Geomorphology MCQs (Geography)

• The most widespread relief feature of India is Plains.

• Plains:
  • India’s most prevalent relief feature is plains 42.2 percent of the landscape.
  • The Indian peninsula consists mainly of plains and occupies a total area of 700,000 sq km.
  • It is referred to as the wide plains of the Indo Gangetic and has main rivers such as the Ganga, Yamuna, Brahmaputra & a total boundary of 15. 200 km.
  • This 43% of the land area is appropriate for irrigation and is for industrial use, covering states such as UP (Uttar Pradesh), Bihar, Rajasthan Haryana & Punjab that together occupies the north Indian plains.

• There are three types of rocks:
  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks
• Igneous rocks form when hot, molten rock crystallizes and solidifies.
• Examples of igneous rocks are basalt, granite and dunite.
• Igneous rocks are divided into:
  • Intrusive;
  • Extrusive
• Intrusive igneous rock forms when magma is trapped deep inside the Earth.
• Extrusive igneous rock is produced when magma exits and cools above the Earth's surface.

Thus, we can say that basalt is an example of igneous rock.

• The Lithosphere is the hard top layer of the earth
  • The word 'Litho' means 'rocky', thus the lithosphere is the rigid outermost layer of the earth.
  • The Lithosphere comprises of crust and upper mantle.

• The word 'bio' means 'life', hence the Biosphere is the part of the planet Earth where life exists.
• The word 'Atmos' means 'vapor', hence the Atmosphere is the layer of gases around the Earth.
• The word 'hydro' means 'water', hence the Hydrosphere is the layer that contains water mass on the Earth.

Concept:

• A rock is any naturally occurring solid mass or aggregate of one or more minerals or mineraloid matter.
• The earth's crust is composed of rocks.
• It is categorized by the minerals included its chemical composition and the way in which it is formed.
• The rock cycle is a basic concept of transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous.
• Each rock type is altered when it is forced out of its equilibrium conditions.

The Earth is divided into four main layers: the solid crust on the outside, the mantle, the outer core, and the inner core.

Interior of the Earth:

Crust:

• The uppermost layer over the earth’s surface is called the crust.
• It is the thinnest of all the layers.
• On the continental masses, It is about 35 km and on the ocean floors, only 5 km.
• The crust is the thinnest layer of the Earth and it amounts to less than 1% of our planet's volume.
• The earth is made up of several concentric layers with one inside another, just like an onion.
• The crust is the outermost layer of Earth and is made up of igneous, metamorphic, and sedimentary rocks.
• The Earth's crust is unstable because of the exogenic forces.

Mantle:

• The Mantle extends up to a depth of 2900 km below the crust.
• It is the widest section of the Earth.
• The uppermost part of the mantle is known as the asthenosphere.

Core:

• The innermost layer is the core with a radius of about 3500 km.
• The central core has a very high temperature and pressure.
• The inner core (1200 km) is in the solid-state whereas the outer core (2300 km) is in the liquid state.
• The innermost layer is the core with a radius of about 3500 km and divided into Inner and Outer core.

So it is clear that the crust is the thinner most layer of the earth.

Hence, option 1 is the correct answer.

Discontinuity:

• All those layers are separated from each other through a transition zone.
• These transition zones are called discontinuities.
• Conrad Discontinuity: It lies between the Upper crust and Lower crust.
• Mohorovicic Discontinuity: It lies between crust and mantle.
• Repiti Discontinuity: It lies between the Upper mantle and Lower mantle.
• Gutenberg Discontinuity: It lies between core and mantle.
• Lehman Discontinuity: It lies between the upper core and lower core.​

• Topsoil (A Horizon): This is the uppermost layer of soil, and it is the zone where most of the biological activity occurs.
  • It is rich in organic matter and is composed of a mixture of mineral particles such as sand, silt, and clay, as well as organic matter, water, and air.
  • The topsoil is crucial for plant growth, and it is where most nutrients and biological activity are found.
  • It is typically darker than the layers below due to the organic matter content.

