Relief is simply the difference in elevation between higher point and lower point on the earth’s surface. The highest point of the earth is the peak of the Mount Everest and the lower point is Mariana trench in Pacific Ocean. The difference in elevation of the earth’s surface is due to endogenic and exogenic process operating in the earth’s crust. Relief is arranged in order according to time, process and the ways are formed (shaping or reshaping).
First order relief would be global scale contrasts between continents and ocean basins, between, say, Africa and the Indian Ocean or North America and the Pacific Basin.
First order relief features are tectonic plates and are the largest in special extent. There are two types of plates; continental plates and Oceanic plates. These are differentiated by their rock and mineral composition. Continental plates are lighter in density and are composed of granitic rock materials rich in silica and aluminum. The oceanic plates are made up of dense, basaltic rock composed of silica and magnesium.
The formation of First order reliefs can be explained by the following theories:
- Continental Drift Theory
- Sea Floor Spreading
- Plate Tectonics Theory
Continental Drift Theory was put forward by the German scientist Alfred Wegner in 1915.
According to the Continental Drift Theory, part of the crust are capable of horizontal movement round the globe causing the continents to slowly change their positions in relation to one another.
The fact that South America is a mirror image of Africa is presented as a proof of the continental drift theory (see video below for an animation showing the migration of both of these continents).
For hundreds of millions of years, all the land of Earth was joined together in one large mass or super continent. Scientists call it Pangaea (meaning “all lands” in Greek). Then about 200 million years ago the land began to drift apart. It broke into two pieces, and scientists have called the continent in the north Laurasia and the continent in the south Gondwanaland (named by Eduard Suess, an Austrian geologist).The two large continents continued to break apart into the smaller continents that exist today. Scientists call this movement ‘continental drift’.
Forces responsible for drifting of continents (According to Alfred Wegner)
According to Wegener, the drift was in two directions:
- Towards the equator due to the interaction of forces of gravity, pole-fleeing force (due to centrifugal force caused by earth’s rotation) and buoyancy (ship floats in water due to buoyant force offered by water)
- Westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener).
- Wegener suggested that tidal force (gravitational pull of the moon and to a lesser extent, the sun) also played a major role.
- The polar-fleeing force relates to the rotation of the earth. Earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth (greater centrifugal force at the equator).
- Centrifugal force increases as we move from poles towards the equator. This increase in centrifugal force has led to pole fleeing, according to Wegener.
- Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters (tides explained in detail in oceanography).
- According to Wegener, these forces would become effective when applied over many million years, and the drift is continuing.
The evidences in support of the continental drift theory:
The similarity in outline of the coastlines of eastern South America and West Africa had been noted for some time. The best fit is obtained if the coastlines are matched at a depth of 1,000 meters below current sea level
When the geology of eastern South America and West Africa was mapped it revealed that ancient rock outcrops (cratons) over 2,000 million years old were continuous from one continent to the other.
- Fragments of an old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today.
- Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia. When these land masses are re-assembled the mountain, belt forms a continuous linear feature.
- Today, glacial deposits formed during the Permo-Carboniferous glaciation (about 300 million years ago) are found in Antarctica, Africa, South America, India and Australia.
- If the continents haven’t moved, then this would suggest an ice sheet extended from the South Pole to the equator at this time – which is unlikely as the UK at this time was also close to the equator and has extensive coal and limestone deposits.
- If the continents of the southern hemisphere are re-assembled near the South Pole, then the Permo-Carboniferous ice sheet assumes a much more reasonable size
- There are many examples of fossils found on separate continents and nowhere else, suggesting the continents were once joined. If Continental Drift had not occurred, the alternative explanations would be:
- The species evolved independently on separate continents – contradicting Darwin’s theory of evolution.
- They swam to the other continent/s in breeding pairs to establish a second population.
Criticism faced by Continental Drift Theory:
- Wegener failed to explain why the drift began only in Mesozoic era and not before.
- The theory doesn’t consider oceans.
- Proofs heavily depend on assumptions that are generalist.
- Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents.
- Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift
A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and deep trenches. Mid-ocean ridges are part of chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the longest mountain chain on Earth. These ridges are spreading centers or divergent plate boundaries where the upwelling of magma from the mantle creates new ocean floor.
