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Earth’s Rocks
EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
Section 1: Rock Cycles
• What is the geological timescale?
Rocks can be viewed as records of time. The geological
timescale represents the relationship between past events,
and describes the timing of these events in relation to
each other.
DIAGRAM 01:
The geological timescale is a combination of relative dates
and absolute dates. Relative dates describe the order
in which the rocks were created, and these are based
largely on changes in the types of fossils in the rock or the
structural relationship between one rock layer and another.
Absolute dates provide a numerical age for the rocks using
techniques such as radiometric dating.
The timescale is split into different units, based on major
geological events, and these different units can be
correlated across continents based on the fossils
they contain.
• Suggested Film
- Rock Cycles
• How does the rock cycle work?
Magma that reaches the Earth’s surface is called lava
Over geological time the three different types of rock are recycled.
Igneous rocks are created within the Earth, or erupted from
volcanoes, and come into contact with the atmosphere where they
are broken down by weathering. The products of this weathering
are eroded and deposited elsewhere, they are buried and then
compacted to form sedimentary rocks. These buried rocks are
subjected to high pressures and temperatures, and can become
deformed and form metamorphic rocks. The metamorphic rock
within the Earth can also melt to become magma, which is the basis
of igneous rocks.
• Suggested Films
- Rock Cycles
- How Hot Is the Earth’s Core?
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
DIAGRAM 02:
Mountains are formed when the Earth’s
crust is pushed upwards
• What is the difference between rocks and minerals?
Rocks and minerals are both formed by geological
processes. Minerals consist of chemical elements or
compounds, and are generally classified by their chemical
structure and specific physical properties. Minerals such
as gold (Au) are composed of a single element, whereas
minerals such as quartz (SiO2) or feldspar are compounds
of elements. Rocks are made up of one or more minerals
and do not need to have a specific chemical composition.
They can instead be classified according to their bulk
composition, which is the overall percentage of different
elements or minerals in the rock. Rocks can therefore
have the same bulk composition, but can contain a
different collection of minerals depending on the conditions
under which they were formed.
• Suggested Films
- Rock Types
- Earthly Treasures: Precious Gemstones
- Earthly Treasures: Diamonds
Amethyst is a gemstone mineral
Extension Question
Q1. What are gemstones?
Gemstones are generally minerals, such as diamond, ruby or sapphire, which have often been cut and polished to
reveal their beauty. However, some rocks are considered to be gemstones too, such as turquoise or lapis lazuli.
Some are organic materials, such as pearl or amber. Perhaps the defining feature of a gemstone is that is has been
reworked from a naturally occurring material to turn it into a precious or semiprecious stone. Local geological conditions
are important; it is the relative rarity of these minerals and the difficulty involved in mining them that
makes them valuable.
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
Extension Question
Q2. How do we compare the hardness of a mineral?
There are several ways to determine the hardness of a mineral, but the most common way is to compare it to the
hardness of other minerals in terms of its ability to scratch them. A scale of mineral hardness called Moh’s scale is
used; the scale ranges from talc at the softest end to diamond at the hardest end.
Section 2: Rock Types
• What is igneous rock?
Igneous rocks form from magma, which consists of liquid rock material
with gas and crystals. Magma is formed deep within the Earth from the
melting of preexisting rocks by geothermal heat, and can solidify within the
Earth to form an intrusive igneous rock, or can be erupted at the surface
as lava to form an extrusive igneous rock.
These smooth granite pebbles are an
example of igneous rock
• Suggested Films
- Rock Types
Igneous rocks are classified by their composition and their texture.
When magma cools slowly, as for intrusive rocks, the minerals have time
to form large crystals, and the resulting rock is coarse-grained like granite.
When lava is exposed to the air it cools rapidly and forms fine-grained
rocks like basalt. These rocks are found in locations such as Iceland
and the Isle of Skye, Scotland.
• What are the characteristics of sedimentary rocks?
Sedimentary rocks are formed as a result of processes acting at the
Earth’s surface and are related to the Water Cycle. They can be formed in
many different environments, by the action of water, wind, gravity or ice.
Sedimentary rocks can form from fragments of preexisting rock which has
been weathered and eroded; these are known as clastic rocks. Sedimentary
rocks, such as limestone and chalk, also form from the accumulation of
organisms or the precipitation of minerals from solution that were once
sea floor deposits. Sedimentary rocks are deposited in layers, producing
a structure known as bedding. They can also preserve evidence of past
environmental conditions in the form of fossils or sedimentary structures,
like ripple marks.
Extension Questions
Q3. What is a fossil?
A fossil is evidence of life in the geological past preserved in sedimentary
rock layers. It can be the remains of plants and animals, or can even be
the preserved traces of animal activity known as trace fossils.
There are clear layers in this
sedimentary sandstone rock
Q4. How do fossils form?
