Earth Structure
and Composition
Layers, Rocks, Minerals and the Rock Cycle
The Layers of the Earth
The Earth is an oblate spheroid – the Solid Earth.
It is composed of a number of different layers as determined
by deep drilling and seismic evidence.
The Four Basic Layers



The crust is the layer that you live on, and it is the most widely
studied and understood.
The mantle is much hotter, has the largest mass, and several layers
(uppermost/rigid mantle, asthenosphere, lower mantle).
The outer core and inner core are even hotter with pressures so
great you would be squeezed into a ball smaller than a marble if you
were able to go to the center of the Earth!
A More Detailed View of Earth’s Structure
The Crust
The Earth's Crust is
like the skin of an
apple. It is very thin in
comparison to the
other three layers.
The crust is only
about 3 - 5 miles thick
under the oceans
(oceanic crust) and
about
25 miles thick under
the continents
(continental crust).
The Crust
The crust is composed of two different rocks.
The continental crust is mostly granite.
The oceanic crust is basalt.
Basalt is much denser than granite. Because of this the less dense
continents ride on the denser oceanic plates.
The Mantle
The Mantle is the largest layer of the Earth (1800 miles thick – 2/3 of
earth’s mass), it is hot (5100 - 3300◦ F), and the source of most magma →
(lava)
The uppermost part of
the mantle is rigid,
and together with
the crust, forms the
Lithosphere
The middle part of the
upper mantle is
composed of very
hot dense rock that
flows like asphalt,
and it is called –
asthenosphere
The lower mantle is
hot and dense.
The Lithosphere
The crust and the uppermost layer of
the mantle together make up a zone of rigid,
brittle rock called the Lithosphere.
The Lithospheric Plates
The crust of the Earth (which is part of the
lithosphere) is broken into many pieces called
lithospheric/crustal plates. The plates "float" on
the soft, semi-rigid or plastic asthenosphere.
The Asthenosphere
The asthenosphere is the semi-rigid part
of the upper mantle that flows like hot
asphalt under a heavy weight.
Convection Currents
The asthenosphere
"flows" because of
convection currents.
Convection currents
are caused by the very
hot material at the
deepest part of the
mantle rising, then cooling
and sinking again –
repeating this cycle over
and over.
When the convection currents flow in the asthenosphere
they also move the lithospheric/crustal plates.
The core of the Earth has a radius of 2100 miles and contains 1/3
of Earth’s mass.
 It is like a ball of very hot metals, with estimated temperatures of
12,400˚F at the center, and 8,600˚F at the outer limits.

The Outer Core
The outer core (1400 miles
thick) is so hot that metals in it
are in liquid state. It is
composed of mainly melted
nickel and iron → Earth’s
magnetic sphere is largely
related to this outer core
The Inner Core
The inner core of the Earth, about 700 miles thick, has
temperatures and pressures so great that the metals are
squeezed together and are not able to move about like a liquid,
but are forced to vibrate in place like a solid.
Core to Crust
Earth Structure: Established Relationships
 Density of materials (rocks, minerals) greatest in the
center (core), and decreases towards the surface
 Gravitational force strongest at the center, decreasing
towards the surface
 Temperature also decreases from center to the periphery
 Pressure decreases from center to periphery as well
 Layers in earth’s internal structure are differentiated by
composition, density, temperature and other characteristics
 Earth’s interior is an immense reservoir of minerals and
geothermal energy
Questions
1. What are the different layers in
Earth’s Structure that are
indicated on this diagram?
2. Which layers together
constitute the Lithosphere?
3. Which layer is characterized by
convection currents?
4. What layers are solid? Which
ones are liquid and/or plastic?
5. Have we ever seen part of the
Mantle? Explain.
Detailed View of Earth’s Structure
Rocks, Minerals
and the Rock Cycle
WHAT YOU LEARN
• To distinguish between minerals and rocks
• How rocks are classified
• How rocks are constantly recycled providing
materials for other rocks
raw
EARTH’S CRUST – Rocks and Minerals
 The earth’s crust is composed of rocks.
 Rocks are primarily composed of minerals (but may also
contain organic materials).
 The granite and basalt rocks of the continental and oceanic
crusts were the original, igneous rocks.
What is a mineral?
•
•
•
•
•
A naturally occurring inorganic substance
With a definite chemical composition and atomic structure
Crystalline in nature, displaying consistent geometric shapes
Commonly grouped into: Silicates, Oxides and Carbonates
Also, metallic and non-metallic minerals
What is a Rock?
 A rock is an aggregate of mineral particles – but may
also contain organic materials
 So, minerals are essentially the building blocks of rocks
 Bedrock → Outcrop → Regolith → Soils
Rock Classification
• Igneous Rocks
form due to the cooling and
crystallization of magma
• Sedimentary Rocks
form through lithification of
sediments from other rocks
• Metamorphic Rocks
form via recrystallization of
other rocks due to heat,
pressure, and chemical
alteration
IGNEOUS ROCKS
Igneous Rocks Are Subdivided into Two Classes:

