Welcome to Shapes in the Clouds!
This EarthCache is located within an area frequently patrolled by Police. Please avoid acting suspiciously whilst completing the Tasks, and, if challenged, please freely explain about geocaching. It may be worth pointing out that it is not a physical box you are looking for, but are simply studying the rocks visible to everyone.
This is an urban EarthCache in which geocachers are invited to examine a marble sculpture in central London. The learning point of this EarthCache is to get the geocacher to become familiar with geological stress and strain, in particular plastic and elastic strain.
Everything you need to answer the questions is available by visiting the location and by reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.
Here are some keywords for this lesson:
foliation = repetitive layering in metamorphic rocks.
elastic deformation = a temporary change in the shape or size of a rock, that is recovered when the deforming force is removed
plastic deformation = a permanent change in the shape or size of a rock that is not recovered when the stress is removed
‘Clouds in the Sky II’ is a 2013 sculpture by the artist, Peter Randall-Page. His 30 year body of work has been inspired and informed by the study of natural phenomena. Randall-Page has an interest in organic form and growth patterns in nature, and explore the dynamic tension between order and randomness. Throughout his career Randall-Page has gained an international reputation through his sculpture, drawings and prints. He has undertaken numerous large scale commissions and exhibited widely including a major solo exhibition at the Yorkshire Sculpture Park. His work is also held in public collections at the British Museum and the Tate.
In this sculpture, the artist “combines geometric order with geological chaos to produce something both visceral and sensual.” In his words, “These fundamental regular forms have been known since antiquity and are the geometric building blocks of our universe.”
Made of ‘highly figured’ Rosso Luana marble, the stone “introduces a cloud like and poetic quality in contrast to the structural discipline of the form itself.” It was quarried in Vagli di Sopra, Tuscany, Italy.
Rosso Luana marble was formed during the Jurassic period, which began 201.3 million years ago and ended 145 million years ago.
OK, so let’s get started...
Introduction to rocks
Minerals make up rocks. Rocks are formed in many different types of environment. These can be on, or within the Earth's crust. There are three types of rock, and each is formed in a different way.
Sedimentary rocks are formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
More on metamorphic rocks
Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic rocks that have been chemically changed (metamorphisised) inside the Earth. Sometimes this happens within the crust and sometimes the upper mantle. Metamorphic rocks can also happen by contact metamorpism by a lava flow flow over surface rocks. At that depth, rocks are subject to great pressure and temperature. Although very great, the temperature is still not high enough to melt the rocks, because otherwise igneous rock would form. The pressure is much greater than that required solely to break the rocks up. In fact, the pressure is so high that it changes the chemical make up of the rocks by forcing the elements in the minerals to change position. Nevertheless, pressure alone won't change the chemical make up, as fluids moving through would do so too. The process of metamorphosis takes millions of years.
Metamorphic rock needs either pressure or temperature, or both of these occurring together, to form.
Metamorphism is an isochemical process, which means the chemical composition is mostly unchanged from that of the protolith (original rock.) The main difference is the recrystallisation of the minerals into a new form. New structural features are often found in the metamorphosed rocks, such as slaty cleavage or schistosity.
Different grades of temperature and pressure will cause the same original rock to form very different metamorphic rocks.
Here, the sculpture is made of Rosso Luana marble. Rosso Luana Marble is a striking and dramatic metamorphic stone. Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism.
Metamorphism causes the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals.
When it is formed from a limestone with very few impurities, marble is white. It is composed primarily of calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. White marbles are usually swirled with darker colors. These veins are remnants of thin layers of clay within the original limestone, which have been transformed into flowing patterns as the rock was heated and compressed. Marble that contains impurities such as clay minerals, iron oxides, or bituminous material can be bluish, gray, pink, yellow, or black in colour.
Stress and Strain: Stress
In the process of plate tectonics, massive slabs of the Earth’s crust (the lithosphere) move unevenly over the planet’s near-spherical surface.
Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. Four types of stresses act on materials.
• A deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move, it cannot deform. This is called confining stress.
• Compression squeezes rocks together, causing rocks to fold or fracture (break). Compression is the most common stress at convergent plate boundaries.
• Rocks that are pulled apart are under tension. Rocks under tension lengthen or break apart. Tension is the major type of stress at divergent plate boundaries.
• When forces are parallel but moving in opposite directions, the stress is called shear. Shear stress is the most common stress at transform plate boundaries.
There are four general types of stress, which are either uniform or non-uniform:
- Uniform stress - this occurs when the force applies equally on all sides of a body of rock.
- Tension, compression and shear, are non-uniform, or directed, stresses.
All rocks in the Earth’s crust experience a uniform stress at all times. This uniform stress is called lithostatic pressure and it comes from the weight of rock above a given point in the Earth. Lithostatic pressure is also called hydrostatic pressure, because this force also includes the weight of the atmosphere and, if beneath an ocean or lake, the weight of the column of water above that point in the Earth. Nonetheless, the proportion of pressure on a rock due to water and air is neglible, apart from at the Earth’s surface.
The only way for lithostatic pressure on a rock to change is for the rock’s position within the Earth to change. Since lithostatic pressure is a uniform stress, a change in lithostatic pressure does not cause fracturing and slippage along faults. In subducting tectonic plates, the increased pressure of greater depth within the Earth may cause the minerals in the plate to metamorphose spontaneously into a new set of denser minerals that are stable at the higher pressure.
Rocks are also subjected to the three types of directed (non-uniform) stress – tension, compression, and shear.
• Tension is a directed (non-uniform) stress that pulls rock apart in opposite directions. The tensional (also called extensional) forces pull away from each other.
• Compression is a directed (non-uniform) stress that pushes rocks together. The compressional forces push towards each other.
• Shear is a directed (non-uniform) stress that pushes one side of a body of rock in one direction, and the opposite side of the body of rock in the opposite direction. The shear forces are pushing in opposite ways.
A rock’s response to stress depends on the rock type, the surrounding temperature, and pressure conditions the rock is under, the length of time the rock is under stress, and the type of stress.
When stress causes a material to change shape, it has undergone strain, also known as deformation. Deformed rocks are common in geologically active areas.
Stress and Strain: Strain
The Earth’s crust is constantly subjected to these stress forces that push, pull, or twist it. In response to stress, the rocks of the Earth undergo strain, also known as deformation.
Strain is any change in volume or shape.
Rocks have three possible strain responses to increasing stress:
• elastic deformation: the rock returns to its original shape when the stress is removed.
• plastic deformation: the rock does not return to its original shape when the stress is removed.
• fracture: the rock breaks.
This diagram demonstrates elastic and plastic deformation
At the Earth’s surface, rocks can be more likely to break. On the other hand, further into the crust, where temperatures and pressures are higher, plastic deformation is a lot more likely. Furthermore, metamorphic processes deeper in the crust can take place over relatively longer periods of time, which is also a factor in plastic deformation.
In response to stress, rock may undergo three different types of strain – elastic strain, plastic strain, or fracture.
• Elastic strain is reversible. Rock that has undergone only elastic strain will go back to its original shape if the stress is released.
• Plastic strain is irreversible. A rock that has undergone ductile strain will remain deformed even if the stress stops. It is caused by the slippage of atoms, or small groups of atoms past each other in the deforming material, without the loss of cohesion. Another term for plastic strain is ductile deformation. Where rocks deform plastically, they tend to fold.
• Fracture is also called rupture. A rock that has ruptured has abruptly broken into distinct pieces. If the pieces are offset—shifted in opposite directions from each other—the fracture is a fault. Another term for fracture is rupture.
Position 1 is the starting point of the rock.
Position 2 is the rock under stress and strain.
Position 3 are the possible outcomes - return to position 1 (elastic,) stay in position 2 (plastic,) or rupture
Plastic and Brittle Strain
Earth’s rocks are composed of a variety of minerals and exist in a variety of conditions. In different situations, rocks may be plastic (ductile) and able to undergo an extensive amount of plastic strain in response to stress, or brittle, which will only undergo a little or no plastic strain before they fracture. Some of the factors that determine whether a rock is plastic or brittle include:
• Composition - Some minerals, such as quartz, tend to be brittle and are thus more likely to break under stress. Other minerals, such as calcite, clay, and mica, tend to be ductile and can undergo much plastic deformation. In addition, the presence of water in rock tends to make it more ductile and less brittle.
• Temperature - Rocks become softer (more ductile) at higher temperatures. Rocks at mantle and core temperatures are ductile and will not fracture under the stresses that occur deep within the Earth. The crust is cold enough that in the right conditions, it can fracture if the stress is high enough.
• Lithostatic pressure - The deeper in the Earth a rock is, the higher the lithostatic pressure it is subjected to. High lithostatic pressure reduces the possibility of fracture because the high pressure closes fractures before they can form or spread.
• Strain rate - The faster a rock is being strained, the greater its chance of fracturing. Even brittle rocks and minerals, such as quartz, or a layer of cold basalt at the Earth’s surface, can undergo ductile deformation if the strain rate is slow enough.
When a rock or mineral is subjected to stress, stress is proportional to strain as long as the elastic limit has not been exceeded. This relationship is known as Hooke's law. Plastic deformation takes place when a rock, mineral, or other substance is stressed beyond its elastic limit. Plastic deformation is that deformation that produces a permanent change in the shape of a solid without that solid having failed by fracturing.
Rocks in the deeper parts of the Earth do not undergo fracturing because the temperatures and pressures there are high enough to make all strain ductile.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
- Please describe the rock (shape, size, colour, texture, foliation.)
- How “pure” do you think this marble is? Does it contain many impurities?
- Has this rock been subject to elastic or plastic deformation?
- Why do you think the rock has been deformed, and hasn’t fractured?
- Based on your answers above, how brittle do you think this rock is?
- Other than lithostatic pressure, what types of stress(es) do you think this rock was subject to?
- What other factors do you think affected the strain on this rock?
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Good luck, and thanks for visiting this EarthCache!
