The document is a slide presentation about calculating the density of oceanic lithosphere and how it changes with age. It introduces weighted averages and shows how to calculate the average density of oceanic lithosphere at different ages by taking a weighted average of crust and mantle densities. The density decreases as the lithosphere ages and the crust becomes a smaller proportion. However, at subduction zones the basalt transforms into denser eclogite, making the lithosphere dense enough to sink - providing the driving force for plate tectonics.
The lithosphere is Earth's outer layer consisting of soil and rock. It ranges from 64-96 km thick and is broken into tectonic plates. The lithosphere includes two types - oceanic lithosphere associated with oceanic crust in ocean basins, and continental lithosphere associated with continental crust. Beneath the lithosphere lies the mantle, which is divided into the asthenosphere and mesosphere, and below that is the core consisting of an inner solid section and outer molten section.
Ophiolites provide evidence for the composition and structure of oceanic crust and the upper mantle. They represent sections of oceanic crust and upper mantle that have been obducted or thrust onto continental margins. Studying ophiolites like the Samail ophiolite in Oman has helped scientists understand the layered sequence of rocks that make up oceanic crust, including extrusive basalts, dikes, and intrusive gabbros.
The document describes the three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks such as granite and basalt form from cooling magma. Sedimentary rocks like sandstone and shale form from compressed sediments. Metamorphic rocks like gneiss and marble form from existing rocks undergoing heat and pressure. Rocks give the lithosphere its solid, rigid properties and help maintain the shape of the Earth despite various forces.
The document describes the different layers that make up the Earth, including the crust, mantle, outer core, and inner core. It provides details on the composition and characteristics of each layer, such as the crust being the outermost solid layer and the inner core being made of solid iron and nickel. It also discusses the lithosphere, which includes the crust and upper mantle, and the types of rocks that make up the different layers, such as basalt in the crust and iron and nickel in the outer core.
The document provides an introduction to plate tectonics and earthquakes. It discusses key concepts such as the structure of the lithosphere and asthenosphere, the three main types of plate boundaries (divergent, convergent, and transform), and examples of each. It also covers earthquake locations in relation to plate boundaries and types of faulting.
Short course discussing a practical approach to Sequence Stratigraphy and attempting to clarify some of the terminological muddle that has accumulated over the past few decades.
Note: Originally presented as in-house short course for Pioneer Natural Resources Company. All material is public domain and/or original sketches/figures by author.
The lithosphere is Earth's outer layer consisting of soil and rock. It ranges from 64-96 km thick and is broken into tectonic plates. The lithosphere includes two types - oceanic lithosphere associated with oceanic crust in ocean basins, and continental lithosphere associated with continental crust. Beneath the lithosphere lies the mantle, which is divided into the asthenosphere and mesosphere, and below that is the core consisting of an inner solid section and outer molten section.
Ophiolites provide evidence for the composition and structure of oceanic crust and the upper mantle. They represent sections of oceanic crust and upper mantle that have been obducted or thrust onto continental margins. Studying ophiolites like the Samail ophiolite in Oman has helped scientists understand the layered sequence of rocks that make up oceanic crust, including extrusive basalts, dikes, and intrusive gabbros.
The document describes the three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks such as granite and basalt form from cooling magma. Sedimentary rocks like sandstone and shale form from compressed sediments. Metamorphic rocks like gneiss and marble form from existing rocks undergoing heat and pressure. Rocks give the lithosphere its solid, rigid properties and help maintain the shape of the Earth despite various forces.
The document describes the different layers that make up the Earth, including the crust, mantle, outer core, and inner core. It provides details on the composition and characteristics of each layer, such as the crust being the outermost solid layer and the inner core being made of solid iron and nickel. It also discusses the lithosphere, which includes the crust and upper mantle, and the types of rocks that make up the different layers, such as basalt in the crust and iron and nickel in the outer core.
