EEES 1010-004, Spring 2007,  Dr. James Martin-Hayden

F. A Tour of Plate Tectonics

1. 7  Major Plates

2. The 3 types of plate boundaries

Basic features found at each types of boundary

Examples of each type of boundary

Geologic phenomena at each type of boundary

3. Summary of Convergent boundaries and crust

4. Introduction to Geologic Time

1. The Major Lithospheric Plates

·       Each continent “rides” on a major lithospheric plate

·       One major lithospheric plate has no continent riding on it, the Pacific Plate

·       This makes a total of 7 major lithospheric plates each with the name of the continent that rides on it with the exception of the Pacific Plate

·       There are 6 or 7 smaller plates between some of the larger ones

·       The Major Lithospheric Plates and directions of movement (Fig. 2.14)

2. Types of Plate Boundaries

Divergent Plate Boundaries: where plates move away from each other new oceanic crust is generated from melting asthenospheric material that cools and forms Basaltic Rock

·       Iron-rich

·       Silicon-poor

·       Dense

·       Young

·       Forming a mid-ocean ridge in each of the four oceans (Fig 2.12 and 2.15)

Mid-Ocean Ridge examples

·       East Pacific Rise

·       Mid Atlantic Ridge

·       Mid Indian Ridge

·       Mid Arctic Ridge

When forming beneath a continent, will tear the continent apart creating rift valleys

            E.g., The Red Sea and East African Rift Valleys (Fig. 2.16)

Geologic Phenomena at Divergent Plate Boundaries

·       Volcanic Activity

·       Fissures Eruptions

·       Lava Floods, and

·       Volcanoes

·       Non-Explosive

·       Basaltic rocks formed (i.e. iron-rich/silica poor)

·       Shallow Earthquake Activity  Fig 9.5

Convergent Plate Boundaries: where plates move toward each other, oceanic crust and the underlying lithosphere is subducted beneath the other plate (with either oceanic crust or continental crust)

Physiographic Features at Convergent Plate Boundaries

Island Arcs (of volcanic Islands ), Oceanic Trenches, Volcano Chain

Geologic Phenomena at Convergent Plate Boundaries

·       Volcanic Activity

·       Explosive, Composite Volcanoes

·       Granitic rocks formed (iron-poor/silica-rich)

·       Shallow earthquakes near trench

·       Shallow and Deep Earthquakes over subduction zone

Transform Plate Boundaries

·       Where plates slide parallel to each other:

·       Transform faults are created

·       Mid-ocean ridges are offset

·       If occurring beneath a continent the continent is sheared and faulted

·       Transform Plate Boundaries

·       Shallow earthquakes are generated

·       Volcanic activity is rare

·       Examples

·       Every mid-ocean ridge is offset along by transform faults

·       San Andreas Fault

http://pubs.usgs.gov/publications/text/San_Andreas.html

3. Summary of Plate Boundaries and Associated Types of Crust Formed

·       Divergent

·       Convergent

·       Three Rock Types

Divergent Plate Boundaries Rifting and Formation of new Basiltic Oceanic Crust

·       Thin (<10 km)

·       Young (<200my)

·       Iron Rich (>5%) /

·       Silica Poor (~50%)

·       Dense (s.g. ~ 3)

·       Low lying (5-11 km deep)

·       Formed at Divergent Plate Boundaries

·       Basaltic Volcanism (non-explosive)

·       Rifting and generation of shallow earthquakes (<33km)

Convergent Plate Boundaries: Formation of Granitic Continental Crust

·       Thick (10-50 km)

·       Old (>200 m.y. and up to 3.5 b.y.)

·       Iron Poor (<1%) /

·       Silica Rich (>70%)

·       Less Dense (s.g. ~ 2.5)

·       High Rising

·       (mostly above see level)

·       Formed at Convergent Plate Boundaries

The “Ring of Fire”
A ring of convergent plate boundaries on the Pacific Rim

·       New Zealand

·       Tonga/Samoa

·       Philippines

·       Japanese Isls.

·       Aleutian Island arc and Trench

·       Cascade Range

·       Sierra Madre

·       Andes Mtns.

Depth of Earthquakes at convergent plate boundaries

·       Shallow quakes at the oceanic trench (<33km)

·       Deep quakes over the subduction zone (>70 km)

The 3 rock types form at convergent plate boundaries

·       Sedimentary Rock: Sediments (e.g., ocean sand, silt, and clay) are compressed cemented (lithified)

·       Metamorphic Rocks: rocks are compressed, heated and change minerals but do not melt

·       Igneous Rocks: When rocks melt, Magma is formed, rises, cools and crystallizes.

Below surfaceà Intrusive.  LavaàExtrusive Isostatic Adjustment

Isostatic Adjustment

·       Why do we see, at the earths surface,

·       Intrusive igneous rocks and

·       Metamorphic rocks

·       Formed many km deep?

·       Thick, light continental crust buoys up even while it erodes

·       Eventually, deep rocks are exposed  at the earth’s surface

·       Minerals not in equilibrium weathered (transformed) to clay

·       Sediments are formed

Fig 1.12

F. Geologic Time

Time in Millions of Years

·       0.01: The end of the last Ice Age

·       4: The First Hominids Evolve

·       65: Dinosaurs go extinct, Imporant Mammals evolve

·       245: Age of the Reptiles (Dinosaurs begin to evolve)

·       545: Complex animals evolve

·       ~3,500: Origin of life

·       4,500: Origin of Earth

Fig. 8-26 (see table 1.1)

Plate Movement, 100s of Millions of Years

·       If a plate moves 1 cm/yr, it will move

·       1 m (about 3 feet) in a century,

·       10 km in a million years,

·       1000 km in 100 million years.

·       E.g., The Atlantic Ocean is 4,830 km wide (between North America and Northern Africa)

·       If the oldest Atlantic Oceanic crust is 225 my

·       How fast is the Atlantic widening?

                        See fig. 19.2