is for [Music] no [Music] [Applause] [Music] [Music] among geology stories are saying that um any questions that you can ask in in in geology uh if you answer PL tectonics it will be a good answer so that is to give an idea of how play tectonics has become a very uh useful framework uh to a scaffold to frame a lot of the processes that we um observe um almost like a unifying theory that tries to pull together several observations from different fields of geology that why is that's why play tectonic has become uh so important
in geology because it allows to uh connect many observations from different fields play tectonics is an interesting Theory because it the idea isn't that old uh it's only been accepted in the geological Community since the 1960s uh when new evidence became available however it was first suggested by a scientist back in uh the early part of the 20th century 1910 1920 about there uh by Alfred Vagner and his suggestion was continental drift that the continents were plowing through the oceans and they were drifting and people said well how can that happen there's no mechanism for
that it doesn't make any sense and so people didn't accept his ideas at the time although there was evidence that it seemed like the continents had moved because you could find tropical plant fossils in the Arctic or Antarctic that was a long a long ways away from the equator and so he had this idea but it wasn't accepted however in the 1950s and 1960s new data became available um from understanding what was happening on the seafloor that seemed to indicate there was very good evidence for spreading close to the spreading ridges uh the um the
spreading centers between uh what we now called uh different plates and so this seemed to be a major Scientific Revolution there had been hypothesis before it had been suggested before that the um the continents for example had lifted or there was this Clash of views or ideas between a fist understanding of the crust of the Earth where the continents and the oceans had been in the same place um since Antiquity and so that was really a fixis uh idea versus a more mobista that these um continents indeed could have even have moved you know significantly
through time and the concept was was resisted at first uh it it looked uh to um outside the box you know and you really needed uh um a solid explanation or solid evidence to support something that looked so dramatic like the idea that continents could move around it wasn't until um the 1960s where um after the second world war there was a lot of study of the ocean it was really the oceans that brought about the revolution of plate tectonics a better understanding the of the ocean the bottoms of the oceans uh their topography their
their composition um surveys um of their magnetic anomalies and so plate tectonics really was a revolution that was born out of the Seas when when we increased our understanding of this uh large part of the surface of the Earth that was a little bit mysterious then all of a sudden it was possible to formulate a new Theory and so perhaps the the revolution really came as a result of an large increase in information about a part of the crust of the earth that hadn't been known very much [Music] for [Music] for for for for fore
we can find um some some um manifestation surface manifestation of the crust of the earth not being homogeneous and of the crust of the Earth being Dynamic um there are some that are very familiar to um everyone and there are some that are more indirect and uh but uh so for example a very interesting manifestation of the fact that the Earth is is fragmented into the crust of the earth is fragmented into plates is um seismic activity when when uh we plot the distribution of major earthquakes um and there are diagrams that are freely accessible
to to the public you know like the USGS as some free diagram you can you can use as illustrations um when we plot um the the distribution of major earthquakes they are not evenly distributed uh on the surface of the Earth but they tend to um concentrate along um certain um in certain areas a second manifestation of uh uh the possibility that the the Earth the crust of the earth is not homogeneous is volcanism also volcanism um is distributed according to certain patterns on the crust of the earth and interestingly U there is also some
overlap between the distribution of major earthquakes and major volcanic activity um and then um data on the composition of the crust of the earth so um compositionally the the crust of the earth is not really uh homogeneous there are um maybe mainly two types uh of um rock types and one type is um like Granite so to say we could assimilate it to Granite and granitic Inc composition and one type we could assimilate it to Bal and so these two main um types also represent um a way in which we could subdivide the the crust
of the earth so the crust is less dense than the mantle and so it's just like having a wooden boat in the water the boat floats on the water if you put things in the boat it floats down lower if you take things out of the boat it floats higher so similarly the continental crust is less dense and it floats high in the mantle and that's the reason the are above water whereas the oceanic crust is more dense it's made out of Basalt and it floats lower in the mantle and because it floats lower that's
where all the oceans are so the ocean basins are above the oceanic crust and the continents are up above the oceans when we put these uh rock type or compositional differences together with the concentration of these Dynamic events and their distribution we start to form a picture of um what the the crust of the earth is made of we start to begin to observe oh this is why we have continents and we have oceans we have oceans because the oceans have a a a composition that is different than the one on the continents oceans have
a more basaltic composition is denser and so tends to sink more in into the mantle and and and um granites are more boyant so you have more elevated areas and so that's uh your oceans will tend to be concentration concentrated on the depressed area and and so that's your major division between continents and oceans but then even within the oceans we start to observe that there is some topography there's specific concentrations of volcanoes and earthquakes and along specific lines and um in the 60s in the 1960s all this inform information started to be pulled together
um other um important observations from uh the world of physics having to do with magnetism the the the way in which rocks maget magnetize and um started to be interpreted all together and it became apparent that uh indeed you could divide the crust of the Earth in distinct uh plates and not only that but these plates um seem to be dynamic [Music] [Music] so geologists have found that there are about half a dozen major plates and then lots of minor plates so for example there's the North American Plate the South American Plate the African plate
