. The accretionary process led to an even greater gravitational attraction. Gravitational energy converted into heat. Radioactive elements are unstable, and over time become more stable. Radioactive decay processes releases heat. U-238 to Pb Initial accretion led to a homogeneous sphere that eventually became density stratified with the heavier iron and nickel sinking to the center of the earth, and the lighter silicates (rocky) floated to the top.Density Stratified planet
3 Earth’s Interior Core Mantle Crust dense Iron and Nickel Inner Core - solidOuter Core - liquidMantleLess dense than coreIron and Magnesium silicatesMostly solidUpper mantle is partially moltenCore – composed of two parts. The core is very dense and composed mostly of Iron and Nickel. The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form. The outer core is liquid iron and nickel. Again, it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core.Mantle is composed of Iron and Magnesium silicates (Si + O). It is mostly solid, but the uppermost part of the mantle is partially molten (slush – where you do have liquid and solid mixed). Now, this is very important in plate tectonics.Crust –This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid. If you can imagine an apple. It would have the thickness of the skin of an apple. There is two types of crust – continental crust and oceanic crust.Important notes:Thin crust + uppermost mantle is rigid. This forms the “lithospheric plates”.partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth.CrustOutermost layerVery thin and rigidContinental – graniteDensity = 2.8 g/cm3Oceanic – basaltDensity = 3.0 g/cm3
4 Evidence of Internal Structure GraniteBasaltDensitycalculate density of EarthSpeculate on probable compositionsMeteoritesUse composition and age to determine composition and age of EarthBy calculation, the Earth’s density is 5.8 gm/cm3. In the Sierras, composed mostly of granite, represents the thin part of Earth’s interior, i.e. skin of an apple. If you calculate the Earth’s surface using granite, it comes to 2.8 gm/cm3. What is this telling us? There are heavier rocks in the Earth. We know the age of the earth is 4.6 Ga. People who study meteorites determined their age to be about 4.5 Ga – same as Earth. Meteorites are composed of Iron and Nickel. It is hypothesized the Earth’s interior is composed of the same elements. P-waves travel 4x faster than S-waves and also travel through different material. These also give us some clue on the properties of the Earth’s interior.Seismic wavesTravel times and direction give indication of internal structure of Earth
5 Types of Seismic Waves P waves S waves Primary waves Push and pull movementTravel fastest (~ 6 km/sec)Travel thru solids and liquidsS wavesSecondary wavesMove side-to-sideSlower (~ 4 km/sec)Travel thru solids only
6 Seismic Waves Through Earth Scientists use waves generated by earthquakes to determine the Earth’s interior. that the outer core of the earth is liquid.Earthquakes generate P-waves and S-waves within the earth. Shadows occur on the opposite side of the earth from the earthquake epicenter because the outer core reflects S-waves, and bends P-waves. S-waves are reflected because they cannot travel through liquids, and they cast a larger shadow than the bent P-waves. Geologists and seismologists determined the size of the outer core by using the 154-degree arc of the S-wave shadow and measurements taken on the surface of the earth.
7 Lithosphere Consists of continental, oceanic and upper part of mantle Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3Why do continents sit higher than ocean basins? It is their composition and density. Continent = Quartz (SiO2) and Feldspars (K, Ca, Na, Al, SiO2). These are light elements. Ocean = Basalt (Fe and Mg). These are heavier elements. Asthenosphere is partially molten. It forms a lubricant to allow plates to move.Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere
8 Continental Drift Evidence Researchers noted geographic fit of continentse.g. Africa and S. AmericaAtlantic formed by separation of Africa from S. AmericaSeuss, 1885, proposed super continent by studying fossils, rocks, mountainsWegener and Taylor, early 1900’s, proposed continental drift and PangaeaSeuss hypothesized a super continent “Pangaea” surrounded by oceans.Evidence supporting the idea that the continents had drifted.Geographic fit of continentsFossilsMountainsGlaciation
9 Continental Drift Geographic Fit About 210 Ma, Pangaea began to break apart.Continents seem to fit together like pieces of a puzzle
10 Continental Drift Fossils Studies were done on land animals that could not swim. Once continents split, evolution occurs. Organisms begin to differentiate. Climate is different, food sources are different, predators are different.Similar distribution of fossils such as the Mesosaurus
11 Continental Drift Mountains If you were to put the puzzles back together, mountain chains match up, i.e., Appalachian Mtns, British Isles, and Caledonian Mtns.Mountain ranges match across oceans
12 Continental Drift Glaciation Glaciers typically occur near the poles. We are in a warm period, but in the past it was cool and ice caps grew. Deposits related to glaciers were deposited. You also find glacial deposits in Africa near the equator. How to you get glacial deposits near the equator?Glacial ages and climate evidence
13 Continental Drift Model Problems Alfred WegenerPresented research to professionalsDid not provide a plausible mechanism to explain how continents driftedWegener was a scientist. Scientists go to scientific meetings to present their research. When Wegener presented his research to other physicists, they did not agree. They wanted to know how do continents drift? There was no driving mechanism to support continental drift. Wegener could not answer “how” so his Continental Drift Model was not accepted.