The document provides an introduction to plate tectonics and earthquakes. It discusses key concepts such as the structure of the lithosphere and asthenosphere, the three main types of plate boundaries (divergent, convergent, and transform), and examples of each. It also covers earthquake locations in relation to plate boundaries and types of faulting.
Short course discussing a practical approach to Sequence Stratigraphy and attempting to clarify some of the terminological muddle that has accumulated over the past few decades.
Note: Originally presented as in-house short course for Pioneer Natural Resources Company. All material is public domain and/or original sketches/figures by author.
This document discusses the concept of significant figures and how to determine the number of significant figures in measurements and calculations. It defines significant figures as the "important digits" that indicate the precision of a measurement. Rules are provided for determining significant figures depending on leading or trailing zeros and whether the number is read from left to right or right to left. Examples demonstrate applying these rules and how to round final answers in calculations like addition, subtraction, multiplication and division based on the least precise measurement used. The key takeaway is that significant figures convey precision and final answers should not be more precise than the least precise input.
This document discusses hypothesis testing. It explains that hypothesis testing is used to determine if data is statistically significant enough to reject or fail to reject the null hypothesis. The key aspects covered are:
- Identifying when hypothesis testing is appropriate
- Distinguishing between the null and alternate hypotheses
- Determining whether to reject or fail to reject the null hypothesis based on comparing a test statistic to a critical value from a distribution table
This document discusses how scientists measure the hydrologic cycle. It describes traditional methods like stream gaging stations, groundwater wells, and SNOTEL stations to monitor streams, groundwater levels, and snowpack. It also discusses newer geodetic methods like GPS and GRACE satellites that can measure subtle changes in gravity or ground movement related to water storage and flow. These comprehensive measurements across different reservoirs help scientists better understand the complex global hydrologic cycle.
The document discusses how the coastline of North America during the Cretaceous Period 80 million years ago, with a Western Interior Seaway dividing the continent, still influences patterns today. It notes that the fertile soil deposited along this ancient coastline attracted slave plantations, and after emancipation the populations remained high in African Americans. As a result, modern voting patterns follow the same curve as the long-gone Cretaceous coastline, with counties with larger African American populations voting predominantly Democrat.
This PowerPoint document provides instructions for an activity to analyze climate and biomes using data on cities from around the world. Students will sort city climate information cards into biome categories, plot locations on a map, and fill out a worksheet characterizing climate and biome for each city. The PowerPoint includes over 50 slides providing detailed climate and location data on cities to support categorizing into biomes.
This document provides instructions for tracking weather systems using maps. Students are asked to print maps showing the location of low pressure centers over time. By examining the date and time stamps, students track one low pressure system as it moves across the United States over several days, recording its location on blank maps. They then connect the locations with a line to show the storm's path. Students also have the option to track additional storms, measure distances traveled between maps to calculate speed, or use software to analyze and animate the map images.
This document provides an overview of traditional and geodetic methods for measuring water resources. It discusses the hydrological cycle and key reservoirs and fluxes. Traditional measurements like gauging stations and SNOTEL stations that measure snowpack are introduced. Geodetic methods using GPS and gravity satellites are presented as newer techniques to measure vertical land motion, snow depth, soil moisture, and groundwater levels. Declining trends in snowpack and streamflow in Montana watersheds are highlighted as impacts of climate change on water resources. Stakeholders in water resources like local residents, industry, and government are identified.
This document defines and compares the three main measures of central tendency: mean, median, and mode. It explains that the mean is calculated by adding all values and dividing by the total number of values, the median is the middle value when the values are arranged in order, and the mode is the most frequently occurring value. The document also notes that outliers can affect the mean more than the median or mode. An example calculation is provided to demonstrate how an outlier impacts each measure. The key takeaway is that the mean, median and mode are important for summarizing large datasets with a single representative value.
Soils are essential to supporting life and human civilization. As populations grow, pressures on soils increase and maintaining soil health is important. Throughout history, human activities like deforestation, overgrazing, and poor irrigation have led to soil degradation problems like erosion, desertification, and salinization. This has negatively impacted societies by reducing agricultural productivity and sometimes causing civilizations to fail. However, more recent initiatives show people rediscovering the importance of soils and taking steps to promote sustainable land use and soil conservation.