Deep-sea trenches are long, narrow basins which extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle.
Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle.
A test of the hypothesis of sea-floor spreading was provided by studies of the Earth’s magnetism.
Age of the sea floor:
- The age of the sea-floor also supports sea-floor spreading.
- If sea-floor spreading operates, the youngest oceanic crust should be found at the ridges and progressively older crust should be found in moving away from the ridges towards the continents. This is the case.
- The oldest known ocean floor is dated at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea trenches.
- Magnetic surveys over the ocean floor in the 1960s revealed symmetrical patterns of magnetic “bands,” (zebra stripes)anomalies parallel to midoceanic rifts .
- The same patterns in relation to midoceanic rifts are present in different oceans.
- The magnetic anomalies coincide with the episodes of magnetic reversals that have been documented from studies on land, indicating that the andesitic rocks that form new oceanic crust in the tensional setting of the rift valley record the earth’s magnetic field as they cool.
- A rock has a normal (positive) polarity when its paleomagnetic field is the same as the earth’s field today.
- The positive magnetism adds to the earth’s magnetic field and creates a higher magnetic measurement at that location.
- Rocks are negatively polarizedwhen the earth’s field is reversed, which reduces the earth’s net field strength.
- Since the ages of these anomalies are known from dating the paleomagnetic reversals on land, the rate of movement of the ocean floor can be calculated.
- The fact that new ocean crust moves away from the midoceanic ridge at speeds that range from 2 to 10 centimeters per year has also been documented using satellite measurements and radar. For example, if it is known that a segment of sea floor that formed 10.0 million years ago is now 50 kilometers (5.0 million cm) away from the crest of the ridge, it can be calculated that it traveled that distance at about 2 centimeters per year.
- By using the calculated ages for episodes of paleomagnetic reversal, scientists can construct sea floor age maps, which confirm that the youngest oceanic crust is presently being formed at midoceanic ridges and that the oldest is about 150 to 200 million years old, or late Jurassic in age.
- This older material is the farthest from the spreading centers and is the next crust to be subducted.
- Sea floor age maps have been proven correct by the age dates calculated from hundreds of rock samples gathered from the ocean floor.
- More proof for sea floor spreading comes from seismic studies indicating that earthquakes occur along the rift valley of a midoceanic ridge and the cross‐cutting fractures that offset it.
- Rift valley earthquakes occur only along transform faults, those portions of the fracture zone located between the offset sections of a ridge and rift valley.
- Because of the way in which the sea floor spreads (that is, away from both sides of a midoceanic ridge), transform faults are the only areas along the fracture zone in which sections of the oceanic crust pass one another in opposite directions.
- The concentration of earthquakes in the transform‐fault sections of the fracture zones further supports the concept of ocean crust moving away from a midoceanic ridge.
Modern plate tectonic theory:
- By the 1960s, the theories of continental drift and sea floor spreading were supported by reliable scientific data and combined to develop modern‐day plate tectonic theory.
- The theory maintains that the crust and uppermost mantle, or lithosphere, is segmented into a number of solid, rigid slabs called lithospheric plates.
- These slabs move slowly over the asthenosphere, the 200‐kilometer‐thick zone of more plastic mantle material that underlies the plates.
- New oceanic crust is created at the crests of the midoceanic ridges and pushed laterally away by new accumulations of crust. It begins to cool as it moves away from the high heat flows at the ridge.
- By the time it is subducted at the convergent boundary with another plate, it is cold and dense enough that it begins to sink back into the mantle.
- Subduction is also probably a function of a down‐turning mantle convection current below the converging plates.
- Plate tectonics(from the Late Latin tectonicus, from the Greek: τεκτονικός “pertaining to building”)is a scientific theory describing the large-scale motion of 7 large plates and the movements of a larger number of smaller plates of the Earth‘s lithosphere, over the last hundreds of millions of years.
- The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geo scientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
- The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken up into tectonic plates. The Earth’s lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates.
- Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform.
- Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually.
- Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
- Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.
- In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.
- Tectonic plates are able to move because the Earth’s lithosphere has greater strength than the underlying asthenoshere.
- Lateral density variations in the mantle result in convection.
- Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones.
- Another explanation lies in the different forces generated by tidal forces of the Sun and Moon.
