The Geological Story of Reversal of the Earth’s Magnetic Field: What Does It Mean For Life on Earth
A cartoon from NASA that shows the white-coloured, invisible magnetic field that is generated by, and surrounds, the Earth. The Earth's magnetic field flips from time to time, when the north pole flips to become the south pole. Image credit: Peter Reid, The University of Edinburgh Image from: http://www.nasa.gov/mission_pages/sunearth/news/gallery/Earths-magneticfieldlines-dipole.html
Let's Imagine: Earth As A Big Magnet
When you use a compass (Figure 1) to guide you through the bush, you depend on the Earth's magnetic field. We all know that one end of the compass needle points north - right? Well, in the northern hemisphere it does, most of the time. But did that needle always point to the north? Well, no, there have been many times in the geological past when the compass needle would have pointed to the south instead of north. How can that be?
Figure 1: When we use a compass to navigate, we are actually tracking the Earth's magnetic field. The Earth behaves like a big magnet.
What geological process can change the direction of the compass needle by changing the direction of the Earth's magnetic field? Should we worry about possible impacts on life on Earth during one of those changes in magnetic field direction? We shall answer those questions in this story about the Ontario Beneath Our Feet.
What Is The Earth’s Magnetic Field?:
The Earth is surrounded by a magnetic field that acts like a giant bar magnet (Figure 2). The Earth has a magnetic north and south pole. If you stand anywhere in Ontario with a compass in your hand, the red needle of the compass points to the north - to the magnetic north pole. The compass needle acts like a pointer that shows us the presence and direction of the Earth's magnetic field. Unlike a bar magnet, the strength and orientation of the Earth's magnetic field changes over time.
Figure 2: A cartoon from NASA that shows the white-coloured, invisible magnetic field that is generated by, and surrounds, the Earth. The Earth's magnetic field flips from time to time, when the north pole flips to become the south pole. Image credit: Peter Reid, The University of Edinburgh Image from: http://www.nasa.gov/mission_pages/sunearth/news/gallery/Earths-magneticfieldlines-dipole.html
The origin of the Earth's magnetic field is not completely understood. Geologists conclude there is movement deep in the Earth in the outer core of the Earth (see Figure 3). The outer core of the Earth is made of liquid nickel and iron. The movement in that VERY hot, liquid part of the Earth is what creates the Earth's magnetic field.
Figure 3: Cartoon that shows the structure of the Earth from Crust, on the surface, to the mantle, deep within the Earth. The Outer Core is made of liquid nickel and iron. Movements there create the Earth's magnetic field. Image from: The Earth, by BBC: http://www.bbc.co.uk/education/guides/zysbgk7/revision
Why Is The Earth’s Magnetic Field Important To Life On Earth?
The Sun shoots out streams of dangerous particles (Figure 4). This is called the solar wind. Sometimes, the Sun blasts a really large flare of material towards the Earth, called a solar flare. Magnetic storms around the Earth result when the solar flare material smashes into the Earth's magnetic field. That storm causes the northern and southern lights: Aura Borealis (northern lights) and Aurora Australis (southern lights). If the Earth was not protected by the magnetic field, the solar wind would push our atmosphere away from the Earth into outer space. That would remove the ozone and oxygen from the Earth's atmosphere. The Sun also releases ultraviolet radiation, which also streams towards the Earth. Ultraviolet radiation kills life as we know it. In fact, there are some domestic water purification systems that use ultraviolet radiation to kill bacteria, making the water safe for us to drink. Ozone protects life on the surface of the Earth from life-killing ultraviolet (UV) radiation from the solar blasts. Without the Earth's ozone shield, the ultraviolet radiation would damage or kill life forms that are exposed - like fish, insects, animals, birds, and humans. The ozone in Earth's atmosphere is critical to sustaining life on the surface of the Earth. Without the Earth's atmosphere and its ozone, life as we know it would not likely survive on the surface of the Earth.
Figure 4: A cartoon that shows the burst of solar particles from the Sun, called a solar wind, towards the planet Earth. The Earth's magnetic field deflects the solar wind and protects the Earth from that blast of solar particles. The Earth's magnetic field acts like a shield to keep the solar particles from stripping away our atmosphere!. Image from European Space Agency: http://www.space.com/23131-earth-magnetic-field-shift-explained.html
The Earth is also hit by cosmic rays, which come from outer space. Cosmic rays act like tiny bullets. They can kill life on the surface of the Earth if there is not a shield to block the cosmic rays.
The Earth's magnetic field is the shield that protects the Earth from the solar wind and the cosmic rays. The Earth's magnetic field deflects the dangerous particles away so most do not reach the Earth’s surface. That protects our atmosphere, with its ozone, and ensures the atmosphere is not stripped away from the Earth.