Fossils can be preserved remains of the parts of organisms, such as shells or bones, or can occur where parts of
the organism are replaced by minerals as it is buried. Sometimes the organism dissolves and its space is filled with
sediment, forming a cast, and sometimes features such as tracks and burrows can be filled with a different type of
sediment to form a trace fossil.
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
• How are metamorphic rocks formed?
When rocks are subjected to high pressures and
temperatures, the texture and composition of the rock
can be drastically altered to form metamorphic rock.
Any type of rock can change into metamorphic rock.
The increase in temperature or pressure occurs for a
number of reasons. Burial of rock deep within the Earth
subjects the rock to high temperatures and pressure
from the rocks above, while horizontal pressure from
the collision of tectonic plates can deform the rocks in a
whole region. The heat from intrusive igneous rocks can
subject the surrounding rock to high temperatures and
change their mineral composition. This type of change is
known as contact metamorphism.
• Suggested Films
- Rock Types
Marble is an example
of a metamorphic rock
- Earthly Treasures: Diamond
Extension Questions
Q5. How can you tell what type of rock you are holding?
Rocks are classified by their composition, in terms of the minerals or the chemical elements they contain, the texture
of the minerals within the rock and the processes that formed the rock. To classify the rock by composition it may be
necessary to look at the rock under a microscope or perform chemical analysis on it. Texture can often be seen by
the naked eye and provide clues about how the rock formed. For example, metamorphic rocks are often banded and
folded with certain minerals aligned along the folds. Sedimentary rocks can contain particles, or clasts, with a distinct
shape which indicates where they were deposited. It can still prove difficult to distinguish between the many rock
types.
Q6. How does landscape differ between major rock types?
Although the Earth has a variety of different landscapes, all of the continents are generally composed of the same
landscape features. The centers of continents are composed of very old (igneous and metamorphic) rocks known
as basement rock. The top of this region is known as the “shield,” and is often a large and relatively flat area. The
shield is often covered by flat or gently sloping layers of sedimentary rock and river drainage produces rolling hills.
In any area, the landscape is determined by how resistant the different sedimentary rocks are to erosion. They can
be folded in mountain belts (e.g. the Alps), formed above a subduction zone, or where two continents collide. If the
folded sedimentary rocks are eroded, the landscape is determined by the stronger igneous and metamorphic rocks
underneath. The more resistant igneous rocks can form mountain ranges, while the surrounding rocks are eroded to
form lowland areas.
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
Section 3: Rocks and Man
• What is rock used for in everyday life?
Rocks are used in their bulk form and are processed to extract the minerals
they contain. Rocks are therefore used in a surprising number of ways in
our everyday lives. Rocks such as granite, limestone and sandstone can
be used for construction, and crushed rocks can be used to build roads and
other infrastructure. Coal is mined and used to generate a large percentage
of our electricity and oil is extracted from rocks and processed for use as
fuel, powering our transport systems.
The smoothness of marble makes it
a popular choice for sculptures
The metals extracted from rocks are used to manufacture things as diverse
as computers, airplanes, drinks cans, and the plumbing and wiring in our
homes. The plates that we eat our food from are often made of clay, which
is a weathering product of rocks, and the salt we put on our food is also
derived from rocks. There are also rare earth elements that are vital to the
electronic components in our cell phones. The supply of these products will
become increasingly important in the future.
• Suggested Films
- Structure of the Earth
- Earthly Treasures: Precious Gemstones
- Earthly Treasures: Gold
• Suggested Activity
- Ask the students to identify the different times in a typical
day when rocks are needed. Use some of the examples
mentioned in the question above as a starting point.
Extension Questions
Q7. How are diamonds formed?
Diamonds are composed of carbon that comes from the melting of preexisting rocks. Natural diamonds form at high
pressures and temperatures that can only be found deep within the Earth, especially where the continental crust is
very thick, before being brought near to the surface in volcanic eruptions. Diamond extraction is difficult and recently
extra measures have been introduced to ensure that the miners are treated fairly and can work safely.
Q8. Why is gold so precious?
The chemical and physical properties of gold make it particularly useful and desirable. It is malleable, which means
it can be shaped easily, it is resistant to chemical reactions, and is also a good conductor of electricity. Gold is
therefore very durable and can be used in many ways.
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
• What are the features of limestone?
Limestone is a sedimentary rock, which makes up around 10% of the
Earth’s land surface. It has regular cracks caused during its formation, and
is permeable, which means that water moves through it easily. While surface
limestone features may be minimal, underground features, such as caves
and drainage systems, are common. The Yorkshire Dales have a range of
large caves formed in carboniferous limestone, including Gaping Gill, which
is over 100m deep. The dissolved rock is re-deposited in some caves to
form stalactites and stalagmites,
Limestone is shaped into distinctive
features as it is worn down
by water and weathering
The term karst is used to describe the topography of limestone landscapes.
