Diastrophism

Chapter 6

Diastrophism

• It pertains to any movement of the solid part of the Earth.

• It is a process that results in deformation of the Earth’s crust.

• The movement may be strong and sudden that we feel the shaking of the Earth’s surface, we call this earthquake.

Direction of Forces

• Upward forces cause the widespread rising of the crust.

• Downward forces cause the widespread sinking of the crust.

• Horizontal forces moving in the same direction (compression) can cause landmasses to crumple break and slip against each other; horizontal forces that move away from each other (tension).

Effects of Forces on the Landmasses

• 1. Folding occurs when a part of the crust crumples, bends upward and downward.

• TWO PARTS OF A FOLD• 1. Crest or Anticline• 2. Trough or Syncline

• 2. Faulting occurs when crust is fractured due to unequal forces acting in opposite directions.

• Faults may be classified as• 1. Normal• 2. Reverse or Thrust• 3. Strike-slip

• 1. Normal Faults occur when tensional forces pull the crust apart.

• The forces move the crust vertically apart and are called dip-slip fault since the displacement (slip) is along the tilt (dip) of the fault line.

• 2. Reverse or Thrust faults are formed due to strong compressional forces.

• This kind of fault is associated to transform faults.

• 3. Strike-slip or lateral faults occur when the blocks move horizontally past each other.

• These kinds of fault is associated with plate boundaries and are called transform faults.

3. Trenching

• 3. Trenching – occurs when large masses of the rocks in the crust slide and slip against each other due to great forces coming from different directions.

Causes of Diastrophism

• Theory of Isostasy – • Isostasy can be explained as the balancing of forces

between the effects of gravity on the mass of a section of earth and the resistance of that mass to sinking into the mantle of the earth.

• The simplest analogy of isostasy is icebergs (this is based on Archimedes’ Principal).

• This explains why the wearing down of mountains and the filling up of the ocean basins have not resulted on a leveled surface over the whole earth.

• As vertical adjustments take place, landmasses are folded, buckled and thrusted.

The Contraction Theory

• Earth is shrinking because it is cooling and the great pressure squeezes parts of the earth into a smaller volume.

• Gravity draws the crust inward causing it to buckle, bend and trench.

Convection Theory

• According to this theory, convection currents are set in the crust and heat comes from the disintegration of radioactive elements.

• As heat accumulates, rocks become plastic and moves upward causing the surface of the earth to bulge.

Continental Drift Theory• Proposed by a German

meteorologist and geophysicist, Alfred Wegener.

• Wegener hypothesized that there was an original, gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth".

• Pangaea was a supercontinent consisting of all of Earth's land masses.

• Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period.

The Plants and Animals Match

• Fossil Evidence in Support of the Theory

• Wegener noted that plant fossils of late Paleozoic age found on several different continents were quite similar.

• He was intrigued by the occurrences of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean.

• He reasoned that it was physically impossible for most of these organisms to have traveled or have been transported across the vast ocean.

• The most compelling evidence that the two continents were once joined.

The Rocks Match

• Broad belts of rocks in Africa and South America are the same type.

• These broad belts then match when the end of the continents are joined.

The Shapes Match

• Sir Francis Bacon first noticed this peculiarity in the 17th century.

• The continents look as if they were pieces of a giant jigsaw puzzle that could fit together to make one giant super-continent.

• The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge.

PLATE TECTONICS• The theory of plate tectonics

(meaning "plate structure") was developed in the 1960's.

• This theory explains the movement of the Earth's plates (which has since been documented scientifically) and also explains the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and many other geologic phenomenon.

• The plates are moving at a speed that has been estimated at 1 to 10 cm per year.

• Most of the Earth's seismic activity (volcanoes and earthquakes) occurs at the plate boundaries as they interact.

Types of Plate Motion

• The ways that plates interact depend on their relative motion and whether oceanic or continental crust is at the edge of the lithospheric plate.

• Plates move away from, toward, or slide past each other.

• Geologists call these divergent, convergent, and transform plate boundaries.

• At a convergent plate boundary, lithospheric plates move toward each other.

• The west margin of the South American continent, where the oceanic Nazca Plate is pushed toward and beneath the continental portion of the South American Plate, is an example of a convergent plate boundary.

• At a divergent plate boundary lithospheric plates move away from each other.

• The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary.

• At a transform plate boundary, plates slide past each other.

• The San Andreas fault in California is an example of a transform plate boundary, where the Pacific Plate slides past the North American Plate.

• Relationship between plate tectonic setting and structural style:

• Tectonic Setting

•  Stress State

•  Types of Structures

•  Examples•  Divergent plates extension•  normal faults, roll over anticlines, tilted

blocks•  North Sea, Red Sea, Basin and

Range Convergent plates compression•  thrust faults, folds, faulted folds•  Andes, Zagros Mts (Iran), Canadian

Rockies Transform plate boundaries strike-slip

•  strike-slip faults, compressional and extensional flower structures

•  San Andreas fault, Alpine Fault (New Zealand).