Dynamic Earth
Class 10
9 February 2006
Video:
Continental Drift: Legacy of Fire
Any Questions?
Homework, Chapter 3

What is the association between tectonic plate
boundaries and volcanism? Can the eruptive style and
the chemical composition of the volcanic deposits or the
rock type be correlated with the type of plate boundaries
(convergent versus divergent)?
More than 90% of the world's volcanic activity occurs at
plate boundaries, and the type of activity can be related to
the type of boundary. Free-flowing basaltic lavas appear
at divergent boundaries; andesitic and rhyolitic lavas
generally erupt violently at convergent boundaries.
Homework, Chapter 3

What kinds of forces result in earthquakeproducing faults occur at the three types of plate
boundaries?
Earthquakes that occur along mid-oceanic ridges result
from tension; those along transform boundaries are
produced by shearing; and those along convergent
boundaries are caused by compression.
Homework, Chapter 3

At what depth do earthquakes form at the three
types of plate boundaries?
Most major earthquakes occur along plate
boundaries. Shallow-focus earthquakes tend to
occur at divergent and transform boundaries.
Deep-focus earthquakes occur at convergent
boundaries. Several shallow-focus earthquakes
occur each year at locations that are not
associated with plate boundaries.
Homework, Chapter 3

At a place along a boundary fault between the Nazca
Plate and the South American Plate, the relative plate
motion is 8 cm/year. The last great earthquake, in
1880, showed a fault slip or displacement of 12 m.
When should local residence begin to worry about
another great earthquake?
With a relative motion of 80 mm per year, the total
motion will be about 8 m in 100 years (or 80 m in 1000
years). This is not that close to the 12 m of slip that
occurred in 1880 and more than 100 years have
passed since that time. They might be more
concerned in 2025 when the total motion may be
nearer 12 m.
Exam Review
First Exam – Tuesday February 14th
 Exam will be “fill in the blank” and
short answer questions
 Questions will come from Lectures
(including videos), reading and
homework

Uniformitarianism
The present is the key to the past
— James Hutton
Geologic processes that we see in
operation today have worked much the
same way over geologic time —
however, rates and intensity of
processes may have changed
Relative vs Absolute Age
Relative: Rock “A” is older (or younger)
than rock “B”
 Absolute = The exact age of a rock
(in years)
 Usually geologists first establish relative
ages then try to get absolute age dates
 Know the difference between absolute
and relative age and be able to give an
example

Radiometric Dating
Use of radioactive decay to determine
the age of a rock
 First proposed ~1896-1902

Radiometric Dating

Key principle: Half Life = time required for
1/2 of the nuclei in a sample to decay
Radioactivity and Absolute Time
Decay of
parent atoms
Growth of
daughter atoms
Radiometric Dating
Shows that the earth is much older than
people had previously suspected
 Earth formed about 4.6 billion years
ago

 But,
oldest rocks on earth are only about 4
billion years old
Relative vs Absolute Age
Usually geologists first establish
relative ages then try to get absolute
age dates
 Determining relative age relies on a
number of geologic principles that
were developed during the 17th to
early 19th centuries

Principle of Superposition
Sedimentary rocks are deposited in a
layer-cake fashion:
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one
above and younger than the one
below
Principle of Original Horizontality

Layers of sediment are deposited in a
nearly horizontal position
Principle of Cross-cutting
Relationships

Something (such as a dike or fault)
that cuts across a layer must be
younger than the layer
Magnetostratigraphy

An alternate method for absolute age
dating that works well with volcanic
rocks
Earth’s
Magnetic
Field
Magnetization
of
Magnetite
The Geologic time scale
Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
 Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages

The Geologic
Column
and
Time Scale
The first ~4 billion years of
Earth’s history was nearly
devoid of life – this time is
known as the Precambrian.
Age of Earth ~4.6 by
Age of oldest rocks ~ 4.0 by
After the Precambrian, life began to
develop very rapidly.
The Paleozoic lasted for 300 my.
Dinosaurs dominated the Mesozoic,
which lasted for almost 200 my.
After the dinosaurs died out,
mammals began to dominate in the
Cenozoic.
These are the main subdivisions
that you should know.
Plate Tectonics
Fundamental Concept and Unifying
Theory in Earth Science
 Idea is > 100 yrs old
 Acceptance only within the past 30
yrs

Plate Tectonics
How and why did it come about?
 During the 1800’s geologists
recognized many strange things that
seemed to imply that the continents
had once been together.