Rainwater is weakly acidic, and this dissolves the rock over time. The
ground tends to be undulating where weaker areas of limestone have been
dissolved more rapidly. The varying color of limestone makes it a popular
building material, although erosion from rainwater can become a
big problem.
• Suggested Films
- Limestone: Features
- Limestone: Formation, Forms and Uses
• What are the impacts of quarrying?
The extraction of rocks and minerals has an obvious impact on the
environment. Mining and quarrying involves removing large quantities of rocks
(overburden) to reach the valuable ores and mineral seams at greater depth.
This process may involve blasting with large machinery and huge vehicles.
Dust can spread some distance from the quarry, and there may be increased
traffic on local roads as large vehicles take away the rock. Associated
industries can also develop, for example cement factories are built close to
limestone quarries to create jobs. Many quarries are eventually restored, with
holes being flooded lakes, or vertical surfaces used for climbing. One of the
most famous restoration projects led to the building of the Eden Project
in Cornwall.
This pumice is an igneous rock, evident
from the many tiny holes present
• Suggested Films
- Quarrying: the Impacts
- Quarrying: Managing Negative Impacts
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
• Quizzes
Rock Types
Basic
Advanced
• Which is not a main type of rock?
• Which of these does not affect rocks’
appearance and properties?
A – igneous
B – sedimentary
A – size of grains
C – continental
B – type of minerals
C – how the rocks were formed
D – metamorphic
D – which hemisphere the rocks
originated from
• Plant and animal remains found in sedimentary
rock are known as
• Sandstone is made up of round grains with tiny
gaps between them which makes it
A – debris
B – detritus
A – soft and porous
C – fossils
B – hard and impermeable
D – diamonds
C – soft and impermeable
D – extremely hard
• Rock produced from cooling magma is
A – metamorphic
• Igneous rock at the surface is known as
B – igneous
A – intrusive rock
C – bionic
B – invasive rock
D – magmorphic
C – extrusive rock
D – extensive rock
• Rocks which have changed through excessive
heat and pressure are said to be
• After undergoing a change under heat and
pressure, sandstone becomes
A – metamorphic
B – thermophilic
A – diamond
C – barometric
B – quartz
D – sedimentary
C – slate
D – shale
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
Rock Cycles
Basic
Advanced
• Approximately how old is the oldest rock ever
found on Earth?
• The breaking down of rocks and the creation
of new ones is called
A – rock regeneration
A – 4 million years old
B – rock renovation
B – 4 billion years old
C – rock cycle
C – 400,000 years old
D – rock system
D – 40,000 years old
• Under intense heat and pressure, limestone
rock can change to
• Rocks that have changed under heat and
pressure are best termed
A – igneous rocks
A – marble
B – metamorphic rocks
B – chalk
C – lava
C – slate
D – magma
D – diamond
• Rising magma pushing rocks up on the surface
is a process known as
• What does cooling magma form?
A – igneous rocks
B – metamorphic rocks
A – faulting
C – sedimentary rock
B – uplift
C – elevation
D – fossils
D – rebound
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
• Answers
Rock Types
Basic
Advanced
• Which is not a main type of rock?
• Which of these does not affect rocks’
appearance and properties?
A – igneous
B – sedimentary
A – size of grains
C – continental
B – type of minerals
C – how the rocks were formed
D – metamorphic
D – which hemisphere the rocks
originated from
• Plant and animal remains found in sedimentary
rock are known as
• Sandstone is made up of round grains with tiny
gaps between them which makes it
A – debris
B – detritus
A – soft and porous
C – fossils
B – hard and impermeable
D – diamonds
C – soft and impermeable
D – extremely hard
• Rock produced from cooling magma is
A – metamorphic
• Igneous rock at the surface is known as
B – igneous
A – intrusive rock
C – bionic
B – invasive rock
D – magmorphic
C – extrusive rock
D – extensive rock
• Rocks which have changed through excessive
heat and pressure are said to be
• After undergoing a change under heat and
pressure, sandstone becomes
A – metamorphic
B – thermophilic
A – diamond
C – barometric
B – quartz
D – sedimentary
C – slate
D – shale
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EARTH SCIENCE • GEOLOGY • EARTH’S ROCKS
Rock Cycles
Basic
Advanced
• Approximately how old is the oldest rock ever
found on Earth?
• The breaking down of rocks and the creation
of new ones is called
A – rock regeneration
A – 4 million years old
B – rock renovation
B – 4 billion years old
C – rock cycle
C – 400,000 years old
D – rock system
D – 40,000 years old
• Under intense heat and pressure, limestone
rock can change to
• Rocks that have changed under heat and
pressure are best termed
A – igneous rocks
A – marble
B – metamorphic rocks
B – chalk
C – lava
C – slate
D – magma
D – diamond
• Rising magma pushing rocks up on the surface
is a process known as
• What does cooling magma form?
A – igneous rocks
B – metamorphic rocks
A – faulting
C – sedimentary rock
B – uplift
C – elevation
D – fossils
D – rebound
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