GEOL& 115
Geology of the National Parks
Yosemite National Park, CA
The Whole Earth and
Plate Tectonics
We need to understand
what takes place inside
Earth.
Classical Divisions of Earth
Chemical Composition vs. Physical Properties
Why does the Earth have
mountains and basins?
• Basic question asked by many: erosion
is evident everywhere, so why haven’t
all mountains eroded and filled in all
basins?
• Alfred Wegener proposed continental
drift in 1912.
Evidence for continental drift
• Continental “fit” – South America and
Africa separated by the Atlantic
• Similar rocks separated by the Atlantic
• Similar climate markers (glacial
striations) separated by the Atlantic
• Similar fossils separated by the Atlantic
Unifying theory of geology?
• Extension of Wegener’s continental
drift theory from the early 20th century
• Needed not only information about
rocks but also breakthroughs in
geochronology and geophysics
• Continental drift failed to provide a
sufficient mechanism (tides and wind
were not enough); plate tectonics does
Towards a synthesis
• Harry Hess proposes sea-floor spreading (“History of Ocean Basins”,
Petrologic Studies: A Volume to Honor A.F. Buddington, 1962, pp.
599-620)
• Fred Vine and Drum Matthews show that magnetic anomalies on the
sea floor are symmetric and consistent with sea-floor spreading
(“Magnetic Anomalies over Oceanic Ridges”, Nature, 1963, pp. 947949)
• Dan McKenzie and Robert Parker generate a map that shows plate
motion on a sphere can be explained by sea-floor spreading (“The
North Pacific: An Example of Tectonics on a Sphere”, Nature, 1967,
pp. 1276-1280)
• Jason Morgan demonstrates how features such as faults, trenches and
ocean rises may occur as a result of plate motion (“Rises, Trenches,
Great Faults, and Crustal Blocks”, Journal of Geophysical Research,
1968, pp. 1959-1982)
Grander synthesis: the Wilson
(supercontinent) cycle
• J. Tuzo Wilson (“Did the Atlantic close and
then re-open?”, Nature, 1966, pp. 676681) suggested that plate tectonics allow
supercontinents to rift apart and reform
over and over on a roughly half billion year
cycle
• Evidence for this: evidence of pre-Pangea
supercontinents, such as Rodinia
Basic points of plate tectonics
1.Plates exist
2.Plates move at different rates and
directions
3.Plate boundaries are where current
geological phenomena occur
4.Plate interiors are tectonically quiet
Differentiated Earth
1. Iron Core
(outer core liquid)
(inner core solid)
2. Fe-Mg Silicate Mantle
3. Silicate Crust
(oceanic and continental)
4. Oceans
5. Atmosphere
The point is
that heat is
trying to
escape
*Compositional zonation
based on density.
Silicic
or
Mafic
Ultramafic
Interior of Earth by
BEHAVIOR
Differentiated Earth
1. Iron Core
(outer core liquid)
(inner core solid)
2. Fe-Mg Silicate Mantle
3. Silicate Crust
(oceanic and continental)
4. Oceans
5. Atmosphere
*Compositional zonation
based on density.
1. Tectonic plates exist
Crust: 2-70 km thick. Oceanic crust is
thinner (8-10km) and denser than
continental crust (35 km on average).
Broken up into many* tectonic plates.
Mantle: 2900 km thick. 80% of Earth’s
volume but only 67% of its mass. Solidish.
Core: Outer core 2200 km thick, liquid iron.
Inner core radius 1200 km, solid iron.
Crust vs. Mantle is a compositional
boundary. Both are made of silicates
(oxygen, silicon, various metals), but the
bulk chemistry is different.
Lithosphere vs. Asthenosphere is a
behavioral boundary. Lithos = rock,
asthenos = soft. Lithosphere is brittle (can
produce earthquakes) and asthenosphere is
ductile (bends instead of breaking).
Tectonic plates are LITHOSPHERE.
2. Plates move
Convection in the liquid outer core
produces the magnetic field.
Convection in the SOLID(ish) mantle
moves the tectonic plates (pieces of
lithosphere) around on the surface and is
responsible for most geologic activity, such
as volcanoes, earthquakes, and the like.
The Layered Earth
• Partially-molten (nearly completely solid) asthenosphere
imparts motion to the lithosphere.
• This convection drives plate tectonics.
Plate Boundaries
Divergent: Plates move apart, new
oceanic crust is formed in between.
Convergent: Plates move together and
either collide (continental-continental) or
one is subducted (oceanic-continental or
oceanic-oceanic). Continents stay on top.
Transform: Plates slide past each other.
3. Plate boundaries have
geologic phenomena
1)
Type of plate boundary:
• Divergent
• Convergent
• Transform
• (Hotspot)
Two Important Factors in Understanding the
Landscapes of National Parks
1)
Type of plate boundary:
• Divergent
• Convergent
• Transform
• (Hotspot)
2)
Type of crust:
•
Continental (thick; more buoyant)
•
Oceanic (thin; less buoyant)
Types of Crust
• Continental Crust
– 20 to 70 km thick
– Silicic – composed of a variety of rocks, mostly enriched in
minerals with high abundances of silicon and oxygen
• Also rich in of aluminum, calcium, sodium, and potassium
– Low density, so “floats” high on the underlying mantle
– Can be quite old, up to 4 Ga
Types of Crust
• Oceanic Crust
– 7 km (4 miles) thick, on average
– Denser than continental crust and thus less buoyant
– Mafic
• Rich in iron and magnesium silicates
– Young, less than 200 Ma