GEOL1303 Review Module 12 Fall21

Summary

In this intriguing lecture by Jennifer Lytwyn, the focus is on understanding Earth's interior by examining seismic waves. This module highlights how seismic rays travel through different layers of Earth, leading to discoveries about the crust, mantle, and core. It explains the interaction of seismic waves with Earth's layers, including the behavior of S-waves and P-waves in different states, and how these observations help infer the physical and compositional structure of Earth's interior. The lecture also covers Earth's magnetic field, internal heat processes, and gravity anomalies, providing a comprehensive overview of the planet's inside story.

Highlights

• Seismic rays help us understand the Earth's interior structure. 📐
• Lithosphere and asthenosphere layers play vital roles in tectonic activities. 🌎
• The 'Moho' boundary marks the crust-mantle transition. 🔍
• Earth's magnetism originates from the liquid outer core. 🔄
• Heat flow inside Earth drives geological processes. 🔥

Key Takeaways

• Seismic waves unravel Earth's hidden secrets and structures. 🌍
• Earth's crust and mantle have distinct layers and compositions. 🌋
• The core of our planet generates Earth's magnetic field. 🧲
• Changes in seismic wave velocities indicate variations in Earth's interior. 📈
• Gravity anomalies reveal the density differences beneath Earth's surface. ⚖️

Overview

In this engaging review, Jennifer delves into the mysteries of Earth's interior, elaborating on how seismic waves shed light on what's beneath our feet. By examining how these waves travel through different rock types and refract as they go, students learn how this informs our understanding of Earth's structure and layers - from the crust all the way to the core.

The lecture highlights the significance of the lithosphere and asthenosphere in understanding plate tectonics. It explains the 'Moho' boundary, which separates the crust from the mantle, and the role of seismic studies in discerning the structural variances between oceanic and continental crusts, alongside deeper layers like the mantle and core.

Further insights are offered into how Earth's magnetic field is generated by the dynamo processes in the liquid outer core. Jennifer also discusses Earth's internal heat flow and how it affects mantle convection, further interpreting gravity anomalies to reveal geological structures hidden beneath the surface.

Chapters

• 00:00 - 01:00: Introduction and Overview of Module 12 The chapter introduces the content of Module 12 and sets the stage for what will be covered. It begins with a check on whether the participants can view the slides, indicating a presentation format.
• 01:00 - 04:00: Seismic Waves and Earth's Interior The chapter 'Seismic Waves and Earth's Interior' focuses on the exploration of Earth's inner composition through the analysis of seismic waves.
• 04:00 - 07:00: Crust and Upper Mantle The chapter discusses the phenomenon of seismic waves and their expanding motion from the earthquake source, known as the hypocenter. These waves were previously explored in another module related to earthquakes, and this chapter acts as a refresher on the topic.
• 07:00 - 11:00: Asthenosphere and Transition Zones This chapter introduces the concept of seismic rays, which originate at the earthquake source and intersect waves at 90 degrees or perpendicularly. The chapter discusses how an infinite number of these rays can be constructed, focusing on picking specific ones for study.
• 11:00 - 18:00: Lower Mantle and Core In the chapter titled 'Lower Mantle and Core', the focus is on the movement of seismic waves through different types of rocks within the Earth's interior. The chapter details how these waves, specifically seismic rays, travel through various rock types and how their paths are affected as they traverse different layers of the Earth. The path of a seismic ray is monitored from its origin until it is detected by a seismograph at a location different from the origin point. This study of seismic ray paths helps in understanding the composition and structure of the Earth's interior, particularly the lower mantle and core.
• 18:00 - 24:00: Earth's Heat and Magnetic Field The chapter discusses the behavior of seismic waves as they travel through the Earth's interior, focusing on how their velocity changes when moving between different geological layers. Specifically, when waves pass from one type of rock layer (type one) to another (type two), an increase in velocity causes the waves to refract or bend upwards towards the boundary separating these layers. This behavior is key to understanding Earth's internal heat dynamics and the nature of its magnetic field.
• 24:00 - 30:00: Seismic Tomography and Gravity Measurement The chapter titled "Seismic Tomography and Gravity Measurement" discusses the relationship between seismic wave velocities and different rock types. In scenarios where a seismic wave travels from a rock type with a slower velocity to one with a faster velocity, the wave accelerates and the ray bends or refracts upwards. Conversely, a decrease in seismic wave velocity causes different bending behaviors. These principles are fundamental in understanding seismic tomography and gravity measurement techniques.
• 30:00 - 37:00: Gravity Anomalies and Interpretation The chapter discusses the behavior of seismic waves as they travel through different layers of the Earth. It focuses on how these waves refract or bend downward when passing between layers. By analyzing the behavior of seismic rays, one can infer changes in the velocity of these waves as they penetrate deeper into Earth's interior.
• 37:00 - 43:00: Conclusion The chapter focuses on studying Earth's interior layers, starting with the uppermost layer named the crust. It highlights that seismic studies reveal differences between oceanic and continental crust, noting that oceanic crust is thinner and has a different composition than continental crust.

