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Geography, geology, geometry and geodesy

What’s the difference between geometry, geology, geography and geodesy?

Geometry

the branch of mathematics concerned with the properties and relations of points, lines, surfaces, solids.

and of course geometry has many practical uses in many careers, such as building gears, drills bits, laying out camera lenses, and so much more.

 

Geology

the science that deals with the earth’s physical structure and substance, its history, and the processes that act on it.

Moho discontinuity

Which includes the study of minerals, crystals and rocks.

Geography

the spatial study of Earth’s landscapes, peoples, places and environments. This includes cartography (map-making.)

There are many types of maps used in geography.

Geodesy

Geodesy combines applied mathematics and earth sciences to measure and represent the Earth (or any planet.)

what is Geodesy

NOAA National Geodetic Survey, from a PPT by Hawaii Geographic Information Coordinating Council

.

Geodesy reasons

from the  National Oceanic and Atmospheric Administration Ocean Service Education page on Geodesy:

Geodesists basically assign addresses to points all over the Earth. By looking at the height, angles, and distances between these locations, geodesists create a spatial reference system that everyone can use.

Building roads and bridges, conducting land surveys, and making maps are some of the important activities that depend on a spatial reference system.

For example, if you build a bridge, you need to know where to start on both sides of the river. If you don’t, your bridge may not meet in the middle.

As positioning and navigation have become fundamental to the functions of society, geodesy has become increasingly important.

More info

https://www.ngs.noaa.gov/INFO/WhatWeDo.shtml

https://oceanservice.noaa.gov/facts/geodesy.html

Precise Geodetic Infrastructure: National Requirements for a Shared Resource (2010) – Geodesy for the Benefit of Society

Seismic waves

Seismic waves are waves of energy that travel through the Earth’s layers.

They are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions.

They are studied by geophysicists called seismologists.

They are recorded by a seismometer/ seismograph, a hydrophone (when in water), or by an accelerometer.

P waves

Earth Science, Tarbuck & Lutgens, Chapter 8

Earth Science, Tarbuck & Lutgens, Chapter 8

Animation

S waves

Earth Science, Tarbuck & Lutgens, Chapter 8

Earth Science, Tarbuck & Lutgens, Chapter 8

Animation

Surface waves

Tarbuck & Lutgens

Tarbuck & Lutgens

Water waves are an example

Rayleigh surface waves

“The Rayleigh surface waves are the waves that cause the most damage during an earthquake. They travel with velocities slower than S waves, and arrive later, but with much greater amplitudes. These are also the waves that are most easily felt during an earthquake and involve both up-down and side-to-side motion.”

How do we measure motions of the Earth?

What is a seismograph?

Intro to be written

Earth Science, Tarbuck & Lutgens, Chapter 8

Earth Science, Tarbuck & Lutgens, Chapter 8

This is a seismograph record.

Earth Science, Tarbuck & Lutgens, Chapter 8

Earth Science, Tarbuck & Lutgens, Chapter 8

 

Mineral Identification

Mineral Identification Kit Properties

How do we distinguish one mineral from another? By any or all these properties:

 

Cleavage and fracture

Hit a rock with a hammer. What happens? The resulting cleavage or fracture can tell us a lot.

cleavage rocks

Crystal habit

The tendency for a mineral to grow into a special shape. This shape depends on the crystal’s structure (see next section), and also on the environment in which that mineral sample developed. (Where was it underground, how much heat, time, pressure?0

Crystal habits shapes

Crystal structure

All crystals are atoms in a three dimensional pattern. Here some examples.

Crystal structure Cubic Hexagonal

Image from Molecular and Solid State Physics, http://lampx.tugraz.at

 

Diaphaneity/transparency

The way that light passes through and reflects from a surface.

Diaphaneity Minerals Light

From left to right – Calcite is transparent, muscovite is translucent, and cinnabar is opaque.

Lustre

The appearance of a freshly cut mineral surface in light.

Mineral Luster Light

Magnetism

Some minerals are naturally magnetic.  These are some of the more common minerals that demonstrate magnetic properties

Hardness

The Mohs scale characterizes minerals by the ability of harder material to scratch softer material.

Mohs hardness scale minerals 2

Odor – So accustomed are we to associate odors with flowers or food that we scarcely appreciate the fact that certain minerals have a characteristic odor.

Selenium – horse-radish.

