Ch 8 4 Earth_s Layered Structure

Warm Up 10/21(22)
When an earthquake occurs, energy radiates in all
directions from its source, which is called the ____.
a. fault
c. seismic center
b. epicenter
d. focus
The hypothesis that explains the release of energy during
an earthquake is called the ____.
a. moment magnitude hypothesis c. Richter hypothesis
b. elastic rebound hypothesis
d. vibration
Most earthquakes are produced by the rapid release of
which kind of energy stored in rock subjected to great
a. thermal
c. chemical
b. elastic
d. Mechanical
1. d, 2. b, 3. b
Earth’s Layered
Chapter 8, Section 4
Layers Defined by Composition
If Earth were made up of the same
materials throughout its interior, seismic
waves would travel in straight lines at
constant speeds
Seismic waves reaching seismographs
located farther from an earthquake travel
at faster average speeds, due to increased
pressure at depth
Earth’s interior consists of three major
zones defined by its chemical composition
– the crust, mantle, and core
Paths of Seismic Rays
The Crust
Crust – thin, rocky, outer layer of Earth;
divided into oceanic and continental crust
The oceanic crust is roughly 7 kilometers
thick and composed of the igneous rocks
basalt and gabbro
The continental crust is 8-75 kilometers
thick (avg. 40 km) and consists mostly of
the granitic rock granodiorite
The rocks of the oceanic crust are younger
then the rocks of the continental crust
Mantle and Core
Mantle – a solid, rocky shell that
extends to a depth of 2890 km
Over 82% of Earth’s mass is
contained in the mantle
The boundary between the crust and
mantle represents a chemical
composition change
Core – a sphere composed of an
iron-nickel alloy
Concept Check
What is the composition of the core?
An iron-nickel alloy
Layers Defined by Physical
Earth’s interior has a gradual increase in
temperature, pressure, and density with
Depending on the physical environment
(temperature and pressure), a material
may behave like a brittle solid, a putty, or
a liquid
Earth can be divided into layers based on
physical properties – the lithosphere,
asthenosphere, outer core, and inner core.
Lithosphere and Asthenosphere
Lithosphere – relatively cool, rigid
shell consisting of the crust and
upper most mantle
Averages about 100 kilometers in
Asthenosphere – soft,
comparatively weak layer beneath
the lithosphere
The rocks within the asthenosphere
are close enough to their melting
points that they are easily deformed
Lower Mantle
From a depth of 660 km down to
near the base of the mantle
More rigid layer that is still prone to
gradual flow
At the bottom of the mantle, rocks
behave more like those in the
Inner and Outer Core
The core, which is composed of an
iron-nickel alloy, is divided into two
regions with different physical
Outer Core – liquid layer 2260 km
thick, flow produces Earth’s magnetic
Inner Core – sphere with a radius
of 1220 km, compressed into a solid
state by immense pressure
Earth’s Layered Structure
Concept Check
Why is the inner core solid?
It is under extreme pressure and is
compressed into a solid as a result
Discovering Earth’s Layers
Moho – boundary separating the crust from
the mantle where the velocity of seismic waves
abruptly increases
Seismic waves from even small earthquakes
can travel around the world
The outer core causes P waves that travel
through it to arrive several minutes after
expected and are bent by 100o away from the
earthquake (shadow zone)
It was further shown that S waves do not travel
through the outer core, making geologists
conclude that the outer core is composed of a
Concept Check
What is the Moho?
The boundary between the crust and
the mantle
Discovering Earth’s Composition
Early seismic data and drilling technology
indicate that the continental crust is
mostly made of lighter, granitic rocks
The crust of the ocean floor has a baslatic
The composition of the mantle and core is
known from more indirect data
Scientists use lava and meteorites to infer
the composition of the mantle and core
Earth’s core is thought to be mainly dense
iron and nickel, similar to metallic
meteorites. The surrounding mantle is
believed to be composed of rocks similar
to stony meteorites
Earth’s Interior Showing P and S
Wave Paths
Read Chapter 8, Section 4 (pg. 233-237)
Do 8.4 Assessment #1-6 (pg. 237)