• Subsoil (B Horizon): This layer is found below the topsoil. It contains fewer organic materials than the topsoil, but it may accumulate minerals, like iron, aluminum, and clay, that are leached (washed out from upper layers) from the topsoil.
  • This layer is often denser and more compact than the topsoil and can act as a storehouse for minerals and water.
  • It's not as fertile as the topsoil and is usually lighter in color.
• Substratum or Weathered Parent Rock Material (C Horizon): This layer is below the subsoil and is characterized by less biological activity.
  • It consists mainly of larger rock fragments along with some very weathered parent rock material.
  • The weathering process, which breaks down rocks both physically and chemically, happens very slowly in this layer.
  • This layer serves as a transition between the soil above and the unweathered bedrock beneath it.
• Unweathered Parent Bedrock (R Horizon): This is the bottom layer, also known as the bedrock or the unweathered rock.
• It lies beneath the soil, and it can be composed of a variety of rock types like igneous, metamorphic, or sedimentary rock.
• This bedrock over geological time, breaks down through weathering and forms the above layers of soil.

• L waves, or Love waves, are one type of surface wave in seismic activity.
  • Named after British mathematician A.E.H. Love, who first mathematically predicted their existence.
  • Love waves move the ground from side to side in a horizontal plane perpendicular to the direction of propagation.
  • As they travel along the Earth's surface, they cause horizontal shearing and produce entirely horizontal motion.
  • L waves are the slowest of all seismic waves and therefore the last to be recorded by seismographs.
  • These waves are particularly damaging to the foundations of structures due to their horizontal motion.

• P waves:
  • P waves, or Primary waves, are body waves that travel through the interior of the Earth.
  • They are the fastest seismic waves and thus the first to be detected by seismographs.
  • P waves cause particles to move in the same direction as the waves, creating a push-and-pull motion.
• S waves:
  • S waves, or Secondary waves, are also body waves that move through the Earth's interior.
  • They are slower than P waves but faster than surface waves.
  • S waves cause particles to move perpendicular to the wave direction, creating an up-and-down or side-to-side motion.
• R waves:
  • Also known as the Rayleigh wave.
  • It has both compressional and shear motions.
  • These waves result from the interaction of P-waves and vertically polarized S-waves with the surface and can exist in any solid medium.

The crust, mantle, outer core, and inner core are the four primary layers that make up the planet.

• The majority of mantle research is done by monitoring the distribution of seismic waves, which are shock waves produced by earthquakes.
• Body wave velocity varies depending on density, temperature, and rock type.
• The upper mantle extends from just beneath the crust to the lower mantle.
• The upper mantle has a thickness of between 200 and 250 kilometers.
• The mantle near the crust has a temperature of 900 to 1600 degrees Fahrenheit. At higher depths, the temperature rises.
• This is made up of fluid and moveable rock. The movement of the Earth's crust's tectonic plates is fueled by this fluidity.
• The change from Solid to Fluid form of this layer makes it a zone of lower velocity for body waves.

• lower crust: it is the lower layer of the earth's topmost layer
• upper crust: it is the upper layer of the eath's topmost layer
• inner core: it is the lower layer of the earth's innermost layer and is solidified.

Therefore, the upper mantle of the earth is called the zone of low velocity.

• The innermost layer surrounding the earth’s center is called Core, which is about 3500 km in radius.
• Core is the most dense layer of the earth with its density range from 9.5 to 14.5 and sometimes even higher.
• It is composed mainly of the Iron and Nickel thus commonly known as Nife (Nickel+Ferrum).
• Core consists of two sub-layers. The inner one is solid and the outer one is semiliquid.
• The layer surrounding the core is known as Mantle, a rock shell about 2900 kms thick and is composed of basic silicates.
• Major constituent elements of mantle are Magnesium and Silicon.
• Hence, this layer is termed as Sima (Silica+Magnesium). The density of this layer varies from 3.3 to 5.7.
• Mantle is surrounded by the outermost layer of the earth, known as lithospheric Crust or simply Crust.
• Its density varies from 2.70 to 2.95.
• Major constituent elements of lithosphere are Silica (Si) and Aluminium (Al).
• Thus this layer is termed as Sial (Silica+Aluminium). It is normally about 8 to 40 kms thick.