Arsenic – garlic.

Sulfides (such as pyrite) – rotten egg.

Antozonite (type of fluorite) – acid, reeking pungent smell.

Anthraconite – tar smell.

 

Specific gravity

How dense something is, compared to the density of water. This has to do with with how many atoms are in a unit volume (and also, with the density of those atoms themselves.)

density 3d

Streak

The streak of a mineral is the color of the powder produced when it is dragged across a flat surface.

cinnabar pyrite streak

 

Tenacity

Tenacity is how a rock sample responds to stress.  Put some force on it, try to crush, bend, break, or tear it. What happens to the sample? Whatever happens as a result is a way of distinguishing one sample from another.

Brittleness

If a mineral is hammered and the result is a powder or small crumbs, it is considered brittle. Brittle minerals leave a fine powder if scratched, which is the way to test a mineral to see if it is brittle.”

Broken Brittle Rock

Malleability

If a mineral can be flattened by pounding with a hammer, it is malleable. All true metals are malleable.”

Malleability malleable

Ductility

“A mineral that can be stretched into a wire is ductile. All true metals are ductile.”

Here, reddish copper is inside a steel cup, with a small hole. Pressure is applied to the copper – and a small amount is pushed out through the hole. A tool grabs this exposed copper, and slowly pulls. As it pulls, more and more copper is drawn out in a tube.

ductile draw into wire

Sectility

“Sectile minerals can be separated with a knife, much like wax but usually not as soft. An example is Gypsum.”

Many metals and some minerals can be cut with knife such as aluminum, gypsum, lead, lithium, and magnesium.

Cutting metal Lithium Knife

.

 

Cleavage and fracture

Many forces can act on tectonic plates, on mountains, even on individual rocks. Those rocks usually stay together as one piece, because the atoms and molecules are holding each other with strong bonds.

If a force becomes stronger than the bonds holding the rock together then the rock breaks apart. It will cleave or fracture.

 

Cleavage

Cleavage planes form along the weakest area of mineral’s structure.

These breaks create flat, planar surfaces.

These surfaces are determined by the structure of its crystal lattice.

These cleavage planes are smooth and are usually reflective.

Note – If a mineral’s structure is equally strong in all directions then it will not have cleavage planes – then it will show fracture (see next section.)

cleavage rocks

Example

Mica has 1d cleavage

Mica has cleavage mineral

Lumen Geology, Identify and classify common rock forming minerals.

Fluorite octahedral cleavage

Calcite has rhombohedral cleavage.

 

Fracture

If a mineral’s structure is equally strong in all directions then it will not have cleavage planes. Then it will just break unevenly.

Fractures have no definite shape.

fracture crystal

Chrysotile has splintery fracture.

Chrysotile has splintery fracture

Quartz has conchodial fracture

Quartz conchodial fracture

Obsidian conchoidal fracture

Obsidian Conchoidal fracture

Limonite, bog iron ore, earthy fracture

Limonite bog iron ore Earthy fracture

Crystals of native copper Hackly fracture (jagged fracture)

Crystals of native copper Hackly fracture (jagged fracture)

Magnetite uneven fracture

Magnetite uneven fracture

 

Samples with both cleavage and fracture

Cleavage and fracture in potassium feldspar

Cleavage and fracture in potassium feldspar

Further examples

GG101 Lab Minerals

 

Terminology

Cleavage terms (only use if cleavage planes can be recognized):

Perfect – Produces smooth surfaces (often seen as parallel sets of straight lines), e.g. mica;

Imperfect – Produces planes that are not smooth, e.g. pyroxene;

Poor – Less regular.

Non-existent.

 

Fracture terms (use in all other cases):

Conchoidal – Fracture surface is a smooth curve, bowl-shaped (common in glass);

Hackly – Fracture surface has sharp, jagged edges;

Uneven – Fracture surface is rough and irregular;

Fibrous – Fracture surface shows fibres or splinters.

This section from

Geololgy, Rocks and Minerals, Univ. of Auckland

Why are some moons spherical while others are shaped like potatoes?

Why are some moons spherical while others are shaped like potatoes?

This blog post was written by Physicist Dr. Matt Caplan, who used to run the QuarksAndCoffee blog. That blog no longer exists, but I’m showing this archived copy of one of his posts for my students.

Short answer: Gravity likes to pull things together, which makes spheres. If the body is small enough gravity isn’t strong enough to deform it, which makes potatoes.

