Jane's Geological Adventure

Jane's Geological Adventure


Over the last couple of years I have been writing a children's book about the rock cycle from the perspective of a zircon crystal called Jane. Zircon is very hard and can survive all sorts of geological processes and is therefore the most commonly used material for geochronology. This crystal was formed in a magma chamber, witnessed insects on the land during the Devonian, was buried and metamorphosed, exhumed and met dinosaurs in the Cretaceous, before being eroded by rivers and buried. It was exhumed again and exposed in the Ice Age before being sampled by a geochronologist, measured and displayed in a museum. This history is not unlike the history of zircons currently found in the Jura Mountains in Switzerland or Cretaceous outcrops in the UK. On her adventure, Jane meets Millie the millipede, Garry the garnet, Mitesh the mica, Anna the ankylosaurs, some baby T-Rex and Hairy Holly the woolly mammoth and various other animals and minerals. 

You can order print copies of the book below and find out more about the rock cycle and science behind the story. 

A scene from the book showing Jane the zircon being transported along a river.




The book is a paper back and is 250x250mm big with 18 double page pictures. All profits from the sale of the book will go towards supporting outreach activities across the Department of Earth Sciences at UCL and help with the GeoBus (https://www.geobus-london.org.uk/). If you would like a copy of the book, please complete the order form below.


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Overview


Jane's Geological Adventure tells the story of a single crystal of zircon as it travels through the geological cycle. Zircon is the ideal mineral to reveal this cycle because it is incredibly hard to destroy zircon. Jane is formed in a magma chamber, gets buried to high pressures and temperatures and transforms into a metamorphic rock as she is buried due to a combination of additional rocks piling up on top and also due to tectonic processes. These same tectonic processes drive her towards the surface and re-expose her to erosion in the Cretaceous. During this phase she is broken from the rock and carried along by a river before entering the ocean. Once she has landed on the sea floor and other sediments have surrounded her, she becomes sedimentary rock. However, her journey is still not finished: she is uplifted once more to the surface. After spending time at the surface and witnessing glacial erosion and changes in the climate, she is collected by a geologist. This text provides additional resources to help understand the rock cycle and the methods geologists use to understand how the Earth works!




Cross section through the modern Alps. At some time in the past, the Jura would have been a shallow sea. These have been uplifted very recently.

The geological history

Creation of minerals and the volcanic eruption


Jane is produced in a cooling magma chamber 400 million years ago and is erupted during a volcanic eruption. A magma chamber is a big volume of molten rock within a volcano that contains lava while it is underground. As magma chambers cool crystals grow, much like how ice crystals grow in a freezer or on a cold night. You can grow your own crystals from salty water at home - Google it! In this case, however, the changing conditions in the magma chamber led to an explosive eruption. She travels through the air and lands on the ground with other volcanic rocks and minerals. During this phase of the story Jane would be part of a magmatic tuff, a rock that is welded together by the intense heat of the rocks that land close to the volcano.

The geological time scale

400 million years ago, during the Devonian period, the Earth would have looked very different. The continental tectonic plates that make up much of Earth's land areas would have been in a different configuration. In fact, most of the land was around the South Pole. The climate was very warm and this helped the plants transition from the oceans to the lands and there were extensive ferns and enormous mushrooms. These warm temperatures also enabled large arthropods (insects, spiders and other creepy crawlies) to develop and take over the land. There were no reptiles or mammals around at this.

Burial and metamorphism


Additional volcanic eruptions would have led to Jane being buried in a thick sequence of volcanic rocks. As Jane was formed in a volcanos, it is clear that she is near tectonic activity. In this case, she is close to a collisional zone where tectonic plates are being pushed together due to convection in the mantle. This tectonic activity leads to Jane being buried under other rocks. In some cases, Jane would be buried even deeper and be subducted in a subduction zone however in this case, she is not buried too deeply.

Here is a garnet mica schist with large red garnets and shiny mica.

She is buried deeply enough, however, for pressures and temperatures to modify the surrounding rocks and produce new minerals. The appearance of the minerals garnet and mica suggest that she was buried to pressures of about 600 MPa (Mega Pascals) or about 20 km and temperatures of approximately 500 C! Just as how temperatures change the properties of ingredients when cooking, these temperatures and pressures changed the properties of the minerals found in the magmatic tuff. For example, a chocolate bar may be stable in your cupboard, but if it is kept in your pocket all day, the chocolate may melt and caramel layers may be squeezed out. If it is cooked in an oven, it may become totally molten. This process is similar to what happens to rocks at depth and this is called metamorphism in geology.

Jane also grows bigger during this phase by incorporating any available material into her crystal structure. These new layers of zircon are called zones and images of zircon often show strong zones.