Hence, The 'Sima' layer of the Earth is made up of Silica and Magnesium.

• There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. They are: -
• Igneous Rocks - Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust.
  • ​The two main categories of igneous rocks are extrusive and intrusive
    • Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. example-pumice, obsidian, andesite, rhyolite, and basalt.
    • Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet. example-Diorite, granite, pegmatite.

• Sedimentary Rocks — Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. example-sandstone, limestone, and shale.
• Metamorphic Rocks —Metamorphic rocks arise from the transformation of existing rock to new types of rock, in a process called metamorphism. example- phyllite, schist, gneiss, quartzite and marble.​

• There are over 100 species of the genus Coffee in the world, and all are native to tropical Africa and some Indian Ocean islands.
• Two species, C. arabica and C. canephora, are commonly grown commercially.
  • Coffea canephora is commonly referred to as “robusta” and makes up about 25 to 40% of the coffee grown for consumption.
• Robusta grows in hotter (22 to 26ºC), more humid climates than arabica coffee (18–21ºC), and at lower elevations (from about 200 to 900 m).
• Robusta has a higher caffeine content (30 to 50% more) than arabica.
• Arabica coffee is self-pollinating, robusta requires cross-pollination by insects or wind.

• Santos port in Brazil is known as "Coffee Port of the world".
• It is located in state of Sao Paulo.

The Earth is divided into three main layers. The dense, hot inner core (yellow), the molten outer core (orange), the mantle (red), and the thin crust (brown), which supports all life in the known universe. Click through this gallery to learn more about the mantle and the rest of Earth's interior.

Source of volcanic material from Mantle:

• Geologists have long believed that solidified volcanic lava, or basalt, originates in the mantle, the molten rock just below the crust.
• The mantle is the most solid bulk of Earth's interior.
• The mantle lies between Earth's dense, super-heated core and its thin outer layer, the crust.
• The mantle is about 2,900 kilometers (1,802 miles) thick and makes up a whopping 84% of Earth’s total volume.
• The upper mantle extends from the crust to a depth of about 410 kilometers (255 miles).
• The upper mantle is mostly solid, but its more malleable regions contribute to tectonic activity.
• Two parts of the upper mantle are often recognized as distinct regions in Earth’s interior: the lithosphere and the asthenosphere.
• The asthenosphere is the denser, weaker layer beneath the lithospheric mantle.
• It lies between about 100 kilometers (62 miles) and 410 kilometers (255 miles) beneath Earth’s surface.
• The temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten.
• The lava that erupts from volcanic fissures is actually the asthenosphere itself, melted into magma.

Hence, the volcanic material has its origin in the mantle.

The Correct is Gneiss.

• Gneiss usually forms by regional metamorphism at convergent plate boundaries.
• It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure

• Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals.
• Syenite is an intrusive igneous rock composed of feldspar and a ferromagnesian mineral.
• Chemically, syenites contain a moderate amount of silica, relatively large amounts of alkalies, and alumina.
• Sandstone is a sedimentary rock composed mostly of quartz sand.
• Sandstone that contains more than 90% quartz is called quartzose sandstone.
• When the sandstone contains more than 25% feldspar, it is called arkose or arkosic sandstone.

• Igneous rocks are the most basic type of rocks. They are formed when magma (molten rock, typically derived from the earth's mantle) solidifies.
• This can happen beneath or above the surface, resulting in 2 subtypes:
  • Plutonic rocks
  • Volcanic rocks
• Plutonic rocks:
  • When magma never reaches the surface and cools to form intrusions (dykes, sills, etc) the resulting rocks are called plutonic.
  • Granite is an example of Plutonic rock.
• Volcanic rocks:
  • When magma does reach the surface during a volcanic eruption, the rocks that form there are called lavas or volcanic rocks.
• Sedimentary rocks:
  • Rocks that are formed by the consolidation of sediments of Pre-existing rocks under the mechanical, chemical, or organic activities of the wind, running water, etc are sedimentary rocks.
• Metamorphic rocks:
  • Rocks that are formed under the influence of heat, pressure or both from pre-existing rocks are called Metamorphic rocks.