Long answer: Put a ball on top of a hill. What happens? It rolls down to the bottom. Why? Because gravity said so. This isn’t just how it works on the earth, but everywhere in the universe. Clearly, gravity is trying to make spheres. If you tried to dig a super deep hole stuff would fall in from the edges to fill it up. And what happen if we start to pile up rocks? Eventually, the pile of rocks reaches the point where it will all crumble down under its own weight. A sphere is the only shape that has no holes to fill or hills to crush. This is why every planet and star in the universe is round.

Of course, the earth and moon and planets aren’t perfect spheres. They’re lumpy. They’ve got hills and valleys and although none of them are that big compared to the planet, they’re still there. This is because gravity is strong enough to destroy (or prevent the formation) of a really big mountain, but not a small mountain. A small mountain’s own rigidness is enough to support its weight against gravity [1].

Mountain failure How High

This image shows two failure modes for mountains. The mountain on the left experiences shear failure, with the stress from the weight above the diagonal line exceeding the breaking point of the material. The mountain on the right fails due to compression of the base material.

Because materials have some intrinsic rigidity there must be bodies whose gravity isn’t strong enough to pull them into a sphere. Rather, the material is stiff enough to keep an oblong shape. After all, satellites and astronauts and cows don’t collapse into spheres in space.

The limit where gravity is strong enough to overcome the material properties of a body and pull it into a sphere is called the Potato Radius, and it effectively marks the transition from asteroid to dwarf planet [2]. It’s about 200-300 km, with rocky bodies having a slightly larger Potato Radius than icy bodies.

You can use some complicated math with material elasticity, density, and gravity to calculate the Potato Radius from scratch, or you could just look at Mt Everest. It turns out that the same physics determining the maximum height of mountains can be used to determine the Potato Radius – after all, they’re both just the behavior of rocks under gravity.

Check this out. The heights of the tallest mountains on Earth and Mars obey an interesting relation:

height gravity mountains equation

If you know the height of Everest and that Mars surface gravity is 2/5ths of Earth, then you know that Olympus Mons (tallest mountain on Mars) is about 5/2× taller than Everest! This relation also works with Maxwell Montes, the tallest mountain on Venus, but not for Mercury. Planetary science is a lot like medicine in this sense- there are always exceptions because everything is completely dependent on the body you’re looking at.

This is more than a curiosity. It tells us something important. The height of the tallest mountain a planet can support, multiplied by that planet’s surface gravity, is a constant.

For this sake of this piece I’ll call it the Rock Constant because that sounds cool. So why am I spending so long on a tangent about mountains in a piece about potato moons? It’s because the Potato Radius and Rock Constant are determined by the same things – gravity and the elasticity of rock! We can use the Rock Constant to estimate the Potato Radius!

Consider an oblong asteroid. Let’s pretend this asteroid is actually a sphere with a large mountain whose height is equal to the radius of that sphere.

potato shaped asteroid or moon

As the radius of a body increases the maximum height of a mountain decreases. If the radius was any bigger the mountain would have to be shorter and our asteroid would be entering ‘sphere’ territory.

Let’s check if the radius of this imaginary asteroid is close to the Potato Radius using our relation for the Rock Constant:

height asteroid gravity equation

Height asteroid 3

And now we have everything we need:

 

height asteroid 4

This works out to about 240 km [1], right in the middle of the 200-300 km range of the more rigorous calculation!

(1) How High Can A Mountain Be? P. A. G. Scheuer, Journal of Astrophysics and Astronomy, vol. 2, June 1981, p. 165-169.

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How high can a mountain grow?

There is some kind of process that builds mountains, but there also must be something limiting that process. After all, we don’t see mountains 20 or 30 miles tall, right? So we must ask, how high can a mountain grow?

We start by asking, what are the highest mountains on Earth?

Which then brings up the next question, what do we mean by “highest”? The answer isn’t obvious because there are three different ways to think about “highest” – see this diagram.

Highest Mountain on the Earth

Given this, we next notice that most mountains on Earth are nowhere near this height. For instance, the highest mountain in New England is Mount Washington New Hampshire 1,900 m (6,300 ft.). The highest mountain in the Rocky Mountains in Mount Elbert in Colorado 4400 m (14,000 ft.)