GEOL1303 Review Module 12 Fall21 Transcription

• 00:00 - 00:30 all you you able to um see the slides
• 00:30 - 01:00 okay yeah I can see them okay well today I'm going to uh briefly review module 12 which uh deals with Earth's interior and as the lecture in module 12 uh explains uh we mainly study the interior by way of seismic
• 01:00 - 01:30 waves and we looked at seismic waves in detail back in the module dealing with earthquakes but just as a refresher uh when when uh earthquake occurs uh these seismic waves spread out from the earthquake Source again that's called the hyp center and spreads out as a series of seismic waves shown in the
• 01:30 - 02:00 diagram and we can construct what are called uh seismic Rays these are lines which originate at the uh earthquake source and uh are constructed such that the uh Rays intersect the waves at 90 degrees or perpendicular and we can construct an infinite number of seismic rays but it's interesting to to pick one particular
• 02:00 - 02:30 Ray path and see how that Ray path changes as it passes through uh Earth's interior and eventually is recorded by a seismograph station on a different location at a different location now so here we have two different rock types type rock types one and two and we're following one particular seismic Ray path as it passes
• 02:30 - 03:00 through uh Earth's interior and notice that when the velocity of the seismic waves increases when passing from one layer in this case type one into another layer uh type two the wave will refract or bend upward towards the boundaries separating the layers if the velocity of the wave increases in a rock type Ty two
• 03:00 - 03:30 so in other words we have slower wave u in rock type one than pass speeding up when entering rock type two and the net result is that the ray is bent upward or refracted upward as a result of that increase in velocity within rock type 2 conversely when the velocity of a seismic wave decreases
• 03:30 - 04:00 when passing from one layer into another the wave will instead refract or bend downward away from the boundaries separating the two layers so just looking at the behavior of the uh seismic Ray uh we can tell get an idea of the change in velocity of the seismic waves as they go deeper into Earth's uh interior
• 04:00 - 04:30 and this in turn allows us toh study the different layers of Earth's interior beginning with the very uppermost uh layer which is a called which is referred to as the crust and seismic studies tell us that oceanic crust tends to be quite a bit thinner and of different composition than continental crust ocean crust is
• 04:30 - 05:00 relatively thin uh ranging anywhere from 3 to 15 kilomet thick and as we saw before consists primarily of balt and gabo whereas continental crust tends to be quite a bit thicker anywhere from 40 to uh 65 kilometers thick it also has a different composition an average composition of granite and Granite tends
• 05:00 - 05:30 to be less dense and hence more buoyant than oceanic crust now below the crust is the solid portion of the upper mantle in this case colorcoded uh Brown and uh this solid portion of the upper mantle combined with the overlying crust compris is What's called the
• 05:30 - 06:00 lithosphere and the lithosphere is considered solid and brittle and approxim and averages about 100 kilometers in thickness and it's the uh lithosphere just combination of the crust and the solid upper most most par portion of the upper mantle this lithosphere that comprises the uh plates of H plate
• 06:00 - 06:30 tectonics below the athus and what's more the boundary between the crust and the upper mantle is referred to as the moho which is readily recognized uh based on seismic studies now below the lithosphere is the
• 06:30 - 07:00 asthenosphere which is mainly composed of a ultr mic rock called a partite if you recall peridotites are composed of the minerals Olivine and pericine for the most part but notice that within the upper portion of the uh asthenosphere the mantle is partially melted anywhere from 1 to 3% partial melt but as we go deeper in into the
• 07:00 - 07:30 asthenosphere eventually near the base of the asthenosphere U the mantle becomes completely solid and as a result uh seismic waves that pass through the upper part of the asthenosphere tend to slow down because of that because of the presence of that partial melt and that's why the athenos spere is also known as the low velocity Zone because mic wav slow down within