• The Gutenberg discontinuity occurs within Earth's interior at a depth of about 2,900 km below the surface at the boundary between Earth's mantle and outer core.
• The Moho lies at a depth of about 35 km below continents and about 7 km beneath the oceanic crust.
• Modern instruments have determined that the velocity of seismic waves increases rapidly at this boundary.
• The Moho was named for Andrija Mohorovicic.

• Mohorovicic (Moho) discontinuity forms the boundary between the crust and the asthenosphere (upper reaches of the mantle) where there is a discontinuity in the seismic velocity.
• It occurs at an average depth of about 8 kilometers beneath the ocean basins and 30 kilometers beneath continental surfaces.
• The cause of the Moho is thought to be a change in rock composition from rocks containing feldspar (above) to rocks that contain no feldspars (below).

• The interior of the earth is made up of several concentric layers.
• Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.
• The crust is a silicate solid, the mantle is a viscous molten rock, the outer core is a viscous liquid, and the inner core is a dense solid.
• Crust
  • The crust is the outermost layer of the earth making up 0.5-1.0 percent of the earth’s volume and less than 1 percent of Earth’s mass.
  • Density increases with depth, and the average density is about 2.7 g/cm3 (the average density of the earth is 5.51 g/cm³).
  • The thickness of the crust varies in the range of 5-30 km in the case of the oceanic crust and 50-70 km in the case of the continental crust.

A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. There are three kinds of rock: igneous, sedimentary, and metamorphic. Igneous rocks.

Therefore, it is clear that the Granite is an igneous rock, not a metamorphic rock.

Concept:

• A rock is any naturally occurring solid mass or aggregate of one or more minerals or mineraloid matter.
• The earth's crust is composed of rocks.
• It is categorized by the minerals included its chemical composition and the way in which it is formed.
• The rock cycle is a basic concept of transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous.
• Each rock type is altered when it is forced out of its equilibrium conditions.

Explanation:

Rocks containing fossils:

• Fossils, the preserved remains of animal and plant life, are mostly found embedded in sedimentary rocks.
• Of the sedimentary rocks, most fossils occur in shale, limestone and sandstone.
• Gentler pressure and lower temperature allow preservation of past life forms.
• Fossils become a part of sedimentary rocks when sediments such as mud, sand, shells and pebbles cover plant and animal organisms and preserve their characteristics through time.

​Thus, rocks that contain fossils are called sedimentary rocks.

• Calderas volcanoes are an example of the most explosive type of volcano.

• Caldera Volcanoes:
  • Calderas Volcanoes are known as the most explosive volcanoes on Earth.
  • When they erupt, they incline to drop on themselves rather than constructing any structure.
  • The fallen depressions are known as calderas.

​

• Volcanoes:
  • A volcano is a vent or fissure in Earth’s crust through which lava, ash, rocks, and gases erupt.
• The other types of volcanoes areShield Volcanoes and Composite Volcanoes.
• Shield volcanoes:
  • These are the largest of all the volcanoes on the earth.
  • These volcanoes are made up of basalt.
  • Eg: Hawaiian shield volcanoes.
• Flood Basalt volcanoes:
  • These volcanoes discharge highly fluid lava that flows for long distances.
  • Most parts of the world are covered by thick basalt lava flows.

