Crustal Composition
Tuesday, July 7, 2026 2:37 PM
Of the 8 elements of the earth’s crust, oxygen is the main element that accounts for 47%,
nearly ½ of the total weight of the crust; silicon is the next most abundant element 28%, nearly
¼ of the total weight of the crust. Together, they make up 75% of the crust by weight. The next
elements are aluminum 8% & iron 5%.
- The metallic elements/bases (calcium, sodium, potassium, magnesium) are also part of
the earth's crust, essential for plant & animal life. This is where our vitamins & minerals
come from.
Minerals
- Earth’s “elements” are substances, composed of the same atoms (which makes up all
matter). Elements combine to form minerals, look at a rock closely & you’ll see that made
of grains/crystals; most minerals have a crystalline structure. These crystals, that vary in
size, are called minerals. Gemstones, like diamonds or rubies, are minerals that have been
polished, there are 3,500 known minerals. We're probably most familiar w/ the few
minerals (gemstones) associated with Zodiac signs
- Earth’s crust is comprised of these elements: oxygen 47% & silicon 28%, combined
together to form the most common of all mineral groups, the silicate minerals. Silicates
make up 98% of all rocks of earth’s crust. Minerals combine to form rocks, which can be
defined as a group of minerals in the solid state & molten minerals are magma in a liquid
state
Rocks
- Rocks are combinations of minerals (usually more than one). With 3,500 known minerals
& many different crystal forms, only 30 or so are common & less than a dozen are
common in crustal rocks. Since rocks can contain a number of combined minerals & w/
3,500 or so known, there is a huge number of different kinds of rocks that could possibly
exist. Geologists have classified rocks on the basis of how they were formed. There are 3
main ways rocks are formed & 3 rock classes/groups
Rock Process/Groups (igneous, sedimentary, metamorphic)
1. Igneous Rocks
- "Fire" in Latin, these are formed by the cooling & hardening of molten magma. This
molten material moves b/c of great forces, deep within or on the earth's crust. This is how
the continents are thought to have initially formed, most rocks consist of silicate minerals
w/ 1-3 of the metallic elements (aluminum, iron, calcium, magnesium, potassium, sodium)
- The appearance of igneous rocks (in crystal size) varies, depending on the rate of cooling.
If the cooling process was slow (i.e. thousands of years), crystals grow large. If the cooling
process is fast, (i.e. a couple of hours), crystals are small. The rates of cooling varies
depending on where the molten magma cooled into a solid rock, either on the surface, or
below
2 types of igneous rock:
a. extrusive igneous:
- Formed by the cooling & hardening of molten magma on earth’s surface. This may happen
as magma escape’s to the surface through fissures (cracks in the crust) or volcanic vents &
flow onto the surface as lava (in a molten state). B/c of the speed it cools, when exposed
to the relatively cool air temperature, they generally form microscopic-sized crystals; they
may have air pockets in them (ex. pumice) or can be quite smooth (ex. obsidian)
- The most common extrusive igneous rock on the surface of the earth (esp. making up the
ocean floor) is basaltic rock
b. intrusive igneous rocks (aka "plutonic" rocks, after Pluto, Roman god of the underworld)
These rocks are formed below the surface (in the lower crust/upper mantle), often at great
depth & cool very slowly, often thousands of years. B/c the crystals have time to form in the
warmer temps within the crust & uppermost mantle, large grain crystals are
developed. Granite is the best example & most common extrusive igneous rock, which forms
much of the continental land mass. Even though it was formed beneath the earth's surface, it
is often exposed to the forces of weather.
2) Sedimentary Rocks
- Made of pieces of existing rocks, accumulations of marine skeletal bones and/or
accumulations of dead vegetation. These materials are deposited in lakes, streams,
depressions or ocean basins (floors), over time as more sediments accumulate, buried
sediments become increasingly compacted (due to the weight of overlying sediment),
squeezing out excess water
- After possible cementation, pore spaces are filled by a cementing agent from the particles
(like silica, calcium carbonate or iron oxide) these materials accumulate in horizontal
layers called strata;
○ the oldest sediments are found on the bottom & newer, younger layers at the top.