• 07:30 - 08:00 the upper asthenosphere again due to the presence of partial melt within the upper portion of this particular layer so here is a cutaway view of the uh Earth and we're looking at two curves the swave curve and the p-wave curves these are curves representing the velocities of these two waves and the
• 08:00 - 08:30 actual velocity values are shown at the volume at the bottom and they're and these values are in kilometers per second so we see that s waves travel through Earth's interior uh at least through the mantle at about half the velocity of a p waves but notice that that but notice um that the curves kind of shift towards the left as indicated by those
• 08:30 - 09:00 white arrows uh this shows us that seismic wave velocities decrease within the upper asthenosphere again because partite uh contains a few percent partial melt but not enough to completely stop S waves now the as we go deeper into the interior we encounter two trans position
• 09:00 - 09:30 zones marked by increases in seismic wave velocities and this is due to the closer Atomic packing enhanced increased densities primarily of the mineral Olivine and those two transition zones occur at depths of around 400 uh kilometers and about 660 kilometers depth and because of the tighter or closer Atomic packing at these depths
• 09:30 - 10:00 this causes an increase in both swave and p-wave velocities now the lower transition zone the one that occurs at about 660 kilomet marks the uh base of the upper mantle and below that lower transition zone we enter the lower mantle or What's called the mesosphere and we see that as we go deeper into the m mantle both the
• 10:00 - 10:30 velocities of both the S waves and p waves increase with depth in the lower mantle because of the increase in density of the mantle material so are there any questions uh I think I'm following for now okay
• 10:30 - 11:00 now PWS Bend outward when traveling through the mantle due to increasing velocity with depth and this increasing velocity again is due to increased uh mantle density with uh depth and the closer Atomic packing of the mantle uh minerals at the very bottom of the lower
• 11:00 - 11:30 mantle we encounter the dble prime layer which comprises the bottom few hundred kilometers of the lower mantle just above the outer core and seismic studies U is exhibit large horizontal indicated there are large horizontal variations within the dble prime layer in in both temperature and the
• 11:30 - 12:00 composition it's thought to be the possible graveyard of deeply subducted Oceanic lithosphere that reaches the core mantle boundary and what's more is thought to be the birthplace of a deep-seated mantle plumes that once form will then rise up towards the surface and impinge eventually on the base of the lithosphere
• 12:00 - 12:30 now below the dble prime layer we encounter the outer core that layer that's uh colorcoded yellow and it's thought that the outer core is composed of a liquid iron and as a result p waves tend to slow down when passing through the liquid outer core S waves cannot pass through the outer core because s-waves do not travel through liquids s-waves
• 12:30 - 13:00 can only travel through solids and as a result PWS Bend downward when entering the outer core because of the decrease in velocity and bend again when leaving the outer core and again entering the mantle creating what's called a p-wave shadow zone which this figure um occurs within
• 13:00 - 13:30 the quadrants of 100 to 140° and what this tells us is that seismograph stations located anywhere on the surface within that quadrant will not detect p waves because of this bending downward within the outer core and hence the it's called The p-wave Shadow Zone
• 13:30 - 14:00 now s-waves cannot travel through the outer core and as a result the s-wave shadow zone is much larger extending from 100 degrees all the way over to 100 degrees such that seismograph stations located on the surface anywhere within this quadrant will not detect any S waves
• 14:00 - 14:30 now the inner core is composed of solid iron and nickel and as a result pwes can speed up again when traveling through the inner core and also s-waves can travel through the inner core because of the solid uh nature of this um layer so any
