Post on 15-Jan-2016
transcript
Developing an Understanding of Plate Tectonics
The Inner Earth
• Layered– Beneath its familiar
surface & thin crust lie a rocky mantle & iron core
• Hot– Core = hotter than
surface of the Sun– Escape of inner heat to
cold outer space causes plates to move
The Inner Earth
• Flows & Churns– Flowing rocks in the
mantle help move plates above
– In outer core, churning dynamo of liquid iron generates Earth’s magnetic field
The Inner Earth
• First 100 million years…– Ever-larger particles in
new Solar System collided & stuck together creating lots of heat
– Earth gradually increased in size, melted completely & layers began to form
The Inner Earth
• Layers Forming– Dense molten iron sank
& created core– Lighter silicate liquid
rose & cooled, forming the mantle
– Later, partial melting of mantle produced the crust• Process continues today!
The Crust—Earth’s Thin Skin
Basalt vs. Granite RockBasalt Both Granite
• Extrusive—magma breaks through the crust of the earth and erupts on the surface (volcanic eruptions)
• “Blood of the earth”• Makes new seafloor crust• Because the magma comes
out of earth (and usually into water) it cools very quickly, and minerals have little opportunity to grow
• Fine grained—individual mineral grains are nearly impossible to see without magnification
• Generally dark in color, thin, & heavy
• High density due to amount of iron, magnesium and other heavy elements
• Igneous rock (cooled from magma)
• Large amounts of silicon & oxygen
• Most common rock on continental land masses
• Ex: Yosemite Valley & Mt. Rushmore
• Intrusive—magma was trapped deep in the crust & took a long time to cool and crystallize into solid rock
• Coarse-textured rock—individual mineral grains are easily visible
• Light compared to basalt• Accumulates into
continent-sized rafts which bob in “sea of basalt”
The Mantle—Deep & Dense
• ~84% of Earth’s volume
• Uppermost 100 km is rigid– Makes up
Lithosphere with crust (plates)
The Mantle—Deep & Dense
• Next layer makes up Asthenosphere (part of Upper Mantle, 100-400 km below surface)– Solid, hot, and soft– Flows like a glacier
• Lower Mantle– Extremely dense
The Core—Iron Center
• Outer Core is molten– Hot enough to be as fluid as water– Motions create Earth’s magnetic field
The Core—Iron Center
• Inner Core is solid metal– Immense pressure solid state
The Unreachable Frontier—Looming Questions
• In 1990, the world’s deepest drill hole penetrated to a depth of 12.3 km (7.6 mi) beneath Russia’s Kola Peninsula.
• More than 99% to center of Earth lay beneath the drill bit.– If the inner Earth is so remote AND inaccessible,
HOW can we possibly learn anything about it??
The Scientific Method?• Geologists do not apply the scientific method in
a traditional sense.– Many traditional scientific hypotheses are tested by
controlled lab experiments that take place over a short period of time (minutes to a few years)
– For geology, there are too many variables• Too large• Very SLOW processes
– Experimenter must outlive the experiment and geologists cannot reasonably run experiments that precisely duplicate processes that are require centuries or millions of years in nature!
The Scientific Method?• Earth is the laboratory for many geoscience research
objectives.• Geologists assess critical questions through careful
observation.– Observing or measuring features in rocks or landscapes that the
hypothesis predicts should be present– Analytical laboratories measure chemical element abundance
in rocks including density and magnetic characteristics– Other experiments simulate elevated temperature and
pressure within Earth’s interior– Computer simulations can “speed up” time to reproduce Earth
processes and variables can be changed one at a time to examine their effects on outcomes.• Constrained by necessary assumptions or simplifications
How Do We Know About the Core—The Unreachable Frontier?
• Geologist gather clues from– Meteorites– Rocks – Diamonds– Earthquake waves– Earth’s magnetic field
A flash of light in the midnight sky announces the arrival of another messenger from space: a meteorite. These extraterrestrial rocks testify to the origin of the Solar System and of the inner Earth. Asteroids — the parent bodies of most meteorites — originated at the same time as the Sun, Earth, and other planets. When fragments of asteroids land here as meteorites, we glimpse the raw materials that formed our planet and the secrets of its core.
Clues from Rocks
• Learn about the inner Earth from special rocks called peridotites—from the upper mantle.– Peridotites record geological processes that take
place in this inaccessible layer of Earth.
• Chondrites—meteorites composed of Solar System’s original dust– 7 elements make up 97% of crust– But, compared to chondrites, Earth’s crust & mantle
are poor in iron– Deduce that iron must be concentrated in core.