How Do Geologists Read The Story of the Earth's Magnetic Field In Rocks?
When hot rocks are born, cool and turn to a solid, like rocks created by a volcano, the solid rock records the orientation and strength of the Earth’s magnetic field. The minerals in the cooling rocks act like a tiny date and time recorder of the Earth's magnetic field. Geologists have instruments that read that magnetic signature frozen in the rock to understand what the Earth’s magnetic field looked like at the time the rock formed. This study of the Earth's magnetic signature recorded in rocks is called Paleomagnitism. The record of the orientation and strength of the Earth's magnetic field shows up as stripes that form on either side, symmetrically, of great rifts or cracks along the bottom of oceans where new volcanic rock is born (Figure 5).
Figure 5: Cartoon that shows the record of the orientation and strength of the Earth's magnetic field in volcanic rocks that were born along great rifts or cracks along the bottom of the oceans. That record is recorded in the minerals that of cooling ocean floor rock called basalt. This rock is formed along both sides, symmetrically, of a rift or crack in the Earth on the bottom of the ocean. The ocean moves away from that crack and the youngest rock is squeezed into the crack. As that young rock cools from liquid to solid, it records the orientation of the Earth's magnetic field. The darker colour browns show periods of time when the Earth's magnetic field was normal, like today. The white bands show periods of time when the Earth's magnetic field was opposite to today. The white bands record what geologists call "reversed" polarity". This was one of the important lines of evidence to confirm that continents drifted around on the Earth's surface - continental drift!. Image from: http://www.regentsearth.com/ILLUSTRATED%20GLOSSARY/Paleomagnetism.htm
What Is A Geomagnetic Reversal And Why Does It Happen?
A magnetic reversal occurs when the north and south poles of the Earth's magnetic field flip or switch positions (Figure 6). This is called a geomagnetic reversal. The history of geomagnetic reversals is preserved in older and younger rocks (see Figure 5). It is not well understood why the Earth’s magnetic field reverses itself.
Figure 6: This cartoon shows what happens to the Earth's magnetic field when it goes through a full reversal. On the left, the red arrow points in one direction. This is considered "normal" and shows the way the magnetic field is oriented today. The right image shows that the Earth's magnetic field has flipped and the red arrow points in the opposite direction. This is considered a "reversal". The Earth's magnetic field has flipped, or reversed, many times through geological time. Image from: http://roma2.rm.ingv.it/en/themes/5/internal_origin_time_variations/20/geomagnetic_polarity_reversals
How Often Does The Earth's Magnetic Field Reverse Itself?
Over the last 5 million years, the Earth’s magnetic field reversed itself at least 20 times (Figure 7). Over the last 20 million years, the reversal happens about every 200 thousand to 300 thousand years; however, it is very difficult to predict when a reversal will occur. Therefore, geologists think in terms of average reversal interval.
Before the Earth’s magnetic poles switch, the magnetic field slowly gets weaker, fades out, and then reappears with the poles reversed. We may be due for another reversal in the next few thousand years, because it has been 780 thousand years since the last reversal, because the Earth’s magnetic field is weakening, and because there are strange patterns in the south Atlantic that suggest dramatic local changes in the magnetic field are taking place.
Figure 7: An example of the rock record that shows reversals of the Earth's magnetic field. This chart shows magnetic reversals over the last 5 million years (Pliocene and Quaternary, late Cenozoic Era). The dark bands show times when the Earth's historic magnetic field matches that of today - normal magnetic field. The white bands show times when the Earth's historic magnetic field was reversed - reversed polarity. Image source: United States Geological Survey: https://en.wikipedia.org/wiki/Geomagnetic_reversal#/media/File:Geomagnetic_polarity_late_Cenozoic.svg
How Quickly Does A Reversal Take Place?
It is difficult to state the exact amount of time it takes for a reversal to take place. Some geologists estimate that a reversal takes between 1000 and several thousand years to complete. From a geologist's perspective, that is VERY fast by geological standards, but it is very slow on a human time scale.
Is The Earth's Magnetic Field Due For A Reversal Soon?
The character of the Earth's magnetic field have been measured continuously since about 1840 (Figure 8). The image below shows the strength of the Earth's magnetic field since the year 1600. The trend of the line is pointing down. The trend peaked at a value of "35" in the year 1700 and is presently at about a value of "15". This downward trend leads some geologists to suggest that the a reversal of the Earth's magnetic field will take place in the future - in year 3575 or about 1500 to 1600 years into the future. However, the ability to forecast reversals in the future is very complicated and there are great uncertainties and cautions with that approach to predict a reversal.
