Lecture Outline

• I. The Rock Cycle
• II. Plate Tectonics
• III. Why do Plates Move?
• IV. Types of Plate Boundaries
• V. Evidence for Plate Tectonics
• VI. Putting it all together

I. The Rock Cycle and Distribution of Major Rock Types

Surface features and rock cycle on a planet with a single rigid shell would differ dramatically from those of the earth.

A) Rock "cycle" consist largely of

• Isolated volcanism in places where volcanoes punctured the crust
• Erosion of topographically high features,
• Deposition of erosional products in topographic lows or ocean basins
• Sediments would remain in flay-lying layers in the ocean basins (i.e. think of the slide showing flat layers in Grand Canyon
• Mountain building would not occur othern than volcanic piles.
• Landscape would eventually become mountainless and slightly above sea level.

B) Distribution of igneous, sedimentary, and metamorphic rocks in such a world would be as follows:

• Fresh igneous rocks would be isolated to volcanic piles
• Sedimentary rocks would be found primarily beneath water,
• Metamorphic rocks would be rare due to inability to raise these rocks to the surface from their hot, high pressure environments deep beneath the surface.

In contrast, our planet has geologic sections such as the following:

SLIDE OF FAULTED, UPLIFTED, AND HIGHLY TILTED SEDIMENTARY ROCKS

Outcrops such as this should convince you that the earth's outer surface is a geologically dynamic place. On Earth, where significant horizontal and vertical motions of the surface occur, the rock cycle is more complex and includes the following elements:

A) Weathering can reduce any rock type to sediment and dissolved matter, which can in turn be transported by water, ice, wind, or gravity to sites of deposition, where the sediments are buried and lithified.

B) Deep burial can expose the lithified sediments to high enough temperatures or pressures can cause them to metamorphose to another rock type that is more stable at that particular temperature/pressure regime.

C) Uplift can expose buried rocks of any type to erosion, or alternatively, any of the rock types can thrust into the mantle to begin their life anew as an igneous rock.

Distribution of Major Rock Types

IGNEOUS - Basalts and gabbros, the most abundant rocks in the earth's outer shell are found mainly in the oceans. Granites and other related rocks are restricted almost exclusively to the continents.

SEDIMENTARY - Most old sedimentary rocks (i.e. more than 200 million years old) are found on continents. Sedimentary rocks younger than 200 Myr are found on both continents and in the oceans, but most are forming today in the oceans.

METAMORPHIC - Most meta-rocks older than 200 million years are found in the continental shields and beneath the blanket of sedimentary rocks on continents. Metamorphic rocks are forming today in the cores of active mountain ranges and in other geologically active areas.

KEY QUESTIONS:

• Why are continents made mostly of granitic rocks and ocean basins mostly of basaltic rocks?
• Why are the rocks in ocean basins almost exclusively younger than 200 million years whereas rocks on continents range up to 4 billion years?
• What induces significant horizontal and vertical displacements of rocks, thereby giving rise to the complex rock cycle described above?

Understanding the answers to these questions requires the study of plate tectonics, which is the paradigm or theory that is the foundation of many earth sciences.

II. Plate Tectonics

Plate tectonics - the theory that the earth's surface is composed of a mosaic of rigid plates that are in relative motion.

The motion of plates over earth's surface influences many important planetary processes, including volcanism, climate, earthquakes, and evolution. Understanding this simple theory thus helps to important geologic, biologic, and atmospheric processes, all of which have some interplay with environmental geology and in particular, natural hazards.

Simple observations that suggest that the earth's surface is dynamic.

• The earth's has a remarkable dual (bimodal) distribution of surface heights. Continents have an average elevations of about 2500 feet above sea level, and the ocean floor has an average depth of more than 5000 feet. Ocean floor is never older than 200 million years, whereas continental rocks can be up to ~4 billion years in age.
• The earth's surface has lots of mountains, which would not occur if the earth were not geologically active because erosion would tend to smooth topography on continents over tens to hundreds of millions of years.
• The ocean floor is full of geologically interesting features that suggest its history has been anything but dull. How does one explain linear fault scars (fracture zones) that are thousands of kilometers long, and linear chains of subsurface volcanoes? What is the 40,000 km-long connected mountain chain in the ocean basins?
• Finally, the most striking evidence that the earth's surface moves comes from looking at the global distribution of shallow earthquakes.

Earthquake - energy that is released during the brittle failure of the earth's crust or mantle.

During an earthquake, two pieces of the crust move suddenly relative to each other along a fault. Thus repeated earthquakes can cause significant movement of crust on one side of a fault relative to crust on the other side.

