Moho (Mohorovicic discontinuity) - boundary between crust and mantle

Transcription

1 Earth Layers Dynamic Crust Unit Notes Continental crust is thicker than oceanic crust Continental Crust Thicker Less Dense Made of Granite Oceanic Crust Thinner More Dense Made of Basalt Moho (Mohorovicic discontinuity) - boundary between crust and mantle

2 How we know about earth s interior 1. studying seismic waves (earthquakes) 2. composition of meteorites (meteorites are made of iron & nickel like the core) Evidence of crustal changes that take a long period of time to occur 1. Sedimentary rocks found at high elevations - sedimentary rocks are formed in water at lower elevations so finding some on top of a mountain means crustal uplift occurred. 2. Marine Fossils found at high elevations - formed in ocean & uplifted to higher elevation by crustal uplift. 3. Tilted or folded strata - layed down horizontally & tilted or folded by pressure later. Earthquake - shaking, vibrating & sometimes violent movement of lithosphere caused by release of energy stored in rocks. Epicenter - location on earth s surface directly above focus Focus - Point along fault where earthquake originates (underground) seismic waves released at the focus. They travel in all directions. Causes of Earthquakes movement along faults (shifting of rock layers near cracks) some associated with volcanic eruptions Most Earthquakes and volcanoes occur at or near plate boundaries

3 Ring of Fire - Region in the Pacific ocean where many earthquakes and volcanoes occur. Faults - crack in Earth s crust where movement occurs during earthquake Types of faults: 1. Transform (strike-slip, lateral) - land on either side slides past each other horizontally (example : San Andreas Fault) 2. Normal - one side moves down in relation to other side 3. Reverse - one side moves up in relation to other side

4 Measuring Earthquakes 1. Richter Scale - Based on amplitude of seismic waves. Measures energy released during earthquake (motion of bedrock is recorded by seismograph) 2. Mercalli Scale - measures intensity of earthquake (the effect on people and buildings) 3. Moment Magnitude (Mw) - Used now. More precise than richter scale. Based on movement along fault. Seismograph - machine that records earthquake Seismogram - the picture made by the seismograph machine (shows P, S, Surface wave arrival times) Primary waves (P or compressional) Travel fastest, arrive first to station Travel through liquids & solids Travel faster thru denser material Secondary waves (S or shear)

5 Travel slower, arrive second to station Can t travel through liquids or gases (absorbed) Scientists infer outer core is liquid because S waves don t go through Travel faster through denser material Surface waves (Love and Rayleigh) - cause the most damage, arrive to station last *Earthquake waves provide us with information about Earth s interior (that outer core is liquid, and that density increases as depth increases) * Earthquake waves travel faster through denser material Shadow zones - caused by refraction (bending) of earthquake waves and outer core not allowing S waves to pass through. Where do Earthquakes and Volcanoes occur? Occur most near plate boundaries, edges of continents, near Ring of Fire Do Not occur randomly We cannot predict earthquakes, but we can predict areas they will occur. Volcanoes, Earthquakes, mountain ranges tend to occur in the same areas Finding Epicenter Distance from seismograph station

6 Determined by difference in the P & S wave arrivals The greater the difference in arrival time, the further from epicenter Use ESRT P arrival time S arrival time - P travel time or - S travel time Earthquake origin time Earthquake origin time Pinpointing the Epicenter Must use epicenter distance from a minimum of 3 stations to locate the epicenter The three circles drawn around the stations will all intersect in one area, pinpointing the earthquake epicenter. (If two stations were used, the epicenter would be narrowed down to two areas). Continental Drift Theory Proposed by Alfred Wegener (Veg ener), German scientist (theory not accepted at first) Pangaea (panjeeuh) - one large land mass, broke apart, drifted - (believed to have happened more than once)

7 Evidence for theory 1. correlation of coastlines (fit together like puzzle pieces) 2. correlation of rocks along continental margins (rocks are same age and type) 3. correlation of fossils on continental boundaries once touching (same age and type of fossils) 4. mt. Ranges continue from one continent to another (formed when together) 5. fossils show land mass had different climate when they lived (climate was different because the land mass was located at a different latitude) ex. coal found in Antarctica and warm coral fossils found in cold areas, evidence of glaciers in Africa.

8 6. mid-ocean ridges (evidence of sea floor spreading at divergent plate boundaries) Ex. Mid-Atlantic Ridge (runs N S ) Mid-Ocean Ridges Sea Floor Spreading 2 evidences for sea-floor spreading Mid-Ocean Ridges Evidence of Sea Floor Spreading Ex. Mid-Atlantic Ridge (runs N S ) 1. Age of bedrock on either side of ridges 2. Polarity reversals shown by rocks on either side of ridges

9 Theory of Plate tectonics Plates - enormous slabs of lithospheric rock riding on partially molten asthenosphere. Plate motion is driven by convection cells in the mantle Convection cells drive the plate movement (look at direction of arrows)

10 3 Types of Plate Boundaries 1. Convergent a. Continental crust converging with oceanic crust = subduction denser oceanic plate slides under less dense continental plate forming ocean trench and volcanic mt chains b. Continent crust converging with continental crust = mt. Building, folding, faulting

11 Oceanic crust converging with Oceanic crust 2. Divergent Plate Boundary (sea floor spreading) 3. Transform - plates slide past each other (ex. San Andreas Fault)

12 Hot spot - magma burns through crust and forms volcanic island chain (like Hawaiian islands). The hot spot stays stationary and the plate moves over it.