Lecture 6: Solid Earth (Outline)

Climate Role of the Solid Earth

Internal Structure of the Solid Earth

Theory of Plate Tectonics

History of Plate Tectonics

Tectonic-Scale Climate Change

Why was Earth ice-free even at the poles 100 Myr ago (the Mesozoic Era)?

What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?

Plate Tectonics

Continental Drifting Theory

Alfred Wegener proposed that all the continents were once assembled into a supercontinent (Pangea) and then broke and slowly drifted to their current positions.

Plate Tectonics

The branch of tectonics that deals with the processes by which the lithosphere plates move and interact with each other is called plate tectonics.

Circulation of the Solid Earth

Twenty Rigid Plates

What can happen to the cold boundary?

The lithosphere has broken into a number of rocky pieces, called plates.

There are six large plates plus a number of smaller one comprise the Earth’s surface (a total of 20 plates).

The plates range from several hundred to several thousand kilometers in width.

Continental and Oceanic Crusts

Some liothspheric plates are composed primarily of oceanic crustal material, whereas others are composed primarily of continental materials.

The continents stand, on average, about 4.5 km above the floor of the ocean basins.

Continental crust is relatively light (density 2.7 g/cm3), whereas oceanic crust is relatively heavy (density close to 3.2 g/cm3).

Temperature Structures in the Solid Earth

Heat In Earth’s Interior

There are two major sources of the heat in Earth’s interior:

(1) Radioactive decay: of potassium, uranium, and thorium.

(2) Residual heat from Earth’s formation: A tremendous amount of energy was transferred to Earth during the accretion of the planet by collisions with planetesimals.

Internal Structure of the Earth

The Crust

     It is inhomogeneous in both thickness and compositions.

The Mantle

     It is uniform in composition and formed of silicate materials

The Core

      * It is dominated by iron, along with small amounts of nickel.

      * It is the source of Earth’s magnetic field.

      * The outer core is in fluid form and the inner core is in solid form.

The Theory of Plate Tectonics

A major problem of the continent drifting theory is: How could the continents drift through the rigid sea floor?

This problem is answered by the seafloor spreading hypothesis: Continents do not plow through the sea floor. Continents and segments of ocean floor are connected into plates that continuously move away from one another at mid-ocean ridges.

Seafloor Spreading

Plate Margins

Interactions between plates occur along their edges. There are three types of plate margins:

Divergent margins

       form mid-ocean ridges (over oceans) and rift valleys (over lands)

(2) Convergent margins

       form deep-sea trenches (two oceanic plates or ocean+continental plates) or high mountains (such as Tibetan Plateau) (two continental plates).

Transform fault margins

       form earthquake faults

Chemical Weathering

The precipitation process in the atmosphere dissolve and remove CO₂ from the atmosphere.

Rocks exposed at Earth’s surface undergo chemical attack from this rain of dilute acid.

This whole process is known as chemical weathering.

The rate of chemical weathering tend to increase as temperature increases.

Weathering requires water as a medium both for the dissolution of minerals and for the transport of the dissolved materials to the ocean

è The rate of chemical weathering increases as precipitation increases.

Negative Feedback From Chemical Weathering

The chemical weathering works as a negative feedback that moderates long-term climate change.

This negative feedback mechanism links CO₂ level in the atmosphere to the temperature and precipitation of the atmosphere.

A warm and moist climate produces stronger chemical weathering to remove CO₂ out of the atmosphere è smaller greenhouse effect and colder climate.

Earth’s Thermostat – Chemical Weathering

Chemical weathering acts as Earth’s thermostat and regulate its long-term climate.

This thermostat mechanism lies in two facts:

(1) the average global rate of chemical weathering depends on the state of Earth’s climate,

(2) weathering also has the capacity to alter that state by regulating the rate which CO₂ is removed from the atmosphere.

Faint Young Sun Paradox

Solar luminosity was much weaker (~30%) in the early part of Earth’s history (a faint young Sun).

If Earth’s albedo and greenhouse effect remained unchanged at that time, Earth’s mean surface temperature would be well below the freezing point of water during a large portion of its 4.5 Byr history.