Correct Answer 1

• The uppermost layer over the earth’s surface is called the crust.
• It is the thinnest of all the layers.
• On the continental masses, It is about 35 km and on the ocean floors, only 5 km.
• The density of the rocks found at the surface rocks: 2.7 - 2.9 gm/cm3
• The density of rocks found at the sea floor is around: 3.0 gm/cm3
• The oceanic crust is denser than the continental crust.
• The crust is the thinnest layer of the Earth and it amounts to less than 1% of our planet's volume.
• The earth is made up of several concentric layers with one inside another, just like an onion.
• The crust is the outermost layer of Earth and is made up of igneous, metamorphic, and sedimentary rocks.
• The Earth's crust is unstable because of the exogenic forces.

Discontinuity:

• All those layers are separated from each other through a transition zone.
• These transition zones are called discontinuities.
• Conrad Discontinuity: It lies between the Upper crust and Lower crust.
• Mohorovicic Discontinuity: It lies between crust and mantle.
• Repiti Discontinuity: It lies between the Upper mantle and Lower mantle.
• Gutenberg Discontinuity: It lies between core and mantle.
• Lehman Discontinuity: It lies between the upper core and lower core.​
• Discontinuities are the transition zones that separated the layers of the earth.
• Earth’s interior is made of different kinds of materials.
• Each of those materials is different from the others by their physical and chemical properties, such as temperature, density, etc.

Cirque:

• They are usually formed at the top of the glacial valley.
• They are deep long basins with steep concave high vertical walls.
• It is a type of glacial landform formed due to erosion by the glaciers.

• A depositional landform is a landform that was created due to glacial deposition.

Stalagmite:

• They are similar to stalactite in shape but are formed from the floor of the cave towards the roof.
• It is a type of landform formed by underground water deposition.

Pillar:

• When a Stalactite and a stalagmite come together, they form a complete pillar.
• It is a type of landform formed by underground water deposition.

Moraines:

• It is a landform formed due to glacier deposition.

• It is formed due to the accumulation of glacial debris or till.

Lithification:

• It refers to the process by which loose and under-consolidated sediment particles transform into hard and solid rocks.
• This process includes a number of geological processes, such as consolidation, deep bury, cementation, recrystallization, and dehydration.

Igneous Rocks:

• As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.
• The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and solidifies.
• When magma in its upward movement cools and turns into a solid form it is called igneous rock.
• The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
• Igneous rocks are classified based on texture which depends upon the size and arrangement of grains or other physical conditions of the materials.
• Granite, gabbro, pegmatite, basalt, volcanic breccia, and tuff are some examples of igneous rocks.

Sedimentary Rocks

• The word ‘sedimentary’ is derived from the Latin word sediment, which means settling.
• Rocks (igneous, sedimentary, and metamorphic) of the earth’s surface are exposed to denudational agents and are broken up into various sizes of fragments.
• Such fragments are transported by different exogenous agencies and deposited.
• These deposits through compaction turn into rocks.
• In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
• Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale, geyserite, chalk, limestone, coal etc.

Metamorphic Rocks

• Metamorphic means ‘change of form’.
• These rocks form under the action of pressure, volume, and temperature (PVT) change.
• Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.
• Metamorphic rocks are formed due to the proximity of sedimentary rocks with molten magma.
• Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes are called dynamic metamorphism.
• Examples- Marble, Quartzite, Schist, etc.

Foliation:

• In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines.
• Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.

• Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms.
• They are formed from the remains of plants and animals.
• They often have distinctive layering or bedding.
• Common sedimentary rocks include sandstone, limestone and shale.
• These rocks often start as sediments carried in rivers and deposited in lakes and oceans.

Thus, we can say that sedimentary rocks are formed from remains of plants and animals, consisting of several layers of strata and limestone is an example of sedimentary rocks.

• A continent is a large continuous landmass, separated from other continents by water or other geographical features.
• Approximately 71% of the Earth's surface is covered by water.
• And the remaining 29% of the Earth's surface is land.
• The continents are, from largest to smallest:
  • Asia, Africa, North America, South America, Antarctica, Europe, and Australia.

• The world’s largest continent is Asia.
• Asia is the most populous continent in the world (30% of the world’s land area and 60% of the world’s people).
• The world’s smallest continent is Australia and is also referred to as an “island continent” because it is surrounded on all sides by the ocean.
• North America is the only continent having five time zones and every type of climate.
• Antarctica is the coldest continent in the world.