This layering is known as “stratification.” Later, the layers can become tilted &
deformed later due to great pressures within earth. Sedimentary rocks are
categorized by their method of formation
Sedimentary Rock Types:
A. mechanically accumulated (aka "broken")
This includes rocks made from fragments of pre-existing rocks that are worn away by weather
& washed into the sea by rivers. The most abundant mechanically accumulated sedimentary
rocks are:
1. Sandstone
- Sand sized grains are deposited & compacted together to form sandstone. It‘s significant
for its ability to hold water (it’s very porous & there are pore spaces between sand grains);
aquifers made of this material. Many were formed from the sand sized sediment that
glacial waters carried beyond their extent during continental glaciation
2. Shale
- The most abundant sediment rocks is shale, made of compacted mud or clay sized
particles. The most abundant b/c this is the chief sedimentary rock produced on the ocean
floor. The tiniest particles are carried by streams & rivers the farthest out to sea, where
they settle on the ocean floor & form compacted layers of material. It hardens to form a
sedimentary rock
B. Chemically Accumulated
- This type rock forms when dissolved substances settle out of water, there are two major
types:
1. Limestone - most limestones result from the accumulation of skeletal remains of
marine organisms. Their bones react w/ the carbon dioxide in the water (from the
atmosphere) to form calcium carbonate. The calcium carbonate accumulates to form
limestone. It's also the stuff used to treat heartburn (i.e. "Tums")
2. Salt (halite) - carried as a dissolved solid in water, until water is evaporated from
it. When salt lakes dry due to changing climatic conditions, the salt rock is left behind
C. organically accumulated
1. Coal
- An organic rock made from squashed remains of vegetation from swampy mud forests
that covered N. America, Asia & Europe 300 million yrs ago. Many of the swamps were
free-standing, w/o an inflow of fresh oxygen carrying water
- This prevented dead vegetation to fully decompose, this multi-layer of plant debris was
accumulated & caused the material to cement by compaction. Coal is mined today b/c of
its value as fuel for electricity
D. Metamorphic Rocks
- Metamorphic rocks are made of other rocks that have been altered/transformed under
high pressure & temps into new forms. When minerals rearrange/recrystallize, any rock
becomes metamorphic
○ Crustal movements along the edges of the lithospheric plates generate alot of heat,
pressure & deform the crust. This heat/pressure deforms & modifies the original
crystalline structure of the rock
- In some cases, it creates new igneous rocks if subducted, melting starts the rock cycle all
over again. Igneous, sedimentary & even other metamorphic rocks can be squeezed and
scorched (heat and pressure) to become new rocks, “metamorphosed.”
○ Finally, metamorphic rocks become very hard & resistant to erosion when they have
been altered
- This forms the foundation of many major mountain ranges, their appearance is usually
foliated, wavy or banded due to the pressure. Some rocks change into other rock types,
like: sandstone becomes quartzite, shale becomes slate, limestone becomes marble,
granite becomes gneiss & coal becomes diamonds.
Rock Cycle
- Like landforms, rocks don’t remain in their original form indefinitely. They're always in the
process of transformation.
Tuesday, July 7, 2026 2:37 PM
Of the 8 elements of the earth’s crust, oxygen is the main element that accounts for 47%,
nearly ½ of the total weight of the crust; silicon is the next most abundant element 28%, nearly
¼ of the total weight of the crust. Together, they make up 75% of the crust by weight. The next
elements are aluminum 8% & iron 5%.
- The metallic elements/bases (calcium, sodium, potassium, magnesium) are also part of
the earth's crust, essential for plant & animal life. This is where our vitamins & minerals
come from.
Minerals
- Earth’s “elements” are substances, composed of the same atoms (which makes up all
matter). Elements combine to form minerals, look at a rock closely & you’ll see that made
of grains/crystals; most minerals have a crystalline structure. These crystals, that vary in
size, are called minerals. Gemstones, like diamonds or rubies, are minerals that have been
polished, there are 3,500 known minerals. We're probably most familiar w/ the few
minerals (gemstones) associated with Zodiac signs
- Earth’s crust is comprised of these elements: oxygen 47% & silicon 28%, combined
together to form the most common of all mineral groups, the silicate minerals. Silicates
make up 98% of all rocks of earth’s crust. Minerals combine to form rocks, which can be
defined as a group of minerals in the solid state & molten minerals are magma in a liquid
state
Rocks
- Rocks are combinations of minerals (usually more than one). With 3,500 known minerals
& many different crystal forms, only 30 or so are common & less than a dozen are
common in crustal rocks. Since rocks can contain a number of combined minerals & w/
3,500 or so known, there is a huge number of different kinds of rocks that could possibly
exist. Geologists have classified rocks on the basis of how they were formed. There are 3
main ways rocks are formed & 3 rock classes/groups
Rock Process/Groups (igneous, sedimentary, metamorphic)
1. Igneous Rocks
- "Fire" in Latin, these are formed by the cooling & hardening of molten magma. This
molten material moves b/c of great forces, deep within or on the earth's crust. This is how
the continents are thought to have initially formed, most rocks consist of silicate minerals
w/ 1-3 of the metallic elements (aluminum, iron, calcium, magnesium, potassium, sodium)
- The appearance of igneous rocks (in crystal size) varies, depending on the rate of cooling.