• Meteorites from same Asteroid (parent material)
• Earth’s rocky mantle & iron core may be separated by a similar hybrid zone.
• Iron meteorite came from asteroid’s core (large crystals of metal)
• When magma rises rapidly to the surface, dense chunks of upper mantle can “hitch a ride”.
• Called xenoliths or “stranger rocks”
Clues from Diamonds
Clues from Diamonds
• Require tremendous temperature & pressure– Hardest material known
• Occur at depths greater than 150km– Come to surface with rapidly rising magma– Kimberlite & lamproite (partial melting of upper
mantle)
Clues from Earthquakes
• Earthquakes release energy that races through planet as seismic waves.– Material type changes speed of waves– Arrival times of different waves around world
provide clues to composition of inner Earth
Strong ground motion associated with the 1964 Alaska earthquake caused the sliding that destroyed these homes.
Clues from Earth’s Magnetic Field
• Like a bar magnet, Earth has a magnetic field with 2 main poles
• Generated by motions within the outer core
• Dynamic electromagnet– Planet’s rotation causes molten
iron-nickel in outer core to circulate– Creates electrical currents &
magnetic field
Principles, Laws, & Theories
• Principles (or laws) are generalizations about how nature is observed to work– Example: Seventeenth century astronomer
Johannes Kepler stated the first law of planetary motion, which states that planets orbit the Sun along an elliptical orbit• Law derives from many undisputed observations but
does not offer an explanation for why planets follow elliptical paths around the Sun.
Principles, Laws, & Theories
• Theories offer well-tested and accepted explanations, not offhand hunches, or why the natural system works this way.– Example: The theory describing gravity forces
between objects offers an explanation for Kepler’s First Law of Planetary Motion
Principle of Uniformitarianism
• Geologic processes and natural laws now operating on and within Earth have acted throughout geologic time; the logic and method by which geologists reconstruct past events.– James Hutton & Charles Lyell published Principles
of Geology in 3 volumes 1830-1835
Applying Uniformitarianism
• Modern Beach & Ancient Rock
• Similarities of modern & ancient features indicate that the ancient rock formed in an environment similar to a beach– Geologists interpret features
preserved in ancient rocks in terms of observed processes
Modern View of Uniformitarianism
• Should rate of processes be uniform through time and limited to the values measured during the history of geologic study?
• Is it possible for processes to have been active in the past that humans have not witnessed or for such processes to have operated at different rates?
Modern View of Uniformitarianism• Many geologic processes occur episodically (e.g. collisions of
meteors and comets with Earth)• Or, over a wide range of scale (e.g., volcanic eruptions, floods)• Geologic observations have only been made over 2 centuries
for 5 millennia of recorded history– If primordial Earth was hotter, could processes driven by thermal
energy have taken place at faster rates?– Would not rates of erosion have been higher before the appearance
of rooted plants on land beginning 400 mya?• Cannot place too many restrictions on applying
uniformitarianism, especially with regard to the rates & conditions of processes that might change over time.
1964 Earthquake—Alaska
Theory of Plate Tectonics
• Theory that Earth’s outer shell (lithosphere) is not seamlessly continuous but is broken into discrete pieces that move slowly relative to one another and change in size over geologic time.
• Most important geologic theory• From 1960s
Plate
• One of several discrete, rigid to semi-rigid, roughly 100-km-thick slabs that make up Earth’s lithosphere 7 Major Plates
• Pacific• North American• South American• African• Eurasian• Indian-Australian• Antarctic
Plate Boundaries
• Motion between adjacent plates describes the type of boundary between plates
Types of Plate Boundaries
• Tectonics—study of the causes of rock deformation
• 3 Basic types of conceivable relative motions between two plates at mutual boundaries1. Divergent plate boundaries2. Convergent plate boundaries3. Transform plate boundaries
Divergent Plate Boundaries
• Linear or curving zones where plates move apart from one another and new lithosphere forms
East Africa Rift Valley
• Interactive Plate Boundary Map
Convergent Plate Boundary
• Curving zone where plates collide nearly head-on into one another, compressing the lithosphere and causing subduction of one plate beneath the other
• Subduction—the process by which a lithospheric plate descends beneath a neighboring plate
Transform Plate Boundary
• Zones where lithospheric plates slide past one another with neither creation nor destruction of lithosphere
What next?
• By understanding Plate Tectonics, we can better understand other geologic processes, such as earthquakes and volcanoes. Studying these processes continues to feed our understanding of Plate Tectonics.