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Planet Earth
Section 1
Section 1: Earth’s Interior and Plate Tectonics
Preview
• Key Ideas
• Bellringer
• What is Earth’s Interior Like?
• Plate Tectonics
• Plate Boundaries
• Divergent Boundaries
Planet Earth
Section 1
Key Ideas
〉 How is Earth’s interior structured?
〉 How has the appearance of Earth changed over time?
〉 What geologic features are common near tectonic plate
boundaries?
Planet Earth
Section 1
Bellringer
A peach can be used as a model for some aspects of
Earth’s structure. Compare the drawing of the cross section
of the peach below with the cross section of Earth, and
answer the following questions.
Planet Earth
Section 1
Bellringer, continued
1. Describe the outer layer of the peach (the skin).
What aspect of Earth’s structure does the outer layer of
the peach represent?
2. The peach pulp is the next layer.
How would you describe it? What aspect of Earth’s
structure does the peach pulp represent?
3. The pit is the innermost part of the peach.
What is the pit like? What aspect of Earth’s structure
does the peach pit represent?
Planet Earth
Section 1
What is Earth’s Interior Like?
〉 How is Earth’s interior structured?
〉 Earth’s interior is made up of several distinct
compositional layers.
• crust: the thin and solid outermost layer of Earth above
the mantle
• mantle: the layer of rock between Earth’s crust and core
Planet Earth
Section 1
What is Earth’s Interior Like? continued
• Earth’s interior gets warmer with depth.
• Earth’s core is nearly as hot as the surface of the sun.
– Earth’s core is composed mainly of nickel and iron.
– The inner core is thought to be solid and metal.
– The outer core is thought to be made of liquid metal.
• core: the center part of the Earth below the mantle
Planet Earth
Section 1
Visual Concepts: Structure of the Earth
Planet Earth
Section 1
Visual Concept: Formation of Earth’s Crust,
Mantle, and Core
Planet Earth
Section 1
What is Earth’s Interior Like? continued
• Radioactivity contributes to high internal temperatures.
– The breakdown of radioactive isotopes uranium,
thorium and potassium give off energy that
contributes to Earth’s high internal temperatures.
– Temperatures in the mantle can reach more than
1250 °C.
– The core may reach temperatures above 5000 °C.
Planet Earth
Section 1
Plate Tectonics
〉 How has the appearance of Earth changed over time?
〉 Alfred Wegener hypothesized that all of the continents
might have been part of one landmass in the past before
they drifted apart.
– Wegener pieced the continents together like a puzzle
and called the supercontinent they formed Pangaea.
– Wegener found nearly identical fossils on widely
separate continents, which supported his idea.
Planet Earth
Section 1
Visual Concept: Continental Drift (Pangaea)
Planet Earth
Section 1
Plate Tectonics, continued
• Evidence for Wegener’s ideas came later.
– Wegener’s theory of continental drift was ignored
until structures discovered on the ocean floor
provided evidence for a mechanism for the
movement of continents.
– Symmetrical bands on either side of a mid-ocean
ridge indicate that the two sides of the ridge were
moving away from each other and new ocean floor
was rising up between them.
Planet Earth
Section 1
Plate Tectonics, continued
• Alignment of oceanic rocks supports the theory of
moving plates.
– Iron in molten rock aligns itself with Earth’s magnetic
field as it cools.
– The Earth’s magnetic field reverses polarity about
every 200,000 years.
– The process is recorded as magnetic bands in rock,
based on the age of the rock.
– Symmetrical bands on either side of the Mid Atlantic
Ridge suggest that the crust was moving away from
the ridge.
Planet Earth
Section 1
Plate Tectonics, continued
• Earth has plates that move over the mantle.
– The crust and upper portion of the mantle are divided
into about seven large pieces called tectonic plates.
• lithosphere: the solid outer layer of Earth that consists of
the crust and the rigid upper part of the mantle
• plate tectonics: the theory that explains how large pieces
of the lithosphere, called plates, move and change
shape
Planet Earth
Tectonic Plates
Section 1
Planet Earth
Section 1
Plate Tectonics, continued
• Scientists do not understand exactly what makes
tectonic plates move.
– One hypothesis suggests that plate movement results
from convection currents in the asthenosphere, the
hot, fluid portion of the mantle.
– Another hypothesis suggests that plate movement
results from the force of gravity acting on the plates.
Planet Earth
Section 1
Plate Boundaries
〉 What geologic features are common near tectonic plate
boundaries?
〉 Volcanoes and earthquakes most often occur where
tectonic plates come together. At plate boundaries, many
other dramatic features, such as mountains and rift
valleys, can also occur.
Planet Earth
Section 1
Plate Boundaries, continued
• Mid-oceanic ridges result from divergent boundaries.
– The border between two tectonic plates is called a
boundary.
– A divergent boundary is a place where two tectonic
plates are moving apart.
– New rock forms between divergent boundaries.
– Magma is liquid rock produced under Earth’s surface.
– A rift valley is a narrow valley that forms where
tectonic plates separate.
Planet Earth
Divergent Boundaries
Section 1
Planet Earth
Magma
Section 1
Planet Earth
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Plate Boundaries, continued
• Oceanic plates dive beneath continental plates at
convergent boundaries.
– Plates slide over each other at a convergent
boundary.
– The area where one plate slides over another is
called a subduction zone. Subduction zones produce
ocean trenches, mountains, and volcanoes.
• subduction: the process by which one lithospheric plate
moves beneath another as a result of tectonic forces
Planet Earth
Convergent Boundaries
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Planet Earth
Section 1
Visual Concept: Convergent Boundary
Planet Earth
Section 1
Visual Concept: Subduction Zone
Planet Earth
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Plate Boundaries, continued
• Subduction of ocean crust generates volcanoes.
– Chains of volcanoes form on the upper plate in a
subduction zone.
– These volcanoes can form far inland from their
associated oceanic trench.
• Islands may form where two oceanic plates meet.
– Magma rising to the surface may form an island arc.
Planet Earth
Section 1
Plate Boundaries, continued
• Colliding tectonic plates create mountains.
– When two plates collide, mountains are formed at the
boundary of the collision.
• Transform fault boundaries can crack Earth.
– Plate movement at transform fault boundaries is one
cause of earthquakes.
• fault: a break in a body of rock along which one block
slides relative to another