14 Seafloor SpreadingContinental drift reexamined in 1960’s with new informationNew theory developed – Seafloor spreadingSupporting evidence for seafloor spreadingWorld seismicityVolcanismThe 1960’s is relatively recent. Geology is a new science compared to Physics, Chemistry and even Biology. People were doing separate studies about earthquakes, volcanoes, age of sea floor, paleomagnetism. Someone put it all together. A new theory developed – Seafloor Spreading. There were a lot of evidence to support Seafloor spreading.Age of seafloorPaleomagnetismHeat flowTheory combining continental drift and seafloor spreading termed “Plate Tectonics”
15 Seafloor SpreadingThe idea of seafloor spreading is that new crust is being formed at spreading centers and old crust is being destroyed in deep trenches. For example, Mid-Atlantic ridge is a chain of underwater mountain chains where new crust is being formed.New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches
16 Evidence for Seafloor Spreading World Seismicity Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.Earthquake distribution matches plate boundaries
17 Evidence for Seafloor Spreading Volcanism Where do volcanoes occur? Subduction zones. Why are they associated with subduction zones? Oceanic crust is moving deep into the Earth. Volcanism also occur at hot spots. Where is the tallest mountain? Mauna Loa. Highest elevation? Mount Everest. If you were to take Mauna Loa and place next to Mount Everest, it is the highest mountain. It is measured at its base deep on the ocean floor.Volcanoes match some plate boundaries; some are hot spots
18 Evidence for Seafloor Spreading Age of Seafloor Age of Earth is 4.5 Ga. Oldest oceanic crust is only 180 Ma. Why is the ocean so young? Old oceanic plate is being subjected or destroyed.Youngest sea floor is at mid-ocean ridgeOldest sea floor away from mid-ocean ridge
19 Evidence for Seafloor Spreading Paleomagnetism Today, Earth’s magnetic field points to the North. Why? They believe that the magnetic field is caused by the liquid outer core moving around the solid inner core by Earth’s spin. The iron (Fe) content in the core produces a magnetic field. Magnetic field is important to us in that it prevents harmful radiation of the Sun to reach us. Every 700,000 years or so, the Earth’s magnetic field reverses. These reversals are recorded in molten magma. As the magma cools, its iron-rich minerals tend to line up with Earth’s magnetic north.Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity)
20 Evidence for Seafloor Spreading Paleomagnetism The record of these alternating reverse and normal polarity helps support the idea of seafloor spreading.Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge.
22 Seafloor Spreading Convection Currents Magma was found were new seafloor was being made.In 1960, proposed as driving force to move continents
23 Theory of Plate Tectonics Heat flow provided the mechanism to move the lithospheric plates.John Tuzo Wilson combined ideas of continental drift and seafloor spreading into “Plate Tectonics”
24 Principles of Plate Tectonics Earth’s outermost layer composed of thin rigid plates moving horizontallyPlates interact with each other along their edges (plate boundaries)Plate boundaries have high degree of tectonic activitymountain buildingearthquakesvolcanoes
25 Plate Boundaries Three types There are three ways that plates can move relative to each other. They can move apart – divergent plate boundary, they can come together – convergent boundary, or they can slide past one another – transform fault.DivergentConvergentTransform
26 Plate Boundaries Divergent Plates move away from each otherNew crust is being formed
27 Divergent Plate Boundaries Examples East African RiftMid-Atlantic Ocean Ridge
28 Plate Boundaries Convergent Three Types:Ocean-continentOcean-oceanContinent-continentPlates are moving toward each otherCrust is being destroyed
29 Convergent Plate Boundaries Examples Mount Fuji, JapanMount Lassen, CaliforniaAndes, South America
30 Plate Boundaries Transform xxxxxxxxxxxxxxxxxxxCrust is neither created nor destroyedPlates slide past one another
31 Transform Plate Boundaries Examples San Andreas FaultCalexico, CaliforniaCarrizo Plains, Central California