- The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.
Types of Plate Boundaries
A divergent boundary
- A divergent boundaryoccurs when two tectonic plates move away from each other.
- Along these boundaries, lava spews from long fissures and geysers spurt superheated water.
- Frequent earthquakes strike along the rift. Beneath the rift, magma—molten rock—rises from the mantle.
- It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates.
- Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor.
- Thus at divergent boundaries, oceanic crust, made of basalt, is created.
- When two plates come together, it is known as a convergent boundary.
- The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench.
- A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench.
- Powerful earthquakes shake a wide area on both sides of the boundary.
- If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt.
- Magma rises into and through the other plate, solidifying into new crust. Magma formed from melting plates solidifies into granite, a light colored, low-density rock that makes up the continents.
- Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.
Transform plate boundary
- Two plates sliding past each other forms a transform plate boundary.
- Natural or human-made structures that cross a transform boundary are offset—split into pieces and carried in opposite directions.
- Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon.
- As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone.
- In contrast to convergent and divergent boundaries, no magma is formed.
- Thus, crust is cracked and broken at transform margins, but is not created or destroyed.
Latest findings made in understanding Plate Tectonics:-
- Axial seamount = It refers to a live recording of volcano mountain. The volcano rising from Juan de fuca ridge demonstrates it. It supports the divergent movement.
- After 2012 Sumatra Indonesia earthquake in Indian ocean the Indo Australian plate broken into many plate. It was mainly due to slipping of plate in interpolated and hence the activation of Barren volcano happened.
- Zealandia:-It’s a new continent. It broke from Antarctica 100 million years and from Australia 80 million yrs ago. Its formation supports movement of plates.
- Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics. Buoyancy is created by heat rising up from deep within the Earth’s core.
- Plate tectonic explains the mechanism of the motion of the tectonic plates while continental drift theory left this question completely unanswered.
- Tectonic plates have been constantly moving over the globe throughout the history of the earth. It is not the continent that moves as believed by Wegener. Continents are part of a plate and what moves is the plate.
- Wegener had thought of all the continents to have initially existed as a super continent in the form of Pangaea. However, later discoveries reveal that the continental masses, resting on the plates, have been wandering all through the geological period, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.
- At the time that Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body. However, concepts of sea floor spreading and the unified theory of plate tectonics have emphasised that both the surface of the earth and the interior are not static and motionless but are dynamic.
- Sea floor spreading:-
- The mobile rock beneath the rigid plates is believed to be moving in a circular manner. The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow
- The ultimate proof of this was the discovery of “magnetic stripes”on the seafloor later in the 1960s: the magnetic domains in oceanic rocks recorded reversal of Earth’s magnetic field over time. The pattern was symmetric to the ridge, supporting the idea of symmetric seafloor spreading. The idea of subduction zoneswas born
- With plate tectonics we have a theory that explains Wegener’s observations and how lithosphere can be produced and consumed so that Earth does not change its size
- Sea floor spreading:-
- Wegener’s continental drift theory lacked was a propelling mechanism. Other scientists wanted to know what was moving these continents around. Unfortunately, Wegener could not provide a convincing answer. The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.
- The following two forces are too small to bring in change :-
- Pole-fleeing or centrifugal force:
- The spinning of Earth on its own axis creates a centrifugal force i.e. force oriented away from the axis of rotation towards the equator. Wegener believed the centrifugal force of the planet caused the super continent to break apart and pushed continents away from the Poles toward the equator. Therefore, He called this drifting mechanism as the “pole-fleeing or centrifugal force”
- Tidal force:-
- Wegener tried to attribute the westward drift of the Americas to lunar-solar drag i.e. by invoking tidal force that is the gravitational forces of the sun and the moon .He also admitted that it is probable that pole- fleeing or centrifugal force and tidal force are responsible for the journey of continents. Wegener failed to devise a sound mechanism for the movement of the continents. For Wegener the drifting mechanism was the most difficult question to solve.
- Plate tectonics is the grand unifying theory of geosciences that explains
- Movement of continents
- Earthquakes, volcanism most major features on Earth’s surface, including mountain building, formation of new lithosphere ,consumption of old lithosphere, mid-ocean ridges
- Pole-fleeing or centrifugal force: