Download The Rock Cycle

Surficial Processes and the Rock Cycle:
What happens when diamonds reach the surface?
Activity 3.1
BACKGROUNDER
The Rock Cycle
The rock cycle identifies the processes that create the various rocks that make up the Earth. There are three main
groups of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. These groups are distinguished by unique
formation processes that give them their distinct identifying characteristics.
Igneous Rocks
Igneous rocks are formed directly from magma, the molten rock located deep within the Earth, and can be
categorized either as extrusive or intrusive igneous rocks. Extrusive igneous rocks result when lava — magma that
reaches the surface — cools on the surface of the Earth (i.e., from volcanoes). Intrusive rocks form when the liquid
magma solidifies within the crust.
The rate of cooling determines the texture of the rock. Rapid cooling results in small or non-existent crystals; these
are typical characteristics of extrusive igneous rocks. Slower cooling allows the growth of larger, distinct crystals,
generally found in intrusive igneous rocks.
Due to the forces involved in volcanic eruptions, and the liquid nature of magma, some extruded igneous rocks can
contain fragments of dust, rock particles, and ash. Their texture can be described as pyroclastic. Not all extrusive
rock, however, is pyroclastic.
Magma exists with a range of chemical compositions; this is evident from the varying chemical compositions of
igneous rocks. At one end of the scale are acid rocks, such as granite, that have a high percentage of quartz. At
the opposite end are the basic rocks, such as basalt, that contain no quartz, and instead, have a high percentage of
pyroxene and calcium plagioclase.
Examples of igneous rocks include granite, pumice, obsidian, and basalt. Kimberlite, the diamond-bearing
rock, is also an igneous rock.
Sedimentary Rocks
The process of erosion is important to the formation of sedimentary rocks. Surface rocks are worn away by erosion
and the loose sediment is transported away by wind, water, or glaciers. The sediment is deposited and collects in a
new place: on land, beaches, and glacial moraines, as well as river, lake, and seabeds. As more sediment arrives, the
process of burial begins and continues with compaction and cementation. Sedimentary rocks are usually made up of
layers, often distinguishable in rocks such as sandstone and shale.
Compaction is the gradual process of packing sediments together to form rock. As the deposits build up, the
increased weight helps the process.
Cementation binds the compacted sediments together with minerals that are often brought in by percolating
groundwater. Common cements are calcite and silica. The strength of a sedimentary rock is determined by the cement
involved in its formation.
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Sedimentary rocks may contain fossils: dead plants and animals that became part of the sediment build-up, and over
time, were compacted within the rocks. Large amounts of organic material, subjected to the pressure and heat of
subsequent sedimentary layers, eventually break down into carbon and hydrogen, which then become petroleum or
natural gas.
Examples of sedimentary rocks include sandstone, shale, limestone, conglomerate, and dolostone.
Metamorphic Rocks
Metamorphic rocks are those that have been changed from their preexisting state by one or more influencing factors.
The principal factors of metamorphism are pressure, temperature, and the chemical composition of fluids in the rock.
These factors change at least one of the minerals, structures, or textures in a preexisting rock. Contact metamorphism
refers to the local effect around igneous intrusions, and is predominantly the result of heat. Regional metamorphism
refers to changes that occur after deep burial of rocks — for example, in subduction — and is usually produced by a
combination of heat and pressure.
The crystals in a metamorphic rock often elongate parallel to the direction of least stress. This alignment creates
stripes in the rock showing a predominate orientation, referred to as “foliations.” Slate, a common example of foliated
metamorphic rock, splits easily into thin slabs.
Examples of metamorphic rock include slate, gneiss, quartzite, and marble.
Granite is an example of igneous rock. Image by Jupiterimages.
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3.1 THE ROCK CYCLE
Surficial Processes and the Rock Cycle:
What happens when diamonds reach the surface?
Activity 3.1
OUTLINE
COMPLEXITY
The Rock Cycle
DISCOVERING DIAMONDS
The diamond-bearing
Overview
The rock cycle is an endless system in which rock material is continually recycled as the
rocks of the Earth are created and destroyed. In this activity, students learn the terms that
apply to the processes and products of the rock cycle, and, using those terms, create a
visual representation of the rock cycle. Students then superimpose the diamond story
onto the rock-cycle diagram.
rock kimberlite is
an igneous rock. The
processes of erosion,
transportation, and
deposition of kimberlites
lead to the dispersal
of indicator minerals
Materials
Glossary Key Words
None required
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Erosion
Transportation
Deposition
Sediment
Burial
Cementation
Compaction
Sedimentary rock
Compression
Igneous rock
Metamorphic rock
and, rarely,
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Melting
Uplift
Continental crust
Subduction
Magma
Oceanic crust
Intrusion
Extrusion
Pyroclastic
Foliation
to alluvial
diamond deposits.
Classroom Instructions
This is a self-directed activity that may be assigned in class or as independent work outside the classroom. It
is suggested that the activity be completed in two stages. Stage one involves identifying rock-cycle terms, then
arranging them in a diagram of the rock cycle; this should be completed as an introduction to Topic 3. Stage two
requires students to superimpose the diamond story onto the rock-cycle diagram. Students will be better equipped to
accomplish this task after the remaining activities in Topic 3 have been completed.
In the accompanying Activities, students will frequently read and refer to “kimberlite,” since it is the primary source
rock for diamonds on Earth. However, as kimberlite is somewhat rare and unusual in its characteristics, it is not
commonly included in introductory Earth science courses. In Activity 2.3 Volcanic Eruptions and Kimberlites,
students discover how kimberlites get to the surface, and should understand that a body of magma has forced its
way up through layers of rock, some of which contain diamonds. Kimberlite is an igneous rock that carries with it
pieces of these secondary rocks, including diamonds. Identifying kimberlites is not possible solely by looking at a
hand-held specimen.
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Safety Precautions
There are no safety issues in this activity.
Assessment Ideas and Sample Responses
1. Identify each rock-cycle term below as either a process or a product of the rock cycle. Create a visual
representation of the rock cycle by arranging the terms to show the connections between the various products
and processes. Use different colours to identify the processes and products in the rock cycle.
This is a visualization task; therefore, there is no single solution. Emphasis should be placed on depicting the correct connections. The terms may be used as many times as required to ensure all possible pathways and connections are shown.
Below is one possible depiction of the rock cycle.
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3.1 THE ROCK CYCLE
Terms are classified as either a process or product in the chart below. Student responses will be colour coded on their
rock cycle diagram as either a process or a product of the rock cycle.
Rock-Cycle Terms
Classification as Process or Product
Erosion
Process
Transportation
Process
Deposition
Process
Sediment
Product
Burial
Process
Cementation
Process
Compaction
Process
Sedimentary rock
Product
Heating
Process
Compression
Process
Metamorphic rock
Product
Melting
Process
Uplift
Process
Igneous rock
Product
Intrusion
Process
Extrusion
Process
Continental crust
Product
Process
Subduction
Magma
Oceanic crust
Cooling
Partial melting
Product
Product
Process
Process
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2. Superimpose the stages in the diamond story onto the rock-cycle diagram.
Students should be able to convey their understanding of the processes required for diamond formation that are
associated with the formation of igneous and metamorphic rocks. Kimberlite is an igneous rock product from
the process of intrusion. Erosion of kimberlites, followed by transportation and deposition, can result in alluvial
diamond deposits and the dispersion of diamond indicator minerals. Burial of kimberlites at the surface also
occurs as the surrounding landscape erodes, and the resulting sediment is transported and deposited.
Diamond Story
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Stages in the Rock Cycle
Diamonds formed
Metamorphism
Indicator minerals formed
Metamorphism, igneous rock
Kimberlite magma forms
Magma, igneous rock
Kimberlite rises and picks up diamonds
Intrusion
Kimberlite pipe erupts at surface
Extrusion
Kimberlite is eroded
Erosion
Indicator minerals are dispersed
Transportation, deposition
Alluvial diamond deposits are formed
Deposition
3.1 THE ROCK CYCLE
Surficial Processes and the Rock Cycle:
What happens when diamonds reach the surface?
Activity 3.1
HANDOUT
The Rock Cycle
DISCOVERING DIAMONDS
Instructions
1. Identify each rock-cycle term below as either a process or a product of the rock cycle.
Create a visual representation of the rock cycle by arranging the terms to show the
connections between the various products and processes. Use different colours to
identify the processes or products in the rock cycle.
This is a visualization task; therefore, there is no single solution. Emphasis should be
placed on depicting the correct connections. The terms may be used as many times as
required to ensure all possible pathways and connections are shown.
Erosion
Transportation
Deposition
Sediment
Burial
Cementation
Compaction
Sedimentary rock
Heating
Compression
Metamorphic rock
rock kimberlite is
an igneous rock. The
processes of erosion,
transportation, and
deposition of kimberlites
lead to the dispersal
of indicator minerals
and, rarely,
Rock-Cycle Terms
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The diamond-bearing
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Melting
Uplift
Igneous rock
Continental crust
Subduction
Magma
Lava
Oceanic crust
Cooling
Partial melting
to alluvial
diamond deposits.
2. Superimpose the stages in the diamonds story onto the rock-cycle diagram.
Stages in the Diamond Story
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Diamonds formed
Indicator minerals formed
Kimberlite magma forms
Kimberlite rises and picks up diamonds
Kimberlite pipe erupts at surface
Kimberlite is eroded
Indicator minerals are dispersed
Alluvial diamond deposits are formed
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