In general, almost everywhere on our planet, the highest that a mountain can be is about half the height of Everest.  This is as tall as a mountain can grow on a lithospheric tectonic plate.

So our next question is, “why is there one set of rules for the highest that a mountain can be almost everywhere on Earth, and why do some locations have exceptions?”

What factors control the height of a mountain?

There is a balance of the forces:

Tectonic plate forces pushes the Earth’s crust upward.

Gravity pulls the mountain downward.

And, when the mountain is high & big enough, the weight of the mountain can crack and shatter the rock inside of it. This causes the mountain to crumble, and settle down to a lower height.

Don’t believe me? Even rock has a maximum amount of strength. Here is a GIF of what happens to solid rock when you put enough pressure on it! 🙂

Source: Unconfined compressive strength test of rock

Thus, if the weight of mountain > yield strength of the base rock then the mountain’s base will crumble.

Then he mountain will compress down to the maximum allowable height.

Of course, when this happens depends on what the mountain is made of. SiO2 is the most common molecule. But there are many minerals that are lighter, or stronger, or both, that can also be found in a mountain.

By the way, this gives us a neat relation – the surface gravity X maximum height of a mountain should be a constant.

Formula lets us relate height of Mt Everest on earth and Olympus Mons on Mars. Or find max deformation of asteroid before gravity pulls it into a sphere.

All the other downward forces on a mountain

Erosion wears the mountain down

How well does the mountain resist weathering/erosion? This depends on what kind of chemicals it is made out of.

Does being in the ocean affect how high a mountain can be?

Consider Mauna Kea, in Hawaii.

Much of Mauna Kea is underwater. It’s base can support more pressure since it’s underwater. Underwater, there is a buoyant force on the object that counteracts the force of gravity. Since nothing counteracts the gravity on Mount Everest, the mountain’s base can only support so much pressure.

Roy Blitz

What else makes mountains rise or grow?

Even while a mountain is eroding, the underlying plate activity may be forcing the mountain to grow higher.

A tectonic plate pushing more directly against another plate will create higher mountains than a plate moving less directly (say, at an angle) against another plate.

How strong are the crustal roots of the mountain?

As a mountain range grows in height, this root grows in depth, and thus the pressure and temperature experienced by the bottom of this root increases.

At a certain point, rocks in the base of this crustal root metamorphose into a rock called eclogite. At that point this rock will be denser than the material supporting the crustal root.

This causes delamination to occur. Depending on the amount of material removed, the rate of new material added, and erosion, scenarios with net increases or decreases in elevation are possible after a delamination event. This sets another limit on how thick a crustal root can get (and thus how high a mountain range grow on the long term).

Source, Reddit comment

Why are there some special spots on Earth where mountains can grow twice as high?

George W Hatcher writes

Mauna Kea rests on oceanic crust, which is denser than continental crust and able to support more weight without displacement. Being mostly inundated with seawater precludes some of the erosional processes to which mountains exposed to the upper atmosphere are subjected.

In addition, the very material of which Mauna Kea is composed (basaltic igneous rocks) is stronger than the variety of rocks that make up the continental crust and uplifted limestone seafloor that can be found atop Everest.

The actual lithospheric limit to mountain height averages about half the height of Everest, which is why Fourteeners are so famous in Colorado. Mountains that exceed this limit have local geologic circumstances that make their height possible, e.g. stronger or denser rocks.

In the case of Everest and the Himalayas, you have a geologic situation that is very rare in Earth history. The Indian plate is ramming into the Eurasian plate with such force that instead of just wrinkling the crust on either side into mountain ranges it has actually succeeded in lifting the Eurasian plate up on top.

So the Himalayas have double the thickness of the average continental plate, thus double the mountain height that would be considered “normal”.

George W Hatcher, Planetary Scientist, Aerospace Engineer

References

How high can a mountain possibly get? Earth Science StackExchange

How High can a mountain get? 2 Earth Science Stack Exchange

How tall can a mountain become on Earth? Quora

What is the theoretical limit to how tall mountains can get on Earth? Reddit

Glacial Buzz Saw Hypothesis: New Scientist article

Examples with math details

Why are some moons spherical while others are shaped like potatoes? Quarks & Coffee

How High Can Mountains Be? Talking Physics

How High Could A Mountain Be? Physics World hk-phy.org

How tall can I make a column of stone? Rhett Allain, Wired magazine columnist

Related lab ideas

Play Doh Modeling Folds: Block Diagrams and Structure Contours

Play-Doh Modeling Folds: Block Diagrams and Structure Contours

Play Doh Introduction to Igneous Intrusions

Play Doh Unconformities

Preparing lessons in case schools close for coronavirus

Be Prepared Preparation

Due to the likely imminent arrival of coronavirus outbreaks in America, many school districts are preparing to fight the pandemic with the most effective tool: social distancing. This may include cancelling school, and other public events, for up to 2 weeks in affected areas.