The document discusses soil classification systems and soil surveys. It explains that soil taxonomy is a hierarchical system used to classify soils based on observable properties like color, structure, and chemistry. Soils are grouped into increasingly broader categories from the most specific level of series up to the broadest level of order. Soil surveys involve soil scientists mapping and describing soils in a given area in order to group soils with similar properties. The classifications aim to convey information about soil formation and management needs.
The document discusses nutrient management and soil fertility. It outlines key nutrients needed by plants and their analogous benefits for human health, including nitrogen for growth, potassium for water uptake and disease resistance, and calcium for growth and strong bones. It also addresses how soil pH impacts nutrient availability and describes common nutrient deficiencies like zinc deficiency that causes stunted growth and yellowing.
This document discusses several issues that can negatively impact soil quality including disturbed and degraded soil, desertification, deforestation, salinization, run-off, mineral extraction, and wind erosion. These processes can damage soil structure and reduce fertility.
The document discusses the major biomes of the world and the soils typically found within each one. It describes the key biomes as tropical rainforests, temperate forests, boreal forests, grasslands, tundra, deserts, shrublands, and wetlands. Each biome is defined by its climate, vegetation, and characteristic soil orders that form as a result of the particular environmental conditions within that biome.
This document discusses the physical properties and formation of soil. It describes how soil characteristics like color, texture, structure, and horizons/profiles influence water movement, storage, erosion, and plant growth. Soil formation is influenced by climate, organisms, topography, parent material, and time in a process known as CLORPT. The physical properties of soil determine how quickly water can infiltrate and percolate through different soil types.
This document discusses various natural and human-caused processes that can degrade soils, as well as best management practices to mitigate soil degradation. It covers topics like erosion from water and wind, desertification, acidification, salinization, effects of deforestation, urbanization, construction projects, land application of manures and wastes, and mining reclamation. Sustainable land management and soil conservation techniques aim to renew resources rather than deplete them over time through practices like maintaining vegetative cover, controlling grazing intensity, and properly applying nutrients from wastes.
This chapter discusses the living components of soil, including bacteria, fungi, protists, and fauna. Bacteria and fungi play important roles in nutrient cycling and forming soil structure. Fungi exist as filaments called hyphae that can form partnerships with plant roots. Protists include amoebas, ciliates, and flagellates that consume bacteria and debris. Larger soil fauna include earthworms, nematodes, springtails, and arthropods that further break down organic matter and improve soil structure through bioturbation. The variety of organisms in soil work together to create a living system that supports plant growth.
This document discusses the 2012-2017 California drought and its impacts. It provides historical context on droughts in California and examines precipitation data. Specific topics covered include:
1. The spatial extent and timing of the 2012-2017 drought across California and how it compares to historical droughts.
2. How precipitation was measured using tools like snow pillows and GPS reflection to track snow levels.
3. The societal impacts of the drought, including mandatory water rationing and transformations to California's landscape and economy.
This document discusses using GPS vertical positioning to monitor groundwater storage changes. It begins by explaining that groundwater mining is a global problem, and that extracting groundwater causes the land surface to rise as the total water storage decreases. It then discusses how GPS networks can detect these vertical position changes at the sub-centimeter level on a daily basis, allowing monitoring of seasonal water changes. Finally, it notes that long-term groundwater pumping can lead to both reversible and irreversible subsidence exceeding several meters, and provides examples from California's Central Valley.
This document discusses methods for characterizing groundwater storage, including traditional well measurements and satellite-based GRACE observations. It defines terrestrial water storage as all water on the land surface, and explains that groundwater often dominates variations in storage. Wells measure groundwater levels, with changes indicating replenishment or depletion over time. GRACE satellites detect changes in mass distribution and associated gravity field variations to infer changes in total water storage, including groundwater, at coarse spatial scales. The document provides examples of using both approaches to monitor groundwater in key aquifers.