Volcanic (Extrusive) Igneous Rocks
 Volcanic extrusive igneous rocks form on earth's surface as lava cools
 Basalt
 Basalt is the most widespread volcanic rock
 It is a dark, fine-grained rock
 Basalt is the rock of the sea floor
 Plutonic (or Intrusive) Igneous Rocks
 Plutonic igneous rocks form deep underground where magma cools
slowly; these rocks have a coarse crystalline texture
 Granite
 Granite is the most widespread of plutonic igneous rocks.
It underlies much of the continental crust.
Igneous Rocks
SEDIMENTARY ROCKS
Most sedimentary rocks are formed of layers of materials
that have washed into lakes, rivers and oceans –
• Sedimentary rocks form strata
• Often layers are tilted by earth movements
• Sedimentary rocks contain fossils
How do sediments turn into hard rock?
 Through Lithification Processes:
 Compaction
 Cementation
 Crystallization
Subclasses of Sedimentary Rocks:
1. Clastic: form from bits and pieces of other rocks
2. Chemical: consist of minerals deposited from a solution
3. Organic: consist of organic matter such as plants and
animal remains
Organically-formed sedimentary rocks form from
the remains of plants and animals (fossil
limestone, coal)
Sedimentary Rocks
METAMORPHIC ROCKS
Metamorphic rocks are rocks that have been changed in form due to
heat, pressure, and chemical alteration.
FOLIATED
Slate
Schist
Gneiss
NONFOLIATED
Marble
Quartzite
Slate: forms when shale is compressed by heat and pressure; splits easily
Schist: dominated by platy or needle-like minerals that form shiny layers
Gneiss: under pressure the minerals in granite recrystallize to form bands of
light and dark minerals
Marble: Limestone recrystallizes into marble – a denser and more
crystalline form of calcite
Quartzite: Sandstone changes into quartzite; Sand grains recrystallize to
form a hard mass of quartz
Metamorphic Rocks
HOW ROCKS RECYCLE ?
 The rock cycle is a general model that describes how
various geological processes create, modify, and
influence rocks
 The origin of all rocks can be ultimately traced back to the
solidification of molten magma
 Magma consists of a partially melted mixture of elements and
compounds commonly found in rocks
 Magma exists just beneath the solid crust of the Earth in an
interior zone, the mantle
 The Rock Cycle shows how rocks of any rock class can be
recycled into rocks of any other rock class.
The Rock Cycle
Stages in the Rock Cycle
 All rock types physically and chemically decomposed by a variety of
surface processes collectively known as weathering
 The debris thus created often transported by erosional processes via
streams, glaciers, wind, and gravity
 When this debris is deposited as permanent sediment, the processes of
burial, compression, and chemical alteration over long periods of time
produce sedimentary rocks
 Geologic processes like tectonic folding and faulting exert heat and
pressure on both igneous and sedimentary rocks, altering them physically or
chemically – rocks modified in this way are termed metamorphic rocks
 Any of the rock types can eventually be returned to Earth's interior by
tectonic forces at areas known as subduction zones
 Once in Earth's interior, extreme pressures and temperatures melt the rock
back into magma to begin the rock cycle again