Chapter 7
Plate Tectonics
Earthquakes
Earthquake = vibration of the Earth produced
by the rapid release of energy.
Seismic Waves
• Focus = the place within the Earth where
the rock breaks, producing an earthquake.
• Epicenter = the point on the ground surface
directly above the focus.
• Energy moving outward from the focus of an
earthquake travels in the form of seismic
waves.
Types of Seismic Waves
1. Body waves
a. P-waves
b. S-waves
Types of Seismic Waves
1.
Body waves
a. P-waves
Primary, pressure, push-pull
Fastest seismic wave
(6 km/sec in crust; 8 km/sec in
uppermost mantle)
Travel through solids and liquids
b. S-waves
Types of Seismic Waves
1. Body waves
a. P-waves
b. S-waves
Secondary, shaking, shear, side-to-side
Slower
(3.5 km/sec in crust; 5 km/sec in upper
mantle km/sec)
Travel through solids only
Types of Seismic Waves
Body waves
a. P-waves
b. S-waves
2. Surface waves
Love and Rayleigh waves
Slowest
Complex motion –
Up-and-down and side-to-side
Causes damage to structures during an
earthquake
1.
Seismogram showing
Seismic Wave Arrivals
Seismographs
• Earthquakes are recorded on an instrument
called a seismograph.
• The record of the earthquake produced by
the seismograph is called a seismogram.
Determining the Earth's
Internal Structure
Earth has a layered structure.
Boundaries between the layers are called
discontinuities.
– Mohorovičić discontinuity (Moho)
between crust and mantle
– Gutenberg discontinuity
between mantle and core
Determining the Earth's
Internal Structure
The layered structure is determined from
studies of how seismic waves behave as
they pass through the Earth.
P- and S-wave travel times depend on
properties of rock materials through which
they pass.
Differences in travel times correspond to
differences in rock properties.
Determining the Earth's
Internal Structure
• Seismic wave velocity depends on the
density and elasticity of rock.
• Seismic waves travel faster in denser rock.
• Speed of seismic waves increases with
depth (pressure and density increase
downward).
Determining the Earth's
Internal Structure
Curved wave paths
indicate gradual increases
in density and seismic
wave velocity with depth.
Refraction (bending of
waves) occurs at
discontinuities between
layers.
S-wave Shadow Zone
Place where no Swaves are received by
seismograph. Extends
across the globe on
side opposite from the
epicenter.
S-waves cannot travel
through the molten
(liquid) outer core.
Larger than the P-wave
shadow zone.
P-wave Shadow Zone
Place where no P-waves
are received by
seismographs.
Makes a ring around the
globe.
Smaller than the S-wave
shadow zone.
The Earth's Internal Layered Structure
•
•
•
•
Crust
Mantle
Outer core
Inner core
Crust
• Continental Crust (granitic)
• Oceanic Crust (basaltic)
Basaltic crustal rocks are more dense than granitic crustal rocks. The Mohorovicic (Moho) discontinuity,
determined by seismic reflection is the boundary between the crust and upper mantle.
Oceanic Crust
•
•
•
•
Basaltic composition
5 - 12 km thick
More dense (about 3.0 g/cm3)
Has layered structure consisting of:
– Thin layer of unconsolidated sediment covers
basaltic igneous rock (about 200 m thick)
– Pillow basalts - basalts that erupted under water
(about 2 km thick)
– Gabbro - coarse grained equivalent of basalt;
cooled slowly (about 6 km thick)
Lithosphere
Lithosphere = outermost 100 km of Earth.
Consists of the crust plus the outermost part
of the mantle.
Divided into tectonic or lithospheric plates that
cover surface of Earth
Asthenosphere
• Asthenosphere = low velocity zone at
100-250 km depth in Earth (seismic wave
velocity decreases).
• Rocks are at or near melting point.
• Magmas generated here.
• Solid that flows (rheid); plastic behavior.
• Convection in this layer moves tectonic
plates.
Isostasy
• Buoyancy and floating of the Earth's crust on
the mantle.
• Denser oceanic crust floats lower, forming
ocean basins.
• Less dense continental crust floats higher,
forming continents.
• As erosion removes part of the crust, it rises
isostatically to a new level.
Isostasy is Isostatic adjustment to erosion and gravity.
The Earth's
Internal
Layered
Structure
Mantle
• Composed of oxygen and silicon, along with iron and
magnesium (based on rock brought up by volcanoes,
density calculations, and composition of stony
meteorites).
– Peridotite (Mg Fe silicates, olivine)
– Kimberlite (contains diamonds)
– Eclogite
• 2885 km thick
• Average density = 4.5 g/cm3
• Not uniform. Several concentric layers with differing
properties.
Core
• Outer core
– Molten Fe (85%) with some Ni. May contain lighter
elements such as Si, S, C, or O.
– 2250 km thick
– Liquid. S-waves do not pass through outer core.
• Inner core
– Solid Fe (85%) with some Ni
– 1220 km radius (slightly larger than the Moon)
– Solid
Core and Magnetic Field
• Convection in liquid outer core plus spin of
solid inner core generates Earth's magnetic
field.
• Magnetic field is also evidence for a
dominantly iron core.
Crustal Structures - Faults
• A fault is a crack in the Earth's crust along
which movement has occurred.
• Types of faults:
– Dip-slip faults - movement is vertical
• Normal faults
• Reverse faults and thrust faults
– Strike-slip faults or lateral faults - movement
is horizontal.
Faults
Fault terminology
Crustal Structures - Folds
• During mountain building or compressional
stress, rocks may deform plastically to
produce folds.
• Types of folds
– Anticline
– Syncline
– Monocline
– Dome
– Basin
Folds
A.
B.
C.
D.
E.
Anticline
Syncline
Monocline
Dome
Basin
Anticline
Aerial view of an anticline
Syncline
Folded strata, Switzerland
Strike and dip
Measuring strike and dip with a Brunton compass
Plate Tectonics
Plate Tectonic theory was proposed in late
1960s and early 1970s. It is a unifying theory
showing how a large number of diverse,
seemingly-unrelated geologic facts are
interrelated.
An outgrowth of the old theory of
"continental drift," supported by much data
from many areas of geology.
The Data Behind Plate Tectonics
Geophysical data collected after World War II provided
foundation for scientific breakthrough:
• Echo sounding for sea floor mapping discovered
patterns of midocean ridges and deep sea trenches.
• Magnetometers charted the Earth's magnetic field
over large areas of the sea floor.
• Global network of seismometers (established to
monitor atomic explosions) provided information on
worldwide earthquake patterns.