Plate tectonics

Plate Tectonics

• Plate tectonics: Earth’s surface is divided into a few large, thick plates that move slowly and change in size

• Plate is a large, mobile slab of rock that is part of Earth’s surface, bounded on four sides by weak plane surfaces

• Plate consists of crust and upper part of the mantle

-The Earth’s crust moves!!!!!-The continents have not always been arranged like they are today.

- The Earth’s crust is divided into segments called plates.

HISTORY OF PLATE TECTONICS

History of Plate Tectonics

• The theory of plate tectonics has largely been developed since 1967 but the history of ideas extend back considerably longer.

• The theory of plate tectonics states that “the Earth’s outermost layer is fragmented into a dozen of large & small plates that are moving relative to one another on top of the mobile Asthenosphere”.

Before the advent of plate tectonics, however, some people already believed that the present day continents were the fragmented pieces of preexisting larger landmasses (supercontinents).

Plate tectonics is the relatively new scientific concept, introduced in the 19th century.

Plate tectonics theory has provided explanation to the distribution of earthquakes & volcanic active zone around the globe.

Abraham Ortelius(1527-1598)

He was the first to note the similarity & suggest an ancient separation in 1556. He suggested that the Americas were “torn away from Europe & Africa by earthquake & floods”.

Alexander Von Humbolt

• The German explorer Humbolt also noted the geometric& geological similarities of the opposing shores of the Atlantic but also suggested that the Atlantic ocean was formed by Catastrophic Events.

Catastrophism vs. UniformitarianismCatastrophism Uniformitarianism

• A principle that states that catastrophic events have been important in Earth history and modify the path of slow change.

• The principle that states that geological change occurs over long periods of time.

• In addition, processes and scientific laws operating today also operated in the past and thus past. Geological events can be explained by forces observable today.

Catastrophism:Catastrophism is the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope. This was in contrast to uniformitarians. Uniformitarians:Uniformitarians (sometimes described as gradualism, in which slow incremental changes, such as erosion, created all the Earth's geological features. Uniformitarians held that the present is the key to the past, and that all things continued as they were from the indefinite past.

James Hutton In the mid of 19th century

concept of “Catastrophism” is replaced by that of “Uniformitarianism” or actualism as proposed by the Scottish geologist James Hutton in 1785.

The Principle of uniformity is commonly stated as: “The present is Key to the

past”

• It means that slow processes going on at & beneath the Earth’s surface today have been going on throughout geological time & have shaped the surface record.

• However the geologic forces acting on the Earth today are the same as those that have acted in the geologic past.

CONTINENTAL DRIFT

Continental Drift & Alfred Wegner’s Theory

• In 1912, the German meteorologist Alfred Wegener (1880-1930), first proposed the theory of continental drift, which state that “parts of the Earth’s crust slowly drift on top of the mobile asthenosphere”.

Wegener hypothesized that all the continents were assembled into supercontinents 225 Million years ago, which he named “Pangaea”, meaning “All earth”.

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

It existed from Permian to Jurassic periods.It began breaking up during the Jurassic period,

forming two supercontinents.

Laurasia• It is the northern portion of supercontinents &

consists of North America, Europe & Asia.

• It is the southern portion of Supercontinent & include South America, Antarctica, Africa, India & Australia.

Gondwanaland

• These two supercontinents were separated by palaeo-Thethys ocean.

• The ocean surrounding them was Panthalassa (literally “all ocean”).

Alfred Supercontinents

Drawbacks of Alfred Wegener’s Theory

• Lack of plausible driving force was missing.

• Wegener suggested that continents drift occurred in response to the centripetal force experienced by the continents because of the Earth’s rotation but it is not true.

Wegener’s theory vindication

• Although Wegener‘s continental theory was rejected during his life time due to lack of reasonable mechanism for continental drift.

• Later on, the British geologist Arthur Holmes championed the theory of continental drift & proposed that continents were moved by convection current powered by the heat of radioactive decay.

However, after his death, new evidence from ocean floor exploration and other studies rekindled interest in Wagener's theory, ultimately leading to the development of the theory of plate tectonics

EVIDENCES OF CONTINENTAL DRIFT

• GEOMETRICAL FIT OF CONTINENTS• Similarity of Rock Sequences and Mountain Ranges• Age provinces• Igneous provinces• Glacial Evidence• Fossil Evidences• Glossopteris flora

• Wegener devoted his life to doggedly pursuing additional evidence to defend his theory.

• The evidence used by Wegener, du Toit, and others to support the hypothesis of continental drift are as follow;

1)GEOMETRICAL FIT OF CONTINENTS

• Wegener, like some before him, was impressed by the close resemblance between the coastlines of continents on opposite sides of the Atlantic Ocean, particularly between South America and Africa. 

• The continents fit together like pieces of a giant jigsaw puzzle.

• He cited these similarities as partial evidence that the continents were at one time joined together as a supercontinent that  subsequently split apart.

Ancient “cratons” within continents match up when they are brought together like a jigsaw puzzle.

Craton is part of a continent that is stable and forms the central mass of the continent.

• As his critics pointed out, though, the configuration of coastlines result from erosional and depositional processes and therefore is continually being modified.  So even if the continents had separated, as Wegener proposed, it is not likely that the coastlines would fit exactly.

• A more realistic approach is to fit the continents together along the continental slope where erosion would be minimal. 

• In 1965 Sir Edward Bullard, an English geophysicist, and two associates showed that the best fit between the continents occurs along the continental slope at a depth of about 2,000m.