• 14:30 - 15:00 questions uh I guess I just have one so um in the lower mantle you said that uh the speed increases because of the increasing density of the materials but um I'm I'm just conf because I know the the lower mantle includes some partial melt so like how do only only at the very base of the lower mantle okay in a dble prime
• 15:00 - 15:30 layer okay that okay that was that clears it up thank you okay now the outer core is the source of uh Earth's magnetic field here uh liquid iron in the outer core is stirred into convective motion by Earth's rotation and internal heat and the circulation of liquid iron in the outer core produces
• 15:30 - 16:00 electric currents uh those are illustrated by the spiraling motion within the outer core and this in turn generates Earth's magnetic field indicated by the uh by the blue v by the blue vectors and we saw before that today the Earth's magnetic field leaves the southern hemisphere wraps around and enters the Earth in the Northern
• 16:00 - 16:30 Hemisphere and that brings this to uh Earth's internal heat engine now the major processes that have contributed to Earth's internal heat um mainly occurred during the early formational history of the planet back 4.5 billion years ago and they included heat that was released by bombarding asteroid siiz Planet tesos
• 16:30 - 17:00 during the formation of the earth there was also the heat released as iron crystallized to formed a solid inner core he emitted by radioactive decay of of unstable Isotopes mainly short-lived um unstable Isotopes and since its early formation Earth has sign significantly cooled uh
• 17:00 - 17:30 since then back in during its early formation the Earth's interior may have been six to seven times hotter than it is today but the planet has since cooled down significantly over its 4.5 billion year history so that uh Earth's internal heat today uh mainly comes from the decay of longlived radioactive isotopes uh these are Isotopes were very very long half lives in the range of
• 17:30 - 18:00 hundreds of millions to billions of years and again they include uranium thorium and pottassium and from there the internal heat is brought to the surface by way of both convection and conduction now how pass how heat passes through the different internal layers
• 18:00 - 18:30 depends on the composition of the layers within the solid iron inner cord it's mostly by conduction within the liquid iron outer core It's a combination of conduction and convection however uh uh convection is the main way in which heat is transferred from the base to the top of the mantle and so the mantle itself is undergoing a large scale circulation
• 18:30 - 19:00 this convective Motion in transferring heat from the base of the mantle up to the uh base of the lithosphere and once the heat reaches the uh base of the lithosphere the heat passes through the lithosphere mainly by way of conduction until reaching the surface and then from there is radiated into the atmosphere here and out into
• 19:00 - 19:30 space now seismic tomography is a technique uh based on thousands and thousands of seismic records which allows a geophysicist to image Earth's mantle by mapping variations and seismic wave velocities uh where slower seismic waves
• 19:30 - 20:00 velocities are equate to hotter regions of the mantle whereas faster velocities equate the cooler denser regions of the mantle and we see in this crosssection of the Earth on the right where the all the colored regions are for the mantle here the core has been Whited out the blues and the uh green areas are places where uh s-wave velocities are faster
• 20:00 - 20:30 than average indicating cooler denser the material whereas the Reds and yellows are places where um s-wave velocities are slower than average indicating areas that are hotter and less dense so such that internal features and processes such as hot mantle super plumes cooler subducting plates and so on can then be imaged uh by computers
• 20:30 - 21:00 that are fed all these thousands and thousands of different seismic records from around the world any uh questions uh so far no okay
• 21:00 - 21:30 now a gravity meter is an instrument that measures variations in Earth's uh gravity and is affected by the density of the Rocks beneath uh towards the left for example we have a section of the crust which includes dense metallic ore and so this dense metallic ore is going to exert a stronger gravitational pull or attraction then would otherwise