Earthquakes show a striking pattern - in the ocean basins, nearly all earthquakes are concentrated in narrow curvilinear zones that tend to connect up and form a global network. Between these seismic zones, few or no earthquakes occur for many thousands of kilometers. The earthquakes represent movement of the earth's crust, thus, the earth's outer brittle shell, the crust, tends to be divided into large rigidly acting areas that move relative to other rigid areas along narrow fault zones between them. Deformation of the earth's crust tends to concentrate along boundaries between large crustal plates that do not deform in their interiors. Earthquakes in continents are often more distributed than in oceanic crust, but nonetheless still tend to concentrate in zones rather being randomly distributed. The fault zones separating large, earthquake-free regions of the earth's crust are called plate boundaries, and the earthquake-free regions are called plates.

III. Why do Plates Move?

The earth's geologically active surface reflects its actively convecting interior. The interior can be viewed as an enormous heat engine that needs to move heat from the core to the cooler surface.

OVERHEAD 2 - Cross-section of the Earth

Basic facts about earth:

• Radius - 3960 miles or 6370 km from surface to center.
• Temperature of core - known imprecisely, but no hotter than 7300 degrees Centigrade and no cooler than 5500 degrees centigrade.
• Chemically, interior s composed of crust, mantle, outer core, and inner core.
• Crust - brittle outer layer - mostly silicates - 5-70 km thickness, average of 35 km.
• Mantle - contains most of earth's volume, flows slowly over geologic time - mostly silicates, thickness of 2900 km.
• Outer core - 2100 km thick - liquid iron, possibly with some sulfur
• Inner core - largely solid iron - about 1400 km thick. Hotter than outer core, but solid due to higher pressure.

This view of the earth was largely known by the 1940s due to studies by seismologists of earthquake waves traveling through the earth. The need to carry heat from the hot core to the cool surface forces the mantle to convect (i.e. to churn) in large cells. The plates can be thought of as a thin, solidified layer atop large convecting cells of hot, fluid mantle material. The buoyant continents remain on the tops of these convecting cells. Plates thus move because the mantle beneath them moves as it carries heat outward from the core.

IV. Types of Plate Boundaries

Plate boundaries refer to areas where plates slide past, beneath, or away from each other. Most objects that collide with something else are damaged at or near their point of contact, but are intact and undisturbed as one gets farther from the collision point. Earth's plates are similar in some respects. Deformation tends to concentrate along plate boundaries. Far from plate boundaries, one sees little or no active deformation. Suggests that plate interiors behave rigidly over geologic time. Boundaries between plates are thus easily defined by looking for evidence of active deformation.

Plate boundaries consist of three end-members:

• Transcurrent or strike-slip - Two plates slide past each other parallel to their boundary. Examples - oceanic transform faults and the San Andreas fault
• Divergent - Two plates move away from each other, creating a void that is typically filled by hot upper mantle. Examples - seafloor spreading centers and the East African rift system.
• Convergent - Two plates colliding with each other. Convergence can be accommodated as follows:
• 1) Subduction of one plate beneath the other into the mantle, creating a subduction zone.
• 2) Overriding of one plate on top of the other or crumpling of one or both plates, resulting in a mountainous region such as the Himalayan mountain belt.
• 3) Lateral escape - Crust caught in a zone of collision between two plates often escapes by moving sideways (laterally) out of the collision zone.

V. Multi-disciplinary evidence for plate tectonics

VI. Putting it all together

Most striking evidence for plate tectonics comes from ocean basins, which were largely unexplored until during and after World War II. Intensive mapping of the seafloor showed that the ocean basins contained an extensive, inter-connected set of sub-surface ridges or mountains that extended around the globe. These ridges had a number of peculiar features.

• (1) They were offset by long, nearly linear faults, giving rise to a pattern similar to that on an alligator's hide.
• (2) The seafloor on either side of a high-standing ridge subsided symmetrically across the ridge, and had a regularly-observed pattern of subsidence for all spreading ridges.
• (3) In the Atlantic basin, the Mid-Atlantic ridge nearly perfectly bisected the ocean basin - which seems highly unlikely if it were by chance alone.
• (4) Ridges also tend to have symmetric pattern of linear magnetic anomalies locked into seafloor on either side. We now recognize these ridges as places where new seafloor is created in the void left between plates that diverge from each other. Thus, as two continents move away from each other, new seafloor forms in between, giving rise to an ocean basin.

How fast do plates move? As slow as a few mm/yr to as fast as 160 mm/yr. Doesn't sound like much, but since plates continue moving over 10-100 Myr time scales, total displacements can reach more than several thousand kms (enough to create the entire Atlantic ocean basin).

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