The document provides an introduction to GPS/GNSS basics, including:
- GPS uses 24-32 satellites in medium Earth orbit that transmit positioning and timing data. Receivers need signals from 4 satellites to calculate a 3D location.
- Ground control stations monitor the satellites and send updates to synchronize their atomic clocks and orbital data.
- GPS determines location by calculating distances to satellites using signal transmission times and triangulating the receiver's position.
- Precise GPS uses permanent stations with stable monuments to collect data over many years, achieving sub-centimeter positioning and millimeter-per-year velocity estimates.
This document discusses the concept of significant figures and how to determine the number of significant figures in measurements and calculations. It defines significant figures as the "important digits" that indicate the precision of a measurement. Rules are provided for determining significant figures depending on leading or trailing zeros and whether the number is read from left to right or right to left. Examples demonstrate applying these rules and how to round final answers in calculations like addition, subtraction, multiplication and division based on the least precise measurement used. The key takeaway is that significant figures convey precision and final answers should not be more precise than the least precise input.
This document discusses hypothesis testing. It explains that hypothesis testing is used to determine if data is statistically significant enough to reject or fail to reject the null hypothesis. The key aspects covered are:
- Identifying when hypothesis testing is appropriate
- Distinguishing between the null and alternate hypotheses
- Determining whether to reject or fail to reject the null hypothesis based on comparing a test statistic to a critical value from a distribution table
This document discusses how scientists measure the hydrologic cycle. It describes traditional methods like stream gaging stations, groundwater wells, and SNOTEL stations to monitor streams, groundwater levels, and snowpack. It also discusses newer geodetic methods like GPS and GRACE satellites that can measure subtle changes in gravity or ground movement related to water storage and flow. These comprehensive measurements across different reservoirs help scientists better understand the complex global hydrologic cycle.
The document discusses how the coastline of North America during the Cretaceous Period 80 million years ago, with a Western Interior Seaway dividing the continent, still influences patterns today. It notes that the fertile soil deposited along this ancient coastline attracted slave plantations, and after emancipation the populations remained high in African Americans. As a result, modern voting patterns follow the same curve as the long-gone Cretaceous coastline, with counties with larger African American populations voting predominantly Democrat.
This PowerPoint document provides instructions for an activity to analyze climate and biomes using data on cities from around the world. Students will sort city climate information cards into biome categories, plot locations on a map, and fill out a worksheet characterizing climate and biome for each city. The PowerPoint includes over 50 slides providing detailed climate and location data on cities to support categorizing into biomes.
This document provides instructions for tracking weather systems using maps. Students are asked to print maps showing the location of low pressure centers over time. By examining the date and time stamps, students track one low pressure system as it moves across the United States over several days, recording its location on blank maps. They then connect the locations with a line to show the storm's path. Students also have the option to track additional storms, measure distances traveled between maps to calculate speed, or use software to analyze and animate the map images.
This document provides an overview of traditional and geodetic methods for measuring water resources. It discusses the hydrological cycle and key reservoirs and fluxes. Traditional measurements like gauging stations and SNOTEL stations that measure snowpack are introduced. Geodetic methods using GPS and gravity satellites are presented as newer techniques to measure vertical land motion, snow depth, soil moisture, and groundwater levels. Declining trends in snowpack and streamflow in Montana watersheds are highlighted as impacts of climate change on water resources. Stakeholders in water resources like local residents, industry, and government are identified.
This document defines and compares the three main measures of central tendency: mean, median, and mode. It explains that the mean is calculated by adding all values and dividing by the total number of values, the median is the middle value when the values are arranged in order, and the mode is the most frequently occurring value. The document also notes that outliers can affect the mean more than the median or mode. An example calculation is provided to demonstrate how an outlier impacts each measure. The key takeaway is that the mean, median and mode are important for summarizing large datasets with a single representative value.