That would result in a “snowball” Earth, which was not evident in geologic record.

Tectonic-Scale Climate Change

The faint young Sun paradox and its possible explanation.

Why was Earth ice-free even at the poles 100 Myr ago (the Mesozoic Era)?

What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?

Tectonic Control of CO₂ Input – The Seafloor Spreading Rate Hypothesis

During active plate tectonic processes, carbon cycles constantly between Earth’s interior and its surface.

The carbon moves from deep rock reservoirs to the surface mainly as CO₂ gas associated with volcanic activity along the margins of Earth’s tectonic plates.

The centerpiece of the seafloor spreading hypothesis is the concept that changes in the rate of seafloor spreading over millions of years control the rate of delivery of CO₂ to the atmosphere from the large rock reservoir of carbon, with the resulting changes in atmospheric CO₂ concentrations controlling Earth’s climate.

Tectonic Control of CO₂ Removal – The Uplift Weathering Hypothesis

The uplifting weathering hypothesis asserts that the global mean rate of chemical weathering is heavily affected by the availability of fresh rock and mineral surfaces that the weathering process can attack.

This hypothesis suggests that tectonic uplifting enhances the exposure of freshly fragmented rock which is an important factor in the intensity of chemical weathering.

This hypothesis looks at chemical weathering as the active driver of climate change, rather than as a negative feedback that moderates climate changes.

Can These Two Hypotheses Explain Tectonic-Scale Climate Changes?

The Polar Position Hypothesis

The polar position hypothesis focused on latitudinal position as a cause of glaciation of continents.

This hypothesis suggested that ice sheets should appear on continents when they are located at polar or near-polar latitudes.

To explain the occurrence of icehouse intervals, this hypothesis calls not on worldwide climate changes but simply on the movements of continents on tectonic plates.

This hypothesis can not explain the climate of the Late Proterozoic Era, when both continents and glaciers appear to have been situated at relatively low latitudes.

It can not explain the warm Mesozoic Era when high-latitude continents were present but were almost completely ice-free.

Climate Changes in the Last 500 Myr

Climate in the past 500 million years have alternated between long periods of warm climate and short periods of cold climate.

During the last 500 million years, major continent-size ice sheets existed on Earth during three icehouse ear: (1) a brief interval near 430 Myr ago, (2) a much longer interval from 325 to 240 Myr ago, and (3) the current icehouse era of the last 35 million year.


	Climate Role of the Solid Earth
	Internal Structure of the Solid Earth
	Theory of Plate Tectonics
	History of Plate Tectonics


	Why was Earth ice-free even at the poles 100 Myr ago (the Mesozoic Era)?
	What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?


	Continental Drifting Theory
	Alfred Wegener proposed that all the continents were once assembled into a supercontinent (Pangea) and then broke and slowly drifted to their current positions.

	Plate Tectonics
	The branch of tectonics that deals with the processes by which the lithosphere plates move and interact with each other is called plate tectonics.


	What can happen to the cold boundary?
	The lithosphere has broken into a number of rocky pieces, called plates.
	There are six large plates plus a number of smaller one comprise the Earth’s surface (a total of 20 plates).
	The plates range from several hundred to several thousand kilometers in width.


	Some liothspheric plates are composed primarily of oceanic crustal material, whereas others are composed primarily of continental materials.
	The continents stand, on average, about 4.5 km above the floor of the ocean basins.
	Continental crust is relatively light (density 2.7 g/cm3), whereas oceanic crust is relatively heavy (density close to 3.2 g/cm3).


	There are two major sources of the heat in Earth’s interior:

	(1) Radioactive decay: of potassium, uranium, and thorium.

	(2) Residual heat from Earth’s formation: A tremendous amount of energy was transferred to Earth during the accretion of the planet by collisions with planetesimals.


	The Crust
	It is inhomogeneous in both thickness and compositions.
	The Mantle
	It is uniform in composition and formed of silicate materials
	The Core
	* It is dominated by iron, along with small amounts of nickel.
	* It is the source of Earth’s magnetic field.
	* The outer core is in fluid form and the inner core is in solid form.