• Since then, other reconstructions using the latest ocean basin data have confirmed the close fit between continents when they are reassembled to form Pangaea.

2)Similarity of Rock Sequences and Mountain Ranges

• If the continents were at one time joined, then the rocks and mountain ranges of the same age in adjoining locations on the opposite continents should closely match.

• Mountain Chains were continuous across the oceans.• Example, the rocks in the Appalachian Mountain Belt, are very similar to those found in British isles, Scandinavia and Africa.

•  

•The continuation of Appalachian fold belt of North America with the Caledonian fold belt of northern Europe was noted.

•Similarly the sedimentary deposits associated with these fold belts also supported theory.

• The grain size & chemical composition of the sediments can be used to determine the nature & direction of the source.

• The source of the sediments in Caledonian fold belt of northern Europe lies to the west in location now occupied by Atlantic ocean, indicate that in past both continents were side by side.

3) Age provinces

• The correlation of the patterns of ages across the southern Atlantic is also noted. Observed similar rock units up to Jurassic.

4) Igneous provinces• The igneous rocks

of similar age can be traced b/w various continents. This consists of both extrusive & intrusive e.g. the Mesozoic dolorites spread through Africa, Antarctica, America.

4) Glacial Evidence

• During the Late Paleozoic Era, massive glaciers covered large continental areas of the Southern Hemisphere.  Evidence for this glaciation includes layers of Till (sediments deposited by glaciers) and Striations (scratch marks) in the bedrock beneath the till.

• Fossil plants found in coals indicate that the Northern Hemisphere had a tropical climate during the time that the Southern Hemisphere was glaciated.

• All of the Gondwana continents except Antarctica are currently located near the equator in subtropical to tropical climates.  Mapping of glacial striations in bedrock in Australia, India, and South America indicates that the glaciers moved from the areas of the present-day oceans onto land.  This would be highly unlikely because large continental glaciers flow outward from their central area of accumulation toward the sea.

• Glacial deposits, including structures that indicate ice flow, direction are located in ancient rocks as shown in the figure below.

• If the continents did not move during the past, one would have to explain how glaciers moved from the oceans onto land and how large-scale continental glaciers formed near the equator.

• But if the continents are reassembled as a single landmass with South Africa located at the south pole, the direction of movement of Late Paleozoic continental glaciers makes sense. 

5) Fossil Evidences

• Some of the most compelling evidence for continental drift comes from the fossil record.

a)Glossopteris flora

• Glossopteris, a tree-like plant from the Permian through the Triassic Period. It had tongue-shaped leaves and was about 12 ft (3.7 m) tall. It was the dominant plant of Gondwana.

Distribution of Glosspteris

b)Mesosaurus

• It is freshwater reptile whose fossils are found in Permian-aged rocks in certain regions of Brazil and South Africa and no where else in the world.

• Because the physiology of freshwater and marine animals is completely different, it is hard to imagine how a freshwater reptile could have swum across the Atlantic Ocean and found a freshwater environment nearly identical to its former habitat.

• It is more logical to assume that Mesosaurus lived in lakes in what are now adjacent areas of South America and Africa, but were then united into a single continent.

Plate Boundary

• Plate Boundary refers to the border between two adjacent lithospheric plates, usually classified by the relative motion taking place, convergent, divergent, or transform.

• Plate boundaries can occur on landmasses (continents) or in marine settings (oceans) or both at the same time.

• Divergent

• Convergent

• Transform

Three types of Plate Boundaries

Three types of Plate Boundaries

Divergent Plate Boundary• Divergent plate boundaries are associated with

the following:a. Tension or extension (pulling apart)b. Normal faultingc. Rifting (as in the mid-oceanic rift zone)d. Creation of magma material inside the rift

zone

Divergent Boundary - Oceanic

Divergent Boundary - Continental

Divergent Boundary – Arabian and African Plates

Arabian Plate

African PlateRed Sea

Divergent Boundary –Iceland

• Spreading ridges– As plates move apart new material is erupted to

fill the gap

Divergent Boundaries

Features/Landforms Associated with Divergent Boundary

• Divergent zone: Oceanic, basaltic magma, spreading center (Mid-Ocean Ridge). Also, Continental: granitic, (East African Rift Zone)

Convergent Plate Boundary• Convergent plate movement is associated with the

following:a. Compressionb.    Reverse faultingc.    Creation of a subduction zoned.    Mountain building processese.    Collisions of plates: – 1. Continent vs. oceanic– 2. Oceanic vs. oceanic– 3. Continent vs. continent

Convergent Boundary – Indian and Eurasian Plates

Indian Plate

Eurasian Plate

Collision Zones:

Continent vs. Oceanic

Oceanic vs. Oceanic

Continental vs. Continental

Convergent Boundary – Oceanic & Continental

Andes Mountains, South America

Convergent Boundary – Oceanic & Oceanic

Aleutian Islands, Alaska

• Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

Himalayas

Features/Landforms Associated with convergent Boundary

1. Continent vs. Oceanic collision zone: subduction zone, deep sea trench associated, volcanic arc, andesitic volcanic rock.

2. Oceanic vs. Oceanic collision zone: deep sea trench associated, volcanic island arc, basaltic volcanic rock.

3. Continent vs. Continent collision zone: Granitic rocks, mountain building processes, no volcanism, no magmatic activities.

• Where plates slide past each other

Transform Boundaries

View of the San Andreas transform fault

Transform Boundary – San Andreas Fault

Features/Landforms Associated with Transform Boundary

• Transform boundaries are associated with the following:

a. Horizontal grinding motionb. Strike-slip faultingc. Lateral offset of rock units