Soils are essential to supporting life and human civilization. As populations grow, pressures on soils increase and maintaining soil health is important. Throughout history, human activities like deforestation, overgrazing, and poor irrigation have led to soil degradation problems like erosion, desertification, and salinization. This has negatively impacted societies by reducing agricultural productivity and sometimes causing civilizations to fail. However, more recent initiatives show people rediscovering the importance of soils and taking steps to promote sustainable land use and soil conservation.
The document discusses soil classification systems and soil surveys. It explains that soil taxonomy is a hierarchical system used to classify soils based on observable properties like color, structure, and chemistry. Soils are grouped into increasingly broader categories from the most specific level of series up to the broadest level of order. Soil surveys involve soil scientists mapping and describing soils in a given area in order to group soils with similar properties. The classifications aim to convey information about soil formation and management needs.
The document discusses nutrient management and soil fertility. It outlines key nutrients needed by plants and their analogous benefits for human health, including nitrogen for growth, potassium for water uptake and disease resistance, and calcium for growth and strong bones. It also addresses how soil pH impacts nutrient availability and describes common nutrient deficiencies like zinc deficiency that causes stunted growth and yellowing.
This document discusses several issues that can negatively impact soil quality including disturbed and degraded soil, desertification, deforestation, salinization, run-off, mineral extraction, and wind erosion. These processes can damage soil structure and reduce fertility.
The document discusses the major biomes of the world and the soils typically found within each one. It describes the key biomes as tropical rainforests, temperate forests, boreal forests, grasslands, tundra, deserts, shrublands, and wetlands. Each biome is defined by its climate, vegetation, and characteristic soil orders that form as a result of the particular environmental conditions within that biome.
This document discusses the physical properties and formation of soil. It describes how soil characteristics like color, texture, structure, and horizons/profiles influence water movement, storage, erosion, and plant growth. Soil formation is influenced by climate, organisms, topography, parent material, and time in a process known as CLORPT. The physical properties of soil determine how quickly water can infiltrate and percolate through different soil types.
This document discusses various natural and human-caused processes that can degrade soils, as well as best management practices to mitigate soil degradation. It covers topics like erosion from water and wind, desertification, acidification, salinization, effects of deforestation, urbanization, construction projects, land application of manures and wastes, and mining reclamation. Sustainable land management and soil conservation techniques aim to renew resources rather than deplete them over time through practices like maintaining vegetative cover, controlling grazing intensity, and properly applying nutrients from wastes.
This chapter discusses the living components of soil, including bacteria, fungi, protists, and fauna. Bacteria and fungi play important roles in nutrient cycling and forming soil structure. Fungi exist as filaments called hyphae that can form partnerships with plant roots. Protists include amoebas, ciliates, and flagellates that consume bacteria and debris. Larger soil fauna include earthworms, nematodes, springtails, and arthropods that further break down organic matter and improve soil structure through bioturbation. The variety of organisms in soil work together to create a living system that supports plant growth.
This document discusses the 2012-2017 California drought and its impacts. It provides historical context on droughts in California and examines precipitation data. Specific topics covered include:
1. The spatial extent and timing of the 2012-2017 drought across California and how it compares to historical droughts.
2. How precipitation was measured using tools like snow pillows and GPS reflection to track snow levels.
3. The societal impacts of the drought, including mandatory water rationing and transformations to California's landscape and economy.
This document discusses using GPS vertical positioning to monitor groundwater storage changes. It begins by explaining that groundwater mining is a global problem, and that extracting groundwater causes the land surface to rise as the total water storage decreases. It then discusses how GPS networks can detect these vertical position changes at the sub-centimeter level on a daily basis, allowing monitoring of seasonal water changes. Finally, it notes that long-term groundwater pumping can lead to both reversible and irreversible subsidence exceeding several meters, and provides examples from California's Central Valley.