• 21:30 - 22:00 be expected and therefore a gravity meter will record what's called a positive anomaly in other words there's excess gravitational attraction above what we would expect for typical uh crust whereas neutral whereas average gravity value is shown in the middle part of the block diagram and notice that um above the dense metallic ore there's a greater
• 22:00 - 22:30 attraction versus just uh the crust itself on the other hand towards the right we act we have a deficit we have a hole in the crust an opening or cave and this the result is that there's not as much of a gravitational attraction over this area compared to average because of the deficit in density the lower density
• 22:30 - 23:00 due to the presence of that cave and that lower than expected gravity value is referred to as a negative anomaly and this way we can get an idea of what's occurring beneath the surface what materials are beneath the surface based on the gravity gravitational traction at that particular uh location
• 23:00 - 23:30 and here's a a map of the United States showing the different gravity values uh here Gravity is in in the form of milligals but um notice that um the Reds and the uh uh oranges are places where uh the gravity is is a Little Bit Stronger whereas we have much weaker gravitational attraction in the
• 23:30 - 24:00 Blues and the uh and the greens and we see within the Basin and Range Province uh there's a negative anomaly there's a deficiency in gravitational attraction and this is because of the this is the area where we have um hotter less dense and tectonically active crust that's pulling apart or rifting and this is accompanied by volcanic
• 24:00 - 24:30 activity and so in this in the Basin of range there's a deficiency in Mass because of this pulling apart and thinning overall thinning of the crust also we have negative gravity anomalies in the Rocky Mountains and the Appalachian Mountains as indicated by the Blues in this case the negative
• 24:30 - 25:00 anomalies um are because the crust has deep roots in these uh areas made up of less dense Rock uh beneath the mountains so we have greater density of rock on both sides whereas within the Mountain Road itself this area has less dense Rock and hence give a negative gravity anomaly or or recorded negative gravity
• 25:00 - 25:30 anomaly and then finally we have this narrow positive anomaly uh right down the center of the midc continent it's called the Midcontinent Rift that and this is due to the presence of dense volcanic rocks that were imp placed into an old Rift uh more than than a billion years ago and so we have excess m within the RIT Valley because of the
• 25:30 - 26:00 presence of dense volcanic rock enhance the uh positive anomaly any questions um I have one so um negative anomalies are not only caused by like a lack of M I guess like distinct material underneath that specific point compared
• 26:00 - 26:30 to the average but like even lesser dense Rock can produce negative anomalies compared to the average well um it's a the gravity recorded within U areas of negative gravity anomaly is is less than what you would expect for the uh main crustal material within the uh region and so something is going on underneath that's making that's that's
• 26:30 - 27:00 um creating a deficiency in Mass uh gravity depends on the mass of the material the denser the material the more uh the stronger the gravitational attraction so when you have a less stce material underneath than what you would expect this would create a negative anomaly because there's a deficiency in
• 27:00 - 27:30 Mass okay as opposed to here you get a stronger gravitational attraction on the margin but a weaker gravitational attraction within the mountain because of this excess of a lower density Rock in this area and conversely if you have an excess density more of higher density and you expect within an area this will
• 27:30 - 28:00 create a stronger gravitational pull and create a positive gravity anomaly and this way we can get a better idea of what's exactly going on beneath earth's surface by interpreting these variations in gravity based on some underlying changes below the
• 28:00 - 28:30 surface but of course you also have seismic studies to get better idea of what's going on um below ground but uh gravity measurements are just one of the different tools that we use in trying to better understand Earth's uh interior does that answer your question
• 28:30 - 29:00 oh yeah yeah it does thank you that's pretty much it for this review I'll stop the recording