the Eurasian plate Australian plate and so forth in addition one of the plates is an ocean Basin so you also have the Pacific Plate and then you have lots of little plates like there's the Indian plate the Indian subcontinent uh makes up its own plate uh there's the Arabian plate and so forth so as you have these different plates and if they're moving there's three different ways that they can move they can either move apart or they can move together or they can move past each other and each of those different motions has a unique
feature that we see the examples of it on the earth so first of all where they move apart uh the best example of that is in the Atlantic Ocean you have a spreading Center where North America and Eurasia are moving apart and where South America and Africa Africa are moving apart and so magma comes up at the spreading Center and forms new oceanic crust as the crust moves apart and here we are in Iceland and Iceland is right on the Mid-Atlantic Ridge so this is a place where the magma comes up and North America and
Europe are moving apart now most of the time this Mid-Atlantic Ridge is underwater but this is one of the unique places on Earth where you actually have the Mid-Atlantic Ridge where you have the spreading Center up above the water and you can see the results of the basalt here [Music] la for for [Music] [Music] [Music] nor m for [Music] [Music] so one kind is where plates move apart another example is in East Africa and there they've started to move apart and you have the East African Rift Zone but they haven't moved apart enough for the
water to come in now the Red Sea is an area where they have moved part enough and you get the ocean but in the East Africa you don't have that happening so the first type of contact between plates is at uh spreading centers the second kind is where you have a subduction zone where the plates are moving together and if one of them is more dense like the oceanic crust it goes underneath a continental crust and probably the best example of that is uh the Pacific Plate or actually the Nazca plate going underneath the South
American Plate and as it goes underneath it's being subducted and it carries water down with it and that water lowers the melting point of things underground and things start melting and so you have Rock melted rock magma that comes up and forms volcanoes in the Andes and if it doesn't come all the way to the surface then it forms granitic rocks that are that are underground so the second part is where you have something like an oceanic crust going underneath a continental crust now what happens if you have two plates meat and they're both continental
crust then it's harder for one to go under the other and the best example of that is where India comes up and hits the Eurasian plate and the two hit and they move up and so that's the explanation for the Himalayas is where these two plates are moving up and um making the Himalayas very hot the Third Kind of contact between plates is where the plates move past each other and the typical example of that is in Southern California where you have the San Andreas fault we're here in the kahone pass and we're in a
very important place we're right on the boundary between two of the Earth's major tectonic plates on this side we have the North American Plate on this side we have the Pacific Plate and they're moving past each other uh the North American Plate on the left side would be moving One Direction with the respect to the Pacific Plate so right here they'd be moving in this direction whenever we have an earthquake so that would move Los Angeles up next to San Francisco so you can see an example of it right here here's a pond it's called
uh it's called a Sag Pond uh and this Pond does not have a stream going into it the main way that it's fed by water is water coming up along the San Andreas fault so because the we're at the boundary between two plates uh there's a weak Zone in the rock where water can easily move up and so we have a very long Pond here that's right along the trace of the San Andreas fault um so once again we have the Pacific Plate moving with respect to the North American Plate and this is right where
it happened the San Andreas fault has quite an effect on the rocks that are nearby as the fault moves the local rocks get folded or they get faulted they get broken and so here we have a good example of a fault that is associated with the San Andreas fault so here you can see two different colors of rock uh a white one on the left hand side and a reddish brownish color uh on the right hand side and you can see a contact between them fairly high up on the hillside hopefully you can see a
fairly good example of the contact between these two rocks so the these two types of rocks used to be in different places but because of the fault they were moved and brought into the position they're in now and you can see that the brownish colored rocks are folded so these are layers that used to be horizontal then they were uplifted and bent because of the San Andreas fault and so you can see the bending here of the different Brown layers so both the fold both the fault and the folding are the result of the forces
from the San Andreas fault and the two plates moving uh past each [Music] other for [Music] [Music] understanding the history of the earth without understanding play tectonics would be uh really difficult because uh if we um come to the conclusion that PL tectonics are really um essential to um provide a framework to understand a lot of the processes that are currently happening or have happened in the past we need to understand how this engine work especially if we think that play tectonics is the um unifying framework that allows to put together a lot of observations
if we don't understand the framework then we will also not understand the observations properly there was a theory the pre the preceding Theory um wasn't uh it was not like it wasn't very articulated or it was considered to be highly deficient there were um some questions about the the details of the physical mechanisms that would control the formation of these geoc inclines or these depressions at the edge of the continents but um it had become the the wellestablished Paradigm if we look backward uh we see that even when we have a very um articulated Theory
that um seems um to respond well to to frame well the data that we have it is always possible that a revolution may occur most likely if there is a new set of data that comes into the picture and and offers A New [Music] Perspective [Music] [Music] for foreign foreign [Music]
