Outline

• Introduction to solid Earth

• Evidence for plate tectonics

• Plate tectonic environments

• Climate and plate tectonics

Climate paradox? Geological evidence indicates that Paleozoic glaciations had a wide distribution, ranging from southern South America to India.

Figure 1. Distribution of Paleozoic glaciations.

Introduction to solid Earth

Solid Earth is composed of:
1. Lithosphere (100 km thick): behaves in a hard, rigid manner. Consists of continental and oceanic crust and upper mantle.
2. Asthenosphere (300 km thick): hot, slowly flowing layer of upper mantle
3. Mantle (2900 km thick)
4. Core (3500 km thick): composed of inner (solid) and outer (liquid)

Oceanic and continental crust differ in composition, age, and thickness. Oceanic crust is thinner (5-10 km) and younger than continental crust (30-70 km) and composed largely of basalt. Continental crust is composed mainly of granite.

Figure 2. The solid Earth.

Why do the continents rise above the ocean crust? Continental and oceanic crust are in isostatic equilibrium.

Figure 3. Evolution of a mountain and isostatic equilibrium (from .

Evolution of Plate Tectonic Theory
1. Alfred Wegener and continental drift: how did continents drift?
2. Harry Hess and sea-floor spreading: why wasn't Earth expanding?
3. John Wilson and plate tectonics

Direct evidence for plate tectonics:
1. Shape of continents

Figure 4. Reconstruction of Pangaea. Is it just a coincidence that Africa and South America see to fit together like pieces of a puzzle?

2. Earthquake activity at faults (see also animations of ruptures)
3. Depth of oceans

Figure 5. World bathymetry (from National Geophysical Data Center). Why are there immense mountain chains and deep trenches on the ocean floor?

Click on the pictures below to see close-up bathymetric maps of trenches and ridges in the ocean (from Virtual Vacationland)

4. Paleomagnetic patterns on ocean floor and ocean crust age. The Earth has a magnetic field, caused by the circulation of liquid iron in Earth's core. Today, because of the magnetic field, compasses point to the north.  However, in the past, the magnetic field has reversed so that a compass would have pointed to the south. Basalts act as paleo-compasses. Iron-rich components within the basalt align with the magnetic field.

Figure 6. Map of the ocean floor magnetism. Notice the symmetry of the magnetic pattern in the Atlantic Ocean. What does this mean?

Plate tectonic settings

Figure 7. Modern plate tectonic map.

Mantle convection

Figure 8. Mantle convection.

Divergent plate boundaries

example: mid-Atlantic Ridge

Figure 9. Evolution of an ocean basin.

Convergent plate boundaries

1. Continent-ocean collision- example: Nazca and South American Plates (Andes)
2. Ocean-ocean collision- example: Phillipine and Pacific Plates (Mariana Trench)
3. Continent-continent collision- example: India-Australian and Eurasian Plates (Himalayas)

Figure 10. Sketch of a convergent plate boundary.

Transform plate boundaries

Example: Pacific and North American plates. The Pacific Plate is moving northward relative to the North American Plate. The San Andreas Fault marks the junction between these plates. In 50 million years, western California will encounter the Aleutian Trench!

Figure 11. Putting it all together. Sketch of the tectonic settings in an ocean basin (e.g., the Pacific Ocean).

Figure 12. The Earth's major mountain chains. What do these mountain chains indicate about the Earth's tectonic past?

Plate tectonics and climate

View a quicktime movie of continental drift through geological time.

How does plate tectonics influence climate?
1. Location of continents
2. Mountain building- alters atmospheric flow
3. Open/close ocean gateways
4. Sea-level change- modifies ratio of land to ocean
5. Altering weathering rates- linked to concentration of CO₂ in atmosphere
6. Altering rates of outgassing- linked to concentration of CO₂ in atmosphere

BLAG hypothesis: Plate tectonics influence global climate by moderating atmospheric CO₂ concentrations

• A 1983 hypothesis, known as the BLAG hypothesis, proposes that climate change in the last several hundred million years have mainly been driven by CO2 input to the atmosphere and ocean by plate tectonic processes.
• See Fig. below CO2 is released into the atmosphere by volcanoes at subduction zones and hot spots and CO2 is released into the ocean by sea-floor spreading.
• The rate of sea-floor spreading controls the delivery of CO2 from rocks into the air which results in long-term climate change controls.
• Fast spreading = more CO2 input.
• BLAG calls on chemical weathering to be a negative feedback control.

Uplift weathering hypothesis: Uplift accelerates chemical weathering, drawing down CO₂, and cooling the global climate.

• The Uplift Weathering Hypothesis
  • In the late 1980s, Maureen Raymo and co-authors hypothesized that chemical weathering was an active control of climate change and not just a negative feedback to BLAG. .
  • The hypothesis asserts that the global rate of chemical silicate weathering is influenced by the availability of fresh surface rocks and minerals that chemical weathering can affect.
  • Mountains and plateaus have steep slopes where mass wasting occurs. Mass wasting includes rock slides and falls, flows of water-saturated debris, and anything else that can move rock. Every event exposes fresh rock underneath.
  • Earthquake frequency is also an important factor as they greatly disrupt rock formations.
  • Steep slopes are also focal point for intense precipitation on the upwind side. Heavy precipitation favors chemical weathering.
  • Mountain glaciers can pulverize rock underneath which increases surface area.
  • Subduction processes do not cause "sudden" changes in global high terrain but continent-continent collisions do (such as India colliding with Asia over the last 55 my.) Earth's climate has cooled over the past ~45 million years ago, roughly in assocation with some major moutain building and uplift of large tectonic block, including the Himalayas.