This document discusses methods for characterizing groundwater storage, including traditional well measurements and satellite-based GRACE observations. It defines terrestrial water storage as all water on the land surface, and explains that groundwater often dominates variations in storage. Wells measure groundwater levels, with changes indicating replenishment or depletion over time. GRACE satellites detect changes in mass distribution and associated gravity field variations to infer changes in total water storage, including groundwater, at coarse spatial scales. The document provides examples of using both approaches to monitor groundwater in key aquifers.
The document provides an introduction to GPS/GNSS basics, including:
- GPS uses 24-32 satellites in medium Earth orbit that transmit positioning and timing data. Receivers need signals from 4 satellites to calculate a 3D location.
- Ground control stations monitor the satellites and send updates to synchronize their atomic clocks and orbital data.
- GPS determines location by calculating distances to satellites using signal transmission times and triangulating the receiver's position.
- Precise GPS uses permanent stations with stable monuments to collect data over many years, achieving sub-centimeter positioning and millimeter-per-year velocity estimates.
2. The paradox of oceanic lithosphere
Oceanic lithosphere plays two very important roles in plate tectonics. First, it is what the oceanic
plates are made of, and thus underlies 70% of Earth’s surface. Second, it is thought to provide the
most important driving force for the motion of the plates as it sinks into the asthenosphere at
subduction zones, dragging the rest of the plate along with it (a process called “slab pull”).
Figure from USGS web site
So how can the density of
the lithosphere be both
greater and less than the
density of the
asthenosphere? This is the
paradox of the oceanic
lithosphere.
Here the oceanic lithosphere is less
dense than the asthenosphere, causing
it float. This is a good thing, because
foundering of the oceanic plate would
destroy the oceans and all life on Earth! Here in the subduction zone the oceanic
lithosphere is denser than the
asthenosphere, causing it to sink. This
(For a review of density, see Endnote 1) tugging force drags the entire plate
along, causing it to move on the surface.
2
3. Objectives of this module
Upon completion of this module you should be able to:
•Explain what the weighted average is, and compare it to the simple non-weighted average;
•Compute the weighted average;
•Compute the density of the oceanic lithosphere given different proportions of mantle and
crustal rock;
•Explain how oceanic lithosphere thickens as it ages, and how its density changes during this
process
•Explain how oceanic lithosphere can be both less dense than the underlying asthenosphere (in
the ocean basins) and more dense than the underlying asthenosphere (in subduction zones)
First, extract the embedded Excel
spreadsheet where you will do your
homework. Remember to immediately Lith_Density
save it under a new, unique name.
Q1. Quick review: what kinds of geologic hazards
are commonly at subduction zone plate
boundaries, like the boundary shown in the
diagram to the left? Go directly to End-of-Module Questions
3
4. Math concept: what is a weighted average?
But here’s the key point: the weights don’t have to all be equal. And If they’re not all
equal, then some terms will get more weight than others in computing the average.
4
5. Weighted average, con’t.
When would the weights be unequal? When we’re taking the average of numbers that don’t
represent individual values, but groups of values. For example, suppose we wanted to calculate the
average age of students at a college given these data:
Number of % of college Average age of Q2. If we didn’t use a
Class weighted average,
students population class (yrs)
what would the
Freshman 135 33.75% 18.25
weights equal for a
Sophomore 107 26.75% 19.37 simple average?
Junior 85 21.25% 20.83 (HINT: we would be
simply averaging four
Senior 73 18.25% 22.09
numbers)
Whole college 400 Go directly to End-of-Module Questions
A simple average wouldn’t be appropriate because it would weigh each class equally in the average,
but the classes aren’t equal—there are more freshman, for example, than seniors. To calculate an
accurate average we need to weigh the averages for each class by the fractions of each class in the
whole population, so that classes—like the freshman class—which contain a higher percentage of
the college’s students, contribute more to the average.
5
6. An example: what is the average asking price of a house in Tampa?
Real estate brokers list homes, and you can use these data to compute the average asking
price. In many cases the data are broken down by the number of bedrooms (groups of values).