	A major problem of the continent drifting theory is: How could the continents drift through the rigid sea floor?

	This problem is answered by the seafloor spreading hypothesis: Continents do not plow through the sea floor. Continents and segments of ocean floor are connected into plates that continuously move away from one another at mid-ocean ridges.


	Interactions between plates occur along their edges. There are three types of plate margins:
	Divergent margins
	form mid-ocean ridges (over oceans) and rift valleys (over lands)
	(2) Convergent margins
	form deep-sea trenches (two oceanic plates or ocean+continental plates) or high mountains (such as Tibetan Plateau) (two continental plates).
	Transform fault margins
	form earthquake faults


	The precipitation process in the atmosphere dissolve and remove CO₂ from the atmosphere.
	Rocks exposed at Earth’s surface undergo chemical attack from this rain of dilute acid.
	This whole process is known as chemical weathering.
	The rate of chemical weathering tend to increase as temperature increases.
	Weathering requires water as a medium both for the dissolution of minerals and for the transport of the dissolved materials to the ocean
	è The rate of chemical weathering increases as precipitation increases.


	The chemical weathering works as a negative feedback that moderates long-term climate change.
	This negative feedback mechanism links CO₂ level in the atmosphere to the temperature and precipitation of the atmosphere.
	A warm and moist climate produces stronger chemical weathering to remove CO₂ out of the atmosphere è smaller greenhouse effect and colder climate.


	Chemical weathering acts as Earth’s thermostat and regulate its long-term climate.
	This thermostat mechanism lies in two facts:
	(1) the average global rate of chemical weathering depends on the state of Earth’s climate,
	(2) weathering also has the capacity to alter that state by regulating the rate which CO₂ is removed from the atmosphere.


	Solar luminosity was much weaker (~30%) in the early part of Earth’s history (a faint young Sun).
	If Earth’s albedo and greenhouse effect remained unchanged at that time, Earth’s mean surface temperature would be well below the freezing point of water during a large portion of its 4.5 Byr history.
	That would result in a “snowball” Earth, which was not evident in geologic record.


	The faint young Sun paradox and its possible explanation.
	Why was Earth ice-free even at the poles 100 Myr ago (the Mesozoic Era)?
	What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?


	During active plate tectonic processes, carbon cycles constantly between Earth’s interior and its surface.
	The carbon moves from deep rock reservoirs to the surface mainly as CO₂ gas associated with volcanic activity along the margins of Earth’s tectonic plates.
	The centerpiece of the seafloor spreading hypothesis is the concept that changes in the rate of seafloor spreading over millions of years control the rate of delivery of CO₂ to the atmosphere from the large rock reservoir of carbon, with the resulting changes in atmospheric CO₂ concentrations controlling Earth’s climate.


	The uplifting weathering hypothesis asserts that the global mean rate of chemical weathering is heavily affected by the availability of fresh rock and mineral surfaces that the weathering process can attack.
	This hypothesis suggests that tectonic uplifting enhances the exposure of freshly fragmented rock which is an important factor in the intensity of chemical weathering.
	This hypothesis looks at chemical weathering as the active driver of climate change, rather than as a negative feedback that moderates climate changes.


	The polar position hypothesis focused on latitudinal position as a cause of glaciation of continents.
	This hypothesis suggested that ice sheets should appear on continents when they are located at polar or near-polar latitudes.
	To explain the occurrence of icehouse intervals, this hypothesis calls not on worldwide climate changes but simply on the movements of continents on tectonic plates.
	This hypothesis can not explain the climate of the Late Proterozoic Era, when both continents and glaciers appear to have been situated at relatively low latitudes.
	It can not explain the warm Mesozoic Era when high-latitude continents were present but were almost completely ice-free.


	Climate in the past 500 million years have alternated between long periods of warm climate and short periods of cold climate.
	During the last 500 million years, major continent-size ice sheets existed on Earth during three icehouse ear: (1) a brief interval near 430 Myr ago, (2) a much longer interval from 325 to 240 Myr ago, and (3) the current icehouse era of the last 35 million year.