Exhumation and erosion


Something changed, however, and brought these metamorphic rocks towards the surface. The new tectonic environment led to the addition of crustal material in large folds below Jane and her crystal friends. This would lead to them being pushed upwards. This is the concept of isostasy and is the same processes that controls the height of icebergs. As water freezes and produces an iceberg, the iceberg gets bigger and bigger. Because the iceberg floats on the water, the addition of more ice makes the bottom of the iceberg deeper and the top higher. In the Earth however, the rocks that are being pushed up are also eroded by rivers and glaciers. Therefore, rocks that may have been deeply buried are pushed upwards and towards the surface. We call the process by which rocks move upwards towards the surface exhumation.  It is this process that exposes the large range of rocks and minerals we find across Earth.

Folds in Stair Hole on the south coast of England.

Once Jane reaches the surface, the Earth has changed. The process of burial and exhumation has taken almost 300 million years and the continents have continued to drift and the animals have continued to evolve. Based on the dinosaurs and early mammals around at this time, Jane has returned to the surface during the Cretaceous period. The continents are starting to look more familiar and the Atlantic Ocean has begun to open up. However, because Jane has been uplifted in an active mountain range, she is soon broken from the bedrock and is transported down a river. She may have been broken off as part of a large boulder during a landslide or she might have been broken off by small animals or plants that turn rocks into sands, for example.

Fluvial transport and deposition


Jane is transported along the river and as she is transported, the bumps and knocks from other grains of sand round-off her sharp crystalline structure.

Once she reaches the end of the river (the river mouth), she enters a shallow tropical ocean and falls to the floor with marine animals, shells, and other sand grains. The marine animals may later become fossils. As time passes and other material lands on top of Jane, the pressures lead to the dissolution of some grains and these may recrystallize, bonding the grains together forming a sedimentary limestone. This dissolution is a bit like how salt dissolves in water. We call the recrystallizing minerals the cement.

Uplift and erosion again


This deposition was still relatively close to the mountain range and due to the ongoing convergence and continued growth of the mountain range, soon the limestones that contain Jane become part of the mountain range. This is because mountain ranges grow by adding rocks from close by into the mountain range. Although Jane would only have been buried for 50 million years or so, the geography, climate and animals would have changed again. The geography would have been very similar to today but the Atlantic Ocean would now be fully open and the Himalayas would have been built forming Earth's largest mountain range. The climate, however, has changed significantly. Whereas the Cretaceous was a time of high global temperatures, the Quaternary (when Jane reappears) was much colder with extensive ice sheets covering many of Earth's mountain ranges and both poles. The dinosaurs have been extinct since the end of the Cretaceous and now there are large mammals with thick fur to keep them warm in the cold conditions.

During the Quaternary, the ice sheets increased in size and then decreased in size with a relative constant frequency. We are currently in an interglacial period and the times when the ice sheets were large are called glacial periods. The last glacial period finished about 15 thousand years ago. It is during this period that Jane sees the landscape change from where the mountains were covered by thick ice sheets to the current interglacial period. Jane might have seen wooly mammoths and sabre toothed tigers during glacial periods. Very recently Jane would have seen a  modern human and this human is a geologist!

Geological Analysis


After Jane has been collected by the geologist, is it likely that tests would be carried out so that Jane can tell her fascinating story through the rock cycle. By looking at Jane with a microscope, different layers within her crystal structure allows geologists to see how she has formed in a magma chamber as part of a volcanic rock, grew in size during metamorphic conditions and was rounded during fluvial transport before becoming part of the sedimentary rock she was found in.

Once her history has been determined, this story can be shared in a museum along with the fossils that were found in the same rock as Jane and other metamorphic minerals that formed during metamorphic conditions.

Natural Analogy


The story of Jane is complex but due to the fact that zircons are virtually indestructible, it is very possible to find similar stories in nature. In particular, the geological history of parts of the European Alps mean that zircon crystals with a similar histories to Jane have been found.

The Alps formed during collision between the European plates and African plates. One of the European micro plates may well have been covered in the volcanic tuffs of Devonian age that were deposited while Jane was erupted. During the Jurassic and the Cretaceous there was ongoing collision between these plates and during this collisional phase rock would have been buried and metamorphosed. We cannot be sure what metamorphic conditions Jane would have experienced during burial because these rocks have been lost from the geological record. However, modern rocks across the Alps display a range of peak metamorphic conditions and so reaching the temperatures Jane experienced is possible. Around the Alps there there are numerous cases of Cretaceous shallow marine sediments, now in the core of the Alps, exposed in the mountains around Santis in Switzerland and along the Austria- Germany boarder.