For example, here are the data for home listings in Tampa, Florida, in December 2010:
Looking at the data, you can see the obvious—larger houses
in general cost more than small ones, and the largest houses
—those that have five or more bedrooms—can cost millions.
You could compute an average of these prices, but what does
Data from trulia.com this mean? The average is heavily influenced by the cost of
the largest houses, but there were only 500 of them.
Instead you
calculate a
weighted Multiply the
average, with weight (W) times
the average
the weights price for each
equal to the size
proportion of The sum of these values is the
listings. weighed average asking price.
Total of all listings using Weight (W) for each size house computed
the SUM() function as the number of listings divided by the total
of all listings. The weights should sum to
one.
6
7. An example: what is the average asking price of a house in Tampa?
Here’s what the Excel cell formulas look like. Study them so you can create your own
spreadsheet to calculate a weighted average. This table is also found on the embedded
spreadsheet file.
Q3. What does the reference
$E$16 mean (it’s found in
the formula for cell G11)
=E11/$E$16 Go directly to End-of-Module Questions
=F11*G11
Copy and paste
the formula in
cell H11 into
these cells
=SUM(E11:E15)
Copy and paste =SUM(H11:H15
the formula in )
cell G11 into
these cells
7
8. Another example: what is the average tuition paid by USF students?
Here’s another example: what is the average tuition
paid by USF students? If you look it up you will find
that the tuition depends on whether a student is in-
state or not, and it’s a big difference!
In state tuition: $5,124
Out of state tuition: $15,933
However, there are far more in-state students than
out-of-staters, so we need to use the weighted
average to compute the average tuition. Here are
the complete data:
Q4. Fill in the rest of the this table,
which computes the weighted average
of tuition for USF students, both in-
state and out-of-state. Note that some
of the computed values are revealed
so you can check your formulas.
Q5. Here is the weighted average.
Is it closer to the in-state or out-of-
state tuition? Why?
Go directly to End-of-Module Questions
8
9. Review: what is lithosphere?
Plates: Lithosphere: Rigid rock
Crust: Intermediate or
mafic rock covered by 100-250 km, 1,300°C
sediment, 7-30km thick
Recall that the lithosphere is
the relatively cool, rigid outer
layer of Earth, and is underlain
Mantle: Asthenosphere: by the asthenosphere, solid
Ultramafic rock Solid rock that can rock that is hot enough to flow
flow like a fluid.
The lithosphere is not the same
thing as the crust, which is the
outermost layer of rock on
Earth defined on the basis of its
chemical composition.
Core: Iron (Fe) Liquid Metal
The lithosphere consists of two
metal
compositional layers: the crust
and the uppermost part of the
mantle. The transition from
lithosphere to asthenosphere
Layering based Layering based on occurs at ≈ 1,300°C, the
on composition style of deformation temperature at which mantle
rock begins to flow.
9
10. Review: what is oceanic lithosphere?
140m.y.
70 m.y.
20m.y.
8
Mid- Crust
Ocean Mantle
Ridge 1,300°C
Asthenosp
here
Click here to see how the
oceanic lithosphere thickens as
it moves away from the ridge
and ages
The oceanic crust is formed at mid-ocean ridges, and consists primarily of mafic volcanic rocks
(basalt). Underlying the crust is a small piece of mantle rock cool enough to be rigid, and these
two components form the oceanic lithosphere.
Typically, about 7 km of volcanic rocks can accumulate at the mid-ocean ridge to form the
oceanic crust before the plate moves away from the source of heat and magma.
As the lithosphere moves away from the mid-ocean ridge and its source of heat, it cools. As it
cools more and more of the mantle rock becomes rigid, and the mantle component of the
oceanic lithosphere thickens. Notice that as it thickens, the crust becomes a smaller and
smaller proportion of the lithosphere.
10
11. Average density of oceanic lithosphere
The average density of the mafic crustal rocks in the oceanic lithosphere is 2,800 kg/m3. The
average density of the ultramafic mantle rock in the oceanic lithosphere is 3,400 kg/m3.
Because it consists of both crust and mantle, the average density of the lithosphere will
therefore be a weighted average of the densities of the crustal and mantle components.
When the oceanic lithosphere is approximately 8 million years old it consists of about 13 km of
mantle overlain by 7 km of crust.
Q6a. Fill in the orange cells in this
table in Excel that will calculate the
average density of the oceanic
lithosphere when it is about 8 million
years old, and consists of 7 km of
crust overlying 13 km of rigid mantle.
Q6b. The density of the asthenosphere below the lithosphere is about 3,350 kg/m3.
Based on the density of the lithosphere you just calculated, will the lithosphere float
in this asthenosphere or sink through it? Enter “float” or “sink” in this cell.
Go directly to End-of-Module Questions
11
12. Average density of oceanic lithosphere as a function of age
As shown on Slide 10, the oceanic lithosphere gets thicker with age, as the mantle component
grows. Now that you know how to compute the average density of the lithosphere using the
weighted mean, you can investigate how the density changes with time.
Q7a. Fill in the orange cells in this Excel table Q7b. In column N Q7c. Once again,
that will calculate the average density of the calculate the density decide whether the
oceanic lithosphere at 9 different ages, from 8 difference between lithosphere will
to 140 million years old. Note that the the lithosphere and float or sink in the
arrangement of the table is a little different the asthenosphere asthenosphere,
from the ones you’ve done before, but the [ρ(asth), shown in and enter “float” or
equations are all the same—just make sure column M]. “sink” in column O.
you enter the cell references properly. I’ve
revealed the density for 25 m.y. old Go directly to End-of-Module Questions
lithosphere so you can check you’re doing it
right. 12
13. Density of oceanic lithosphere under subduction zones
Most geologists think that the motion of
the plates is driven by the sinking of
oceanic lithosphere at subduction zones
(the force is indicated with the arrow).
What is different about the oceanic
lithosphere here as opposed to on the
surface?
Basalt Eclogite
Q8. Calculate the density of the subducting oceanic
lithosphere, and decide whether it floats or sinks.
Go directly to End-of-Module Questions
Here’s the big difference: basalt,
which forms near the surface and is
stable there, transforms at depth into
a new rock called eclogite. Eclogite is
much denser than basalt. Endnote 2
Notice that all the densities are larger because of the
greater pressure at 150 km. The greater pressure
compresses the minerals so they occupy less volume.
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14. End-of-Module Assignment
Answer all questions in the spaces provided in the embedded spreadsheet (Slide 3), which you
should have saved with a different name (e.g., “YourName-density.xls”).
2.Answer questions 1-8 on Slides 3, 5, 7, 8, 11, 12, and 13.
•How does the density of the subducted oceanic lithosphere change as it warms up? How
would this change the “slab pull” driving force for plate tectonics?
•Continental crust and lithosphere is much thicker than oceanic lithosphere. The average
thickness of the continental crust is 30km, and the average thickness of the continental
lithosphere is 200km. Calculate the average density of the continental crust assuming that the
crustal rocks have a density of 2,700 kg/m3. Show all your work in the spreadsheet.
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15. Endnotes
• Density is a measure of the amount of mass per volume. The modern metric unit of density
is kilograms-per-square-meter, kg/m3, though many people are more familiar with the older
grams-per-cubic-centimeter (g/cm3). Water at normal surface conditions has a density of
1,000 kg/m3 or 1.00 g/cm3. Return to Slide 2.
• The difference between basalt and eclogite is the mineral composition. Basalt consists
primarily of three minerals: olivine, plagioclase, and pyroxene. When exposed to high
pressures, the olivine and plagioclase transform into garnet in the rock eclogite. Here are
the densities of the pertinent minerals:
Mineral Density (kg/m3)
Olivine 3,300
Plagioclase 2,700
Pyroxene 3,400
Garnet 3,500
You can see that a rock made of pyroxene + garnet (eclogite) will be denser than a rock
made of olivine, plagioclase, and pyroxene (basalt). Return to Slide 13.
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