As such, teachers may be asked to prepare 2 weeks worth of lessons for students to do at home as stand alone work. How do we efficiently prepare for this? I have been uploading resources to my website and creating worksheets based on them. In a pinch I could email our students 2 weeks worth of worksheets in PDF format, and print out packets for students with little or no internet access.

Teachers can supplement the work they assign  by creating a YouTube channel with daily mini-lectures. Students could listen & ask questions from their homes.

Teachers of course don’t need their own website. They can use any of their favorite science websites, such as The Physics Classroom.

However, we shouldn’t need something like the coronavirus to begin preparing. It is just good practice for teachers to create a library of ready-to-go, self-contained lesson plans for each topic. Instead of waiting for an emergency, make this part of your weekly schedule: Each week pick your best lesson, and write a self-contained worksheet for it.

As you create handouts (whether destined for PDF or paper) please think about readability, and about students who are slow readers or who have IEPs:

  • Have a brief, clear introduction so the student knows what we are learning and why. See below for details.

  • Use a large enough font.

  • Use double-spacing.

  • Have some space between each section.

  • Break long paragraphs into a smaller paragraphs.

  • Add a color graphic to help explain the concept in each section.

A packed page is a poorly-designed page. Trying to shove a class onto just a page or two is especially irrelevant since we are no longer limited by paper. Students can view as many pages as we need on their PCs, tablets, or phones.

Writing a brief, clear introduction

* Content objective:

Briefly discuss what we are learning, and/or why we are learning this. This may involve teaching new ideas, procedures, or skills.

* Vocabulary objective – What are the critical words in this lesson?

These include not only new terms that you introduce, but supposedly “common” words that one assumes the students “already know.” (The problem is that many students don’t always know what these terms means. Please click the link for more information.)

Tier II vocabulary words: High frequency words used across content areas. They are key to understanding directions, understanding relationships, and for making inferences.

Tier III vocabulary words: Low frequency, domain specific terms.

* Build on what we already know.

Very few lessons start completely from scratch. Most will include some vocabulary & scientific concepts that were learned in earlier grades. So in this section of your introduction, briefly make connections to prior concepts.

How are you preparing for this?

 

Reliable sources of information

CDC: Centers for Disease Control – Coronavirus Disease 2019 (COVID-19)

Massachusetts Department of Public Health

US FDA Food and Drug Administration Coronavirus Disease 2019

Coronavirus disease: Myth busters – WHO World Health Organization

How much area would renewable energy require?

Many people believe that we must use either fossil fuels or nuclear power for energy production, because renewable energy (solar, wind) takes up far too much land area.

For instance, consider Ivanpah’s Land Footprint: World’s Largest Thermal Project Requires 92 Times the Acreage of Babcock & Wilcox “Twin Pack”, by Ben Heard, 3/13/2014, The BreakThrough

The author points out that the land footprint of a solar power plant is 92 times larger than the land footprint of a small nuclear reactor.  He thus concludes that we need nuclear fission power, not solar.

Although I certainly agree that there are safe and responsible ways to generate power from nuclear fusion, the case against solar and wind power is overstated and doesn’t hold up to close analysis.

It turns out that the power needs of the entire United States could be filled by renewable energy that uses less than 1% of land area in the nation. Further, that land could even be dual-purpose. There are many places where people grow crops under and between solar panels or wind turbines.

Prof. Katharine Hayhoe writes

People worry about how much land we’d need to supply the US with clean energy. Well, @elonmusk and I have independently calculated it and we both come up with something roughly comparable to the area we currently use for maple syrup or golf. A square about 100-120 miles per side.

