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!

• Animations

Animation

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

Pangaea

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

• Animation

• Animation

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

• Animation

• Animation

• Animation

Transform Plate Boundary

• Zones where lithospheric plates slide past one another with neither creation nor destruction of lithosphere

• Animation

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.