How do we do it?


The age of the eruption of zircon can be measured using geochronology (see the methods section below) and radioactive decay. In particular, the concentration of parent and daughter atoms can be measured from the centre of the crystal. In many cases this is achieved using a laser that can be focused to a very small area and the tiny fragments of the crystal that are released by the laser can be measured on a mass spectrometer (check out the image below of the mass spectrometers at the London Geochronology Centre). This is a tool to measure the composition of various things and can count the number of different types of atoms. By measuring the amount of uranium and lead in the centre of the zircon, the age of eruption can be calculated.

Post from RICOH THETA. - Spherical Image - RICOH THETA



The age of metamorphism can also be estimated using geochronology however this time instead of dating the centre of the crystal we can date the edges of the crystal. This is because during metamorphism, fluids that are released as other crystals break down form new layers around crystals of zircon. We can date each of these layers, or zones, using a mass spectrometer and therefore date when the metamorphism occurred. Although we can measure when metamorphism occurred, it is very hard to estimate the metamorphic conditions during this time. One approach to do this is to measure what the zones of the crystal are made of. If different zones are made of slightly different atoms, this provides clues about what other crystals were growing from the same fluid. For example, if garnets were growing at the same time as zircons, the zircons would have fewer of the atoms that  produce the garnets.

The phase of exhumation that rocks experience can be determined using the thermochronometric system (see the methods section below) based on the production of helium from the decay of uranium. Again, the amount of helium can be measured on a mass spectrometer.

The age of the sediments that Jane was found in can be determined by looking at the fossils that are found in the same layers as Jane. These fossils have been observed in many locations where there are good geochronological constraints and the time at which Jane reached the sea can be established.

The most recent stages of Jane's journey are the easiest to establish. The action of glaciers eroding the surface is clear from the presence of deeply incised glacial valleys. The animals that would be found during this time period are known from the bones of animals that have fallen into caves or from the animals that are found frozen in ice sheets today.

Methods: Geochronology

Geochronology is based on the physics of radioactive decay and is named based on the Greek "geo" meaning Earth and "khronos" meaning time. As crystals grow, such as zircon crystals, atoms (the building blocks of everything) of uranium and other radioactive elements are incorporated into the crystal structure. Radioactive elements are unstable, much like something that is delicately balanced. In order for the elements to become stable something has to change and this change results in the elements producing other atoms. These radioactive atoms (parent atoms) will decay to produce other atoms and these atoms are called daughter atoms. This decay process is random but because this is random, when there are more unstable atoms, the chance of a decay event will be higher. Because of the large numbers of atoms involved, this process is very predictable and therefore this predictability allows geochronologists to date the formation of crystals. In particular, if the amount of uranium can be measured in a crystal of zircon, this provides the rate at which lead, for example, is produced. If the amount of lead in the crystal can be measured, we can calculate how much time is needed to accumulate this much lead. This provides the age of the crystal.


A good example of this type of approach is an egg timer. If you saw an egg timer with the sand flowing from the upper chamber to the lower chamber, could you calculate when it was turned upside down? You could measure how quickly sand was accumulating in the lower chamber by measuring the height of the sand immediately and in a minute or two. Once this has been established, it is straightforward to estimate how much time is required to fill the lower chamber to its current level. Of course this is based on the assumption that the lower chamber was empty when the egg timer was flipped upside down and also that the flow of sand between the chambers is constant. If there are two egg timers of different sizes that were turned upside down at the same time, we can calculate the `age' for both egg timers and check for agreements. This is exactly the situation for geochronology in that there are multiple different geochronological tools that are based on different parent and daughter elements.


Methods: Thermochronology


Thermochronology is based on the same principles as geochronology however the daughter product can escape from the crystal. For example, one of the daughter products of the decay of uranium are helium atoms. Helium is a common gas and is used in floating balloons. Helium atoms can escape from crystals of zircon and the ability of these atoms to escape depends on temperature. At high temperatures, helium can escape but a low temperatures helium can't escape. This is a lot like an egg timer that has a lower chamber with a hole in it - all the sand that falls into the lower chamber can escape. However, if this hole is closed at some point due to a change in temperature, the egg timer will record the time since that hole was closed. Therefore, by measuring the amount of helium in zircon and calculating how long this helium would take to accumulate based on the amount of uranium, we can measure the time since the rock cooled below a specific temperature. For helium in zircon this temperature is about 200 C. As temperatures increase in the earth at about 30 C every kilometre depth, 200 C is a few kilometres deep. In this way, thermochronology allows us to measure the rate at which rocks come from depths in the Earth to the surface.