As a 🇨🇦 let me hasten to clarify that I’m not advocating for removing maple syrup production but rather for co-production of energy on land that is also used for farming or pollinator ecosystems 😁 For example, @FreshEnergy runs this amazing clearinghouse, The Center for Pollinators in Energy

Lands Use Area comparison USA

This graphic from Here’s How America Uses Its Land, By Dave Merrill and Lauren Leatherby, Bloomberg, 7/31/2018

“Land use classifications are based on data published in 2017 by the U.S. Department of Agriculture’s Economic Research Service in a report called the Major Uses of Land in the United States (MLU). Data from the report provide total land-use acreage estimates for each state across six broad categories. Those totals are displayed per 250,000 acres.”

Ramez Naam has a similar analysis, How Much Land Would it Take to Power the US via Solar?

Guest writer Ben Heard [see above] complains that solar’s land footprint (specifically at the Ivanpah plant) is 92 times that of a small modular nuclear reactor… What Heard’s Breakthrough Institute article doesn’t tell you is how tiny that land footprint, in the grand scheme of things, actually is. Do the math on the numbers he presents: 1087 Gwh / yr, or 0.31 Gwh / acre / year.

At that output, to meet the US electricity demand of 3.7 million Gwh per year, you’d need about 48,000 square kilometers of solar sites. (That’s total area, not just area of panels.) That may sound like a stunningly large area, and in some sense, it is. But it’s less than half the size of the Mojave desert. And more importantly, the continental United States has a land area of 7.6 million square kilometers. That implies to that meet US electrical demand via this real world example of Ivanpah, would require just 0.6 percent of the land area of the continental US.

Asked about this on twitter, Heard replied that larger size nevertheless is a disadvantage. It threatens ecosystems and endangered species, for instance. And this is a legitimate point, in some specific areas. (Though certainly far less so than coal and natural gas.)

But, for context, agriculture uses roughly 30% of all land in the United States, or 50 times as much land as would be needed to meet US electricity needs via solar.

 

Discovering lost continents under the Earth

Never mind those stories about a “lost continent of Atlantis.” True, there likely is a lost island that is the basis of the Atlantis tale. Yet we know for a fact that there is no sunken Atlantean continent on the bottom of the Atlantic ocean or Mediterranean Sea. We can now literally see the entire bottom of the Atlantic seafloor in high resolution, and there’s no missing continent.

But although those tales of Atlantis as a lost continent are incorrect that doesn’t mean that some kind of amazing lost continents don’t exist. In fact, several do and we’re going to learn about them today!

We normally think of the Earth’s interior as looking like this

internal%20earth%20structure

But now we know there are details:

To review what a continent is, and how one is defined, see Continents and plate tectonics

The Farallon Plate

Some text from Farallon plate, Karin Sigloch *08, Art of Science 2009

This is a three-dimensional structure of the Earth’s mantle under western North America, down to a depth of 1500 kilometers.

The Pacific coastlines and mountainous western states are plotted above an expansive, seismically fast structure, colored in purple, known as the Farallon plate.

It is a vast piece of ancient ocean floor that has been slowly sinking back into the mantle over the past 150 million years. The tectonic stresses caused by the Farallon’s movements deep underground have thrust up the mountain peaks and plateaus of the West. They continue to drive its volcanoes and earthquakes, thus shaping the surface appearance of an entire continent.

The Farallon Plate was an ancient oceanic plate that began subducting under the west coast of the North American Plate – then located in modern Utah – as Pangaea broke apart during the Jurassic period.

It is named for the Farallon Islands, which are located just west of San Francisco, California.

Over time, it was subducted under the southwestern part of the North American Plate. The remains of it today are

the Juan de Fuca, Explorer and Gorda Plates, subducting under the northern part of the North American Plate;

the Cocos Plate subducting under Central America;

and the Nazca Plate subducting under the South American Plate.

http://en.wikipedia.org/wiki/Farallon_Plate

‘Lost’ Tectonic Plate Found Beneath California
http://www.livescience.com/27994-lost-tectonic-plate-california.html

How do we see inside the Earth

Continents of the Underworld

Large low-shear-velocity provinces, or LLSVPs

continents of underworld Large low shear velocity provinces LLSVPs

By Olena Shmahalo at Quanta Magazine, source data from Sanne Cottaar.

 

In Continents of the Underworld Come Into Focus, Quanta Magazine, Josjua Sokol writes

Decades ago, scientists first harnessed the echoes of earthquakes to make a map of Earth’s deep interior. They didn’t just find the onion layers you might remember from a grade school textbook — core and mantle covered by a cracked crust. Instead, they saw the vague outlines of two vast anomalies, unknown forms staring back from the abyss.

