[Music] for part two of our lesson line of evidence that support plate movement last time we discussed the continental drift theory proposed by alfred wegener we also discussed the different evidence that support the theory of continental drift but many scientists that time did not believe his theory because alfred wegener failed to explain or give a concrete evidence on how the plates move today we will discuss the seafloor spreading the theory and how it helps to prove the continental drift many people in the past believe that ocean floor is flat like the desert but in 1930s where sounding gear sold sonar or the sound navigation ranging was developed it leads to the discovery of the standing features of the ocean floor sonar is a device that uses sound wave to detect objects so how does a sonar device work a sonar device send pulses of sound wave down through the water when these pulses hit objects like fish vegetation or the bottom they are reflected back to the surface the sonar device measures how long it takes for the sound wave to travel down hit an object and then bounce back it's the same echolocation system bats and dolphins use this information enables the device to judge the depth of an object it also measures the strength of the returning pulse the harder the objects the stronger the return pass ocean one of the scientists who studied the mid-atlantic ocean ridge and used sonar during world war ii was hari hamon hesse a professor of geology at princeton university in usa hari and his team carefully examined maps of the mid ocean ridge system has begun to think about the ocean floor in relation to the problem of continental drift theory that leads him to the idea of seafloor spreading seafloor spreading is the process where new oceanic crust is formed a mid-ocean ridge and slowly pushes the old crust toward the subduction zone process ocean ridge old grass papuantasa subduction zone according to this theory hot less dense material below the earth's grass rises towards the mid ocean ridge this material flows sideways carrying the sea floor away the ridge and creates a crack in the crust the magma flows out of the crop cools down and become the new seafloor [Music] over time the new oceanic crust pushed the old oceanic crust from far from the ridge and subducted move upward pushing the old crust to subduction zone among a molten materials and that moves at us decreased temperatures the idea of sifler spreading is supported by the following evidence first is the age of rocks scientists find out that the youngest rocks are found near the ridge while oldest rocks found far from the ridge this provides evidence that seafloor spreading a new crust is being created at the ridge age i'm younger rocks are found near the mid ocean ridge molten materials or um is being created at the ridge so the old crust is pushed and destroyed at the subduction zone older crust abduction zone kapaka is a subduction zone [Music] so temperature [Music] ocean ridge destruction or that they destroyed the man old cross the subduction zone [Music] remember the formation of new crust at the mid ocean ridge is not always as fast as the as the destruction of old crust at the subduction zone sabai nangayare floor spreading at destruction the formation of new crust at the mid ocean ridge is not always as fast as the destruction okay the onsen subduction zone perro hindi makkah singh billis process so if the seafloor spreading is faster than subduction the ocean gets wider kapakado young sea floor floor spreading ocean ridge [Music] destruction at the subduction zone than the seafloor spreading massaging smaller dao young ocean ocean and atlantic ocean subduction is faster than seafloor spreading the ocean gets smaller ibiza had mass mobilizing destruction subduction zone cases of formation on new crust so atlantic ocean amman it is getting wider ibizabihen must information on new crust compare to the destruction of older crust at the subduction zone another strong evidence that support the seafloor spreading is the magnetic reversal magnetic reversal is also called as magnetic cliff of the earth on a magnetic pole on earth on earth it has north magnetic pole and south magnetic pole a north magnetic pole is a south pole antarctica atang south magnetic today this is the orientation of the magnetic poles on earth magnetic north pole becomes the magnetic south pole of the earth this change in polarity of the magnet of the magnet is called magnetic reversal tobacco on magnetic polar earth it is because of the change in direction of the flow of materials in the outer core as the magma becomes the new crust the rocks solidify and follow the magnetic poles of the earth at that time magmas amid ocean ridge magma imoxo solidify or titigas material stay on a line according some magnetic pole on earth babagon a magnetic pole this will form patterns on both sides of the mid ocean ridge this pattern is a proof that mid ocean reaches is moving and that seafloor spreading is really happening it's a continental drift theory so seafloor spreading shows that the ocean are active site of plate activity it only it also shows that new crust is formed at the mid ocean reach an old grass is destroyed at the subduction zone seafloor spreading also explains how the plates move due to the convection curtain in the mantel the hildito na patunaya nang siflor spreading nato and continental grief theory