Download Slide 1

Before Wegener
•
1620 – Sir Francis Bacon (in Novum Organum): match in shape of
coastlines of South America and Africa
•
1650 – Archbishop James Ussher: Bible based calculation of time of
Creation at sunset on the night before 10/23/4004 B.C.
•
1669 – Nicolaus Steno, orig. Niels Stensen, (in Prodromus): principle of
original horizontality: sedimentary layers originally form in horizontal
position; law of superposition: sedimentary layers will young upward unless
later disturbed
•
Late 1700s – James Hutton (a.k.a. “Father of Modern Geology”): wrote The
Theory of the Earth (1799), considered Earth as a heat engine, described
unconformities [Siccar Point], believed Earth is old (uniformitarianism)
•
1959 – Bruce Heezen and Marie Tharp (bathymetric map w/ midocean
ridges)
Plate Tectonics Theory
•
the Earth's rigid crust is divided into plates that float on and move over the
weaker, ductile upper mantle
•
plates separate at spreading centers, marked by midocean ridges and
continental rifts; hot molten material from the mantle creates new oceanic
crust as it fills the void and cools
•
plates slide past one another along transform faults
•
convergent boundaries are of three types:
– ocean-ocean subduction: deepsea trench and island arc
– ocean-continent subduction: trench and volcanic belt on land
– continent-continent collision: high mountains, wide plateau
•
plumes are narrow mantle upwellings that burn holes in the plates leading to
seismic and volcanic activity far from plate boundaries, though they are
found on plate boundaries as well (e.g. Iceland)
Plate boundaries
• divergent – extensional regimes where plates separate
at midocean ridges and new crust is created
• conservative – strike-slip regime where plates slide past
each other along transform faults
• convergent – compressional regime of two main types:
– subduction zone where a trench marks the location where one
plate dives beneath another
– continental collision zone with large mountain ranges
Crustal thickness
•
Types of seismic waves
•
Body waves:
– travel through the interior of the Earth.
– follow raypaths bent by the varying density and
– transmit the first-arriving tremors of an earthquake: primary (P-waves) and
secondary (S-waves).
•
P waves
– longitudinal (compressional)
– generally travel slightly less than twice as fast as S waves and can travel through
any type of material.
– Typical speeds are 1450 m/s in water and about 5000 m/s in granite.
– not as destructive as the S waves and surface waves that follow them.
•
S waves
– transverse (shear)
– speed is about 60% of that of P waves in a given material.
– have several times larger amplitude than the P waves
•
Surface waves
– travel more slowly than body waves.
– low frequency, long duration, and large amplitude, they can be the most destructive
type of seismic wave.
– two types of surface waves: Rayleigh waves and Love waves
– Love waves slightly faster
• http://sunshine.chpc.utah.edu/labs/seismic
/index.htm?ASPIRE_Session=c22ba996a
bd21f5c930f09a7c174a736
Seismograms
Traveltimes
1. P-wave is always the first
arrival
2. No surface waves
arrivals before ~10deg
(~1000km)
(important figure!)
Locating earthquakes
Triangulation:
- 3 stations
- at each station the difference
between the P and S-arrival is
different
- the farther from the source,
the larger the difference
Richter Scale