If the cooling process was slow (i.e. thousands of years), crystals grow large. If the cooling
process is fast, (i.e. a couple of hours), crystals are small. The rates of cooling varies
depending on where the molten magma cooled into a solid rock, either on the surface, or
below
2 types of igneous rock:
a. extrusive igneous:
- Formed by the cooling & hardening of molten magma on earth’s surface. This may happen
as magma escape’s to the surface through fissures (cracks in the crust) or volcanic vents &
flow onto the surface as lava (in a molten state). B/c of the speed it cools, when exposed
to the relatively cool air temperature, they generally form microscopic-sized crystals; they
may have air pockets in them (ex. pumice) or can be quite smooth (ex. obsidian)
- The most common extrusive igneous rock on the surface of the earth (esp. making up the
ocean floor) is basaltic rock
b. intrusive igneous rocks (aka "plutonic" rocks, after Pluto, Roman god of the underworld)
These rocks are formed below the surface (in the lower crust/upper mantle), often at great
depth & cool very slowly, often thousands of years. B/c the crystals have time to form in the
warmer temps within the crust & uppermost mantle, large grain crystals are
developed. Granite is the best example & most common extrusive igneous rock, which forms
much of the continental land mass. Even though it was formed beneath the earth's surface, it
is often exposed to the forces of weather.
2) Sedimentary Rocks
- Made of pieces of existing rocks, accumulations of marine skeletal bones and/or
accumulations of dead vegetation. These materials are deposited in lakes, streams,
depressions or ocean basins (floors), over time as more sediments accumulate, buried
sediments become increasingly compacted (due to the weight of overlying sediment),
squeezing out excess water
- After possible cementation, pore spaces are filled by a cementing agent from the particles
(like silica, calcium carbonate or iron oxide) these materials accumulate in horizontal
layers called strata;
○ the oldest sediments are found on the bottom & newer, younger layers at the top.
This layering is known as “stratification.” Later, the layers can become tilted &
deformed later due to great pressures within earth. Sedimentary rocks are
categorized by their method of formation
Sedimentary Rock Types:
A. mechanically accumulated (aka "broken")
This includes rocks made from fragments of pre-existing rocks that are worn away by weather
& washed into the sea by rivers. The most abundant mechanically accumulated sedimentary
rocks are:
1. Sandstone
- Sand sized grains are deposited & compacted together to form sandstone. It‘s significant
for its ability to hold water (it’s very porous & there are pore spaces between sand grains);
aquifers made of this material. Many were formed from the sand sized sediment that
glacial waters carried beyond their extent during continental glaciation
2. Shale
- The most abundant sediment rocks is shale, made of compacted mud or clay sized
particles. The most abundant b/c this is the chief sedimentary rock produced on the ocean
floor. The tiniest particles are carried by streams & rivers the farthest out to sea, where
they settle on the ocean floor & form compacted layers of material. It hardens to form a
sedimentary rock
B. Chemically Accumulated
- This type rock forms when dissolved substances settle out of water, there are two major
types:
1. Limestone - most limestones result from the accumulation of skeletal remains of
marine organisms. Their bones react w/ the carbon dioxide in the water (from the
atmosphere) to form calcium carbonate. The calcium carbonate accumulates to form
limestone. It's also the stuff used to treat heartburn (i.e. "Tums")
2. Salt (halite) - carried as a dissolved solid in water, until water is evaporated from
it. When salt lakes dry due to changing climatic conditions, the salt rock is left behind
C. organically accumulated
1. Coal
- An organic rock made from squashed remains of vegetation from swampy mud forests
that covered N. America, Asia & Europe 300 million yrs ago. Many of the swamps were
free-standing, w/o an inflow of fresh oxygen carrying water
- This prevented dead vegetation to fully decompose, this multi-layer of plant debris was
accumulated & caused the material to cement by compaction. Coal is mined today b/c of
its value as fuel for electricity
D. Metamorphic Rocks
- Metamorphic rocks are made of other rocks that have been altered/transformed under
high pressure & temps into new forms. When minerals rearrange/recrystallize, any rock
becomes metamorphic
○ Crustal movements along the edges of the lithospheric plates generate alot of heat,
pressure & deform the crust. This heat/pressure deforms & modifies the original
crystalline structure of the rock
- In some cases, it creates new igneous rocks if subducted, melting starts the rock cycle all
over again. Igneous, sedimentary & even other metamorphic rocks can be squeezed and
scorched (heat and pressure) to become new rocks, “metamorphosed.”
○ Finally, metamorphic rocks become very hard & resistant to erosion when they have
been altered
- This forms the foundation of many major mountain ranges, their appearance is usually
foliated, wavy or banded due to the pressure. Some rocks change into other rock types,
like: sandstone becomes quartzite, shale becomes slate, limestone becomes marble,
granite becomes gneiss & coal becomes diamonds.
Rock Cycle
- Like landforms, rocks don’t remain in their original form indefinitely. They're always in the
process of transformation.