Over the years, better maps kept showing the same bloblike features. One huddles under Africa; the other is beneath the Pacific. They lurk where the planet’s molten iron core meets its rocky mantle, floating like mega-continents in the underworld. Their highest points may measure over 100 times the height of Everest. And if you somehow brought them to the surface, God forbid, they contain enough material to cover the entire globe in a lava lake roughly 100 kilometers deep.

In these regions, earthquake waves seem to slow down, suggesting that the blobs are hotter than the surrounding mantle. How do we know this? Rock expands when heated. That causes waves to travel sluggishly through warm regions, said Garnero, like the slower vibrations moving through a loose guitar string.

The slowing waves gave these features their formal name: large low-shear-velocity provinces, or LLSVPs — an unmagical abbreviation that may have contributed to the topic’s low profile. “We are also to blame,” said Sanne Cottaar, a seismologist at the University of Cambridge, “for misnaming this feature so badly.”

 

Learning Standards

tba

Why is the Earth still hot?

I. The formation of the Earth created a huge amount of heat

The Earth is thought to have formed from the collision of many rocky asteroids, perhaps hundreds of kilometers in diameter, in the early solar system.

Formation of Solar System

As the proto-Earth gradually bulked up, continuing asteroid collisions and gravitational collapse kept the planet molten.

Heavier elements – in particular iron – would have sunk to the core in 10 to 100 million years’ time, carrying with it other elements that bind to iron.

Radioactive potassium may be major heat source in Earth’s core,  Robert Sanders, UC Berkeley News, 12/13/2003

II. More heat generated when dense material sank down towards the center of Earth

When the Earth was first formed this material was not solid; some was hot enough to become viscous (like silly putty) or even liquid (like lava.)

The denser material was mostly iron and some radioactive metals.

This dense metal slowly sank towards the center, while less dense rock floated upwards.

This process itself created a lot of friction, which created a lot of heat.

“Gradually, however, the Earth would have cooled off and become a dead rocky globe with a cold iron ball at the core if not for the continued release of heat by the decay of radioactive elements like:

potassium-40, uranium-238 and thorium-232, which have half-lives of 1.25 billion, 4 billion and 14 billion years, respectively.

About one in every thousand potassium atoms is radioactive.”

III. Heat from the decay of radioactive elements.

Most metals we know are stable. Think of Nickel, Iron, Copper and Gold. If you put them in a box so that they don’t get exposed to oxygen, then they don’t rust, and never change. Millions of years from now they will still be around.

What’s inside metal atoms? Electrons, protons and neutrons. In a metal atom, the number of these particles will normally never change.

Example: Iron-56 26 protons, 30 neutrons, 26 electrons.
But some very large atoms are special: they not stable – they do change, all by themselves. These are called radioactive elements.

Uranium-238 92 protons, 146 neutrons, 92 electrons

-> spontaneously will change into

Plutonium-239 94 protons, 145 neutrons, 94 electrons + heat
– – –

In sum, there was no shortage of heat in the early earth, and the planet’s inability to cool off quickly results in the continued high temperatures of the Earth’s interior. In effect, not only do the earth’s plates act as a blanket on the interior, but not even convective heat transport in the solid mantle provides a particularly efficient mechanism for heat loss.

The planet does lose some heat through the processes that drive plate tectonics, especially at mid-ocean ridges. For comparison, smaller bodies such as Mars and the Moon show little evidence for recent tectonic activity or volcanism.

We derive our primary estimate of the temperature of the deep earth from the melting behavior of iron at ultrahigh pressures.

We know that the earth’s core depths from 2,886 kilometers to the center at 6,371 kilometers (1,794 to 3,960 miles), is predominantly iron, with some contaminants.

How? The speed of sound through the core (as measured from the velocity at which seismic waves travel across it) and the density of the core are quite similar to those seen in of iron at high pressures and temperatures, as measured in the laboratory. Iron is the only element that closely matches the seismic properties of the earth’s core and is also sufficiently abundant present in sufficient abundance in the universe to make up the approximately 35 percent of the mass of the planet present in the core.

The earth’s core is divided into two separate regions: the liquid outer core and the solid inner core, with the transition between the two lying at a depth of 5,156 kilometers (3,204 miles).