Figure 8: Illustration of the state of the Earth's magnetic field since about the year 1600 to present day. You can think of this image showing the strength of the magnetic field. Because the strength has dropped from a high of close to "35" in year 1700 to present day value of less than "15", some geologists suggest this means the Earth's magnetic field is getting ready to reverse. Image from: https://en.wikipedia.org/wiki/Earth%27s_magnetic_field
How Does A Magnetic Reversal Affect Life On Earth?
Some birds, sea turtles and bees use the magnetic field to navigate. They may become confused if the Earth's magnetic field reverses. But what about human life? The Earth’s magnetic field protects life on the surface of the Earth from harmful ultraviolet and cosmic radiation from the Sun and beyond. Would life on Earth end if there was a reversal of the Earth's magnetic field? Could the solar wind remove enough of the Earth's atmosphere during a reversal of the Earth's magnetic field to threaten oxygen-breathing life on Earth?
Let us look at the history of life on Earth. The Earth was born about 4.54 billion years ago. The oldest fossil evidence of bacterial life on Earth comes from rocks that are 3.7 billion years old. In Newfoundland and Labrador, multicellular life forms, known as Ediacaran biota, occur as unique fossils in rocks that are 565 million years old. The human family tree is based on the study of fossils that indicate our ancestors have been around for 6 million years. Homo sapiens , or humans like ourselves, have been around for 200,000 years.
Humans, homo sapiens, have not experienced a reversal of the Earth's magnetic field, but life on Earth has. The ancestors of Homo sapiens have experienced a reversal of the Earth's magnetic field and we do not see a catastrophic collapse of our ancestors or other life forms in the last 6 million years. Similarly, when geologists look back through 4.5 billion years of geological time, some geologists state that they do not see great periods of extinction that correspond with reversals of the Earth's magnetic field.
But there still is some debate. During the past 5 mass extinctions over the last 600 million years, up to 97% of all species on Earth perished. During some of these extinction events, the Earth’s magnetic field flipped many times. Some studies suggest that the amount of oxygen in the atmosphere decreased during the magnetic reversal because the solar wind blew the un-shielded Earth’s atmospheric ozone and oxygen out into space. This decrease in concentration of atmospheric oxygen during some of the geomagnetic reversals may have been a key factor that killed numerous species by asphyxiation. This may have led to new life forms. Some suggest that the increased radiation arriving at the Earth's surface may also have favoured the development of new life forms with eyes and shells, which were capable of digging into the mud on the ocean floors.
So, the debate continues with some evidence suggesting that reversals of the Earth’s magnetic field may be more important than previously thought and may have contributed to some of the major extinctions on Earth, along with meteorite impacts, supernovae explosions, and volcanic eruptions. Clearly, the next reversal will be quite interesting.
Summary: Reversal of the Earth's Magnetic Field
The Earth's magnetic field protects the life on Earth from life-killing ultraviolet radiation from the Sun and from cosmic radiation. Over the history of the Earth, the magnetic field has reversed every several hundred thousand years. It has been about 780 thousand years since the last reversal and the present magnetic field is showing some signs that a reversal is due. There is a lot of debate if a magnetic reversal is dangerous to life on Earth. Some evidence suggests there is little impact on life. Other evidence suggests that there may be a relationship between life extinctions and magnetic reversals. Assuming humans have not led to our extinction before the next reversal, the next magnetic reversal will be quite exciting for life on Earth.
So, the next time you watch the northern or southern lights, pause and reflect on the important role played by the liquid iron and nickel, buried deep in the Ontario Beneath Our Feet, in creating the Earth's magnetic field, the role played by the magnetic field in creating that fabulous light show we know as the Aura Borealis (northern lights) and Aurora Australis (southern lights), and for protecting life on the surface of the Earth. How cool is that!
Some Additional Information Sources:
A) Yong Wei and others (2014): Oxygen escape from the Earth during geomagnetic reversals: Implications to mass extinction; Earth and Planetary Science Letters, Vol. 394, Pages 94–98.
B) Magnetic Storm - Earth's Invisible Shield; by NOVA - a video on YouTube.
C) Cheyenne MacDonald (2016): Mass extinction 550 million years ago was caused by Earth's magnetic field becoming hyperactive and flipping; Dailymail.com
D) Joseph G. Meerta, Natalia M. Levashovab, Mikhail L. Bazhenovb, Ed Landingd: Rapid changes of magnetic Field polarity in the late Ediacaran: Linking the Cambrian evolutionary radiation and increased UV-B radiation; Gondwana Research, Volume 34, June 2016, Pages 149–157.
E) The Earth: BBC online educational page about the Earth.
Andy Fyon, March 4/16; revised Sept 23/16; Oct 27/16; April 6/17