Early geologists saw
similarities between
the coasts of Africa
and South America.
This map from 1858
shows that Africa
and South America
can be fit together
very well.
If the northern
continents are fit
together, rock
units match very
well.
Again, a
nice match
if the
continents
are fit
together
Why was Continental Drift
not accepted?
Because it was difficult to understand how continents could move.
 What did we learn to make us
more willing to accept the idea
that the continents have moved?

Mid-ocean Ridges
Drilling into the seafloor provided more
evidence supporting sea-floor spreading


Drilling ship Glomar Challenger
Micropaleontology
of sediments
Dating of the
underlying lavas
Age of Seafloor Crust
Formation of
Magnetic
Anomalies
Water boiling
in a pan on
your
stove is an
example of
convection
Convection
Convection within the Earth
Transports or drives plate motion
Ridge push and slab pull
Gravity pushes plates away from uplifted MOR
and pulls the plates down into Earth’s interior at
subduction zones
Mosaic of Earth’s Plates
Rates of plate motion

Mostly obtained from magnetic
anomalies on seafloor

Fast spreading: 10 cm/year

Slow spreading: 3 cm/year
What do we find right at the spreading axis?
Black smoker:
Hydrogen sulfide
Giant tube worms
and clams live on
the Black smokers
How does it work?
 Crabs live on dead

worms, bacterial mats,
and “snow”
A complete complex
food chain is
established
Were submarine hot springs the
origin of life on Earth?
Maybe?
Seismology

Study of the propagation of
mechanical energy released by
earthquakes.

When energy is released in this
fashion, waves of motion (like the
effect of a pebble tossed into a pond)
are set up in the Earth.
Earthquakes

earthquake: movement of rock
bodies past other

fault: locus of the earthquake
movement

faults come at all scales, mm to
separation of lithospheric plates
(e.g., San Andreas).
Earthquake terms
focus: site of initial rupture
epicenter: point on surface above
the focus
Seismic Waves Radiate from the
Focus of an Earthquake
Know what is needed to locate an epicenter
Elastic Rebound Theory
Elastic Rebound Theory
Elastic Rebound Theory
Elastic Rebound Theory
What Causes Earthquakes?


Sudden release of energy accumulated in
deformed rocks
At shallow depths, rocks are brittle and
deform elastically
When subjected to sufficient stress, they
fracture forming a fault
 When the fault is locked, stress builds


When the friction along the two sides of a
fault is overcome, stress is released and an
earthquake results
Distribution of earthquakes

Not random

Focused in linear zones
Most earthquakes occur at
plate boundaries
Plates
 Rigid
Lithosphere with definite
boundaries
 Can
have both oceanic and
continental crust or just one
kind.
Note that
crust under
continents
is thicker
(~45 km)
than under
oceans
(~8 km).
Types of plate boundaries
Divergent:
mid-ocean ridges
 Convergent: collision zones
volcanic arcs


Strike-slip:
San Andreas Fault
(California)
Anatolian Fault
(Turkey)
Divergent
(Spreading)
Convergent
(Subduction
Zone)
Transform
Three Types of Plate Boundaries
Earthquakes Associated with Divergent and
Transform Margins
Normal Fault
Earthquakes in
subduction
zones
Convection within the Earth
Lithosphere is created at spreading
centers and destroyed at Trenches
(Subduction Zones)
Know mechanisms of plate transport
Subduction zones
Ocean-continent
convergence
Ocean-ocean
convergence
Continent-continent
collision
Growth of Continents



Addition of volcanic arc material to
continent (e.g. Andes, as on the video).
Collisions: Continents
Collisions: Exotic terranes
Elastic Rebound Theory
Works for 1964 Alaska and 2004 Sumatra
earthquakes
Tsunami
Series of very
long-wavelength
waves on the
ocean
/ “tidal wave”
=
Has nothing to do
with tides
Generation of a Tsunami
Tsunami waves
Very small out in the open ocean
 Amplitude of only ~ 1 meter
 Very long wavelengths (up to 100 km)
 Travel very fast (as much as 500 mph)

Tsunami waves
When waves reach shallow water, they
"feel" the shallow bottom, just like
ordinary waves, and they slow down (to
20-30 mph)
 Because of the massive energy, this
slowdown causes them to build up very
high (up to 50-100 m)

Runup

Maximum height above sea level
reached by a tsunami when it
reaches shore
Inundation

Horizontal distance from the normal
water's edge reached by a tsunami
Big Island Field Trip

Need deposit of $50 on or before
February 17th