Therefore, If we can measure the melting temperature of iron at the extreme pressure of the boundary between the inner and outer cores, then this lab temperature should reasonably closely approximate the real temperature at this liquid-solid interface. Scientists in mineral physics laboratories use lasers and high-pressure devices called diamond-anvil cells to re-create these hellish pressures and temperatures as closely as possible.

Those experiments provide a stiff challenge, but our estimates for the melting temperature of iron at these conditions range from about 4,500 to 7,500 kelvins (about 7,600 to 13,000 degrees F).

As the outer core is fluid and presumably convecting (and with an additional correction for the presence of impurities in the outer core), we can extrapolate this range of temperatures to a temperature at the base of Earth’s mantle (the top of the outer core) of roughly 3,500 to 5,500 kelvins (5,800 to 9,400 degrees F) at the base of the earth’s mantle.

The bottom line here is simply that a large part of the interior of the planet (the outer core) is composed of somewhat impure molten iron alloy. The melting temperature of iron under deep-earth conditions is high, thus providing prima facie evidence that the deep earth is quite hot.

Gregory Lyzenga is an associate professor of physics at Harvey Mudd College. He provided some additional details on estimating the temperature of the earth’s core:

How do we know the temperature? The answer is that we really don’t–at least not with great certainty or precision. The center of the earth lies 6,400 kilometers (4,000 miles) beneath our feet, but the deepest that it has ever been possible to drill to make direct measurements of temperature (or other physical quantities) is just about 10 kilometers (six miles).

Ironically, the core of the earth is by far less accessible more inaccessible to direct probing than would be the surface of Pluto. Not only do we not have the technology to “go to the core,” but it is not at all clear how it will ever be possible to do so.

As a result, scientists must infer the temperature in the earth’s deep interior indirectly. Observing the speed at which of passage of seismic waves pass through the earth allows geophysicists to determine the density and stiffness of rocks at depths inaccessible to direct examination.

If it is possible to match up those properties with the properties of known substances at elevated temperatures and pressures, it is possible (in principle) to infer what the environmental conditions must be deep in the earth.

The problem with this is that the conditions are so extreme at the earth’s center that it is very difficult to perform any kind of laboratory experiment that faithfully simulates conditions in the earth’s core.

Nevertheless, geophysicists are constantly trying these experiments and improving on them, so that their results can be extrapolated to the earth’s center, where the pressure is more than three million times atmospheric pressure.

The bottom line of these efforts is that there is a rather wide range of current estimates of the earth’s core temperature. The “popular” estimates range from about 4,000 kelvins up to over 7,000 kelvins (about 7,000 to 12,000 degrees F).

If we knew the melting temperature of iron very precisely at high pressure, we could pin down the temperature of the Earth’s core more precisely, because it is largely made up of molten iron. But until our experiments at high temperature and pressure become more precise, uncertainty in this fundamental property of our planet will persist.

What will happen with the Earth cools?

When the Earth’s core finally does cool – billions of years from now – then Earth will solidify and there will be no more plate tectonics. Therefore there will be

  1. No more earthquakes

  2. No more volcanic eruptions

  3. no more island building

  4. No more mountain building

The Earth’s surface will eventually be eroded down to a flatter surface, marred only by new impact craters. Earth will then be a geologically dead planet, like the Moon.

Some scientists estimate that “The planet is now cooling about 100°C every 1 billion years, so eventually, maybe several billions of years from now, the waning rays of a dying sun will shine down on a tectonically dead planet whose continents are frozen in place.”

 

How do we know what lies at the Earth’s core?

How we know what lies at the Earth’s core. BBC

Addressing misconceptions

If the Earth’s core is radioactive why is there no radiation at the surface?

Click the link to read the article, but short version, there indeed is radioactivity here on the Earth’s surface!

External resources and discussions

What percent of the Earth’s core is uranium? earthscience.stackexchange.com

Claim: Radioactive decay accounts for half of Earth’s heat, and related, What Keeps the Earth Cooking? Berkeley Lab scientists join their KamLAND colleagues to measure the radioactive sources of Earth’s heat flow

A fascinating although somewhat controversial article, Andrault, Denis & Monteux, J. & Le Bars, Michael & Samuel, H.. (2016). The deep Earth may not be cooling down. Earth and Planetary Science Letters. 443. 10.1016/j.epsl.2016.03.020.

 

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