The correct option is 1.

• Plunge pools are formed due to the action of running water.
  
  the deep pool below it.
• Horns are formed due to the action of glaciers.
  • Horn results when glaciers erode three or more arêtes
• Playas are depression formed due to the deflating action of winds.
  • They are found mainly in the desert.
• Whent the water combined with CO2 moves down through the force of limestone a number of long furrows with walls are formed such limestone features are called as lapies.
  • It is a part of karst topography.

• Iron is the second most abundant metal in the earth's crust (5.0%).
• Aluminium is the most abundant metal found in the earth's crust (8.1%).
• Oxygen comprises of 46.6% of the total weight of the earth's crust.

Elements in earth's crust

• Tectonic Plates
  • A tectonic plate (also called a lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
  • The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
  • The concept of Tectonic Plates was first introduced in 1967.
  • A tectonic plate maybe a continental plate or an oceanic plate, depending on which of the two occupies the larger portion of the plate.
  • The Pacific plate is largely oceanic whereas the Eurasian Plate is continental.
• The Major Plates
  • The Antarctic (and the surrounding oceanic) plate
  • The North American plate (with western Atlantic floor separated from the South American plate along with the Caribbean islands)
  • The South American plate (with western Atlantic floor separated from the North American plate along with the Caribbean islands)
  • The Pacific plate
  • The India-Australia-New Zealand plate
  • Africa with the eastern Atlantic floor plate
  • Eurasia and the adjacent oceanic plate
• Minor plates
  • Cocos plate: Between Central America and Pacific plate
  • Nazca plate: Between South America and Pacific plate
  • Arabian plate: Mostly the Saudi Arabian landmass
  • Philippine plate: Between the Asiatic and Pacific plate
  • Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
  • Fuji plate: North-east of Australia
  • Juan De Fuca plate: South-East of North American Plate
  • Hence, Option 4 is correct.

• Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.
• There are two types of body waves.
  • They are called P and S-waves.
• P-waves:
  • They move faster and are the first to arrive at the surface.
  • These are also called ‘primary waves’.
  • The P-waves are similar to sound waves.
  • They travel through gaseous, liquid and solid materials.
  • P waves are characterized by a back-and-forth motion of particles parallel to the direction of wave propagation.
  • On the other hand, S waves, also known as secondary waves or shear waves, cause particles to vibrate perpendicular to the direction of wave propagation, creating an up-and-down or side-to-side motion. Hence statement 2 is correct
• S-waves:
  • They arrive at the surface with some time lag.
  • These are called secondary waves.
  • An important fact about S-waves is that they can travel only through solid materials.

Seismograph:

• In a seismograph, P waves are recorded earlier than S waves.
• P waves, also known as primary waves or compressional waves, are the fastest seismic waves and can travel through both solids and liquids.
• They arrive at a seismograph station before the arrival of S waves, which are slower and arrive after the P waves.
• Therefore, P waves are recorded earlier than S waves in a seismograph. Hence statement 1 is correct

• Earth’s crust is divided into two types: Oceanic Crust and Continental Crust.
• The transition zone between these two types of crust is sometimes called the Conrad discontinuity.
• Oceanic Crust:
  • Oceanic crust, extending 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly composed of different types of basalts.
  • Geologists often refer to the rocks of the oceanic crust as “Sima.”
  • Sima stands for silicate and magnesium, the most abundant minerals in oceanic crust.
  • The ocean floors are formed by basaltic rocks comprising ‘Sima’. Hence statement 2 is incorrect.
  • Oceanic crust is dense, almost 3 grams per cubic centimeter (1.7 ounces per cubic inch).
  • Oceanic crust is constantly formed at mid-ocean ridges, where tectonic plates are tearing apart from each other.
  • As magma that wells up from these rifts on Earth’s surface cools, it becomes young oceanic crust.
• Continental Crust:
  • Continental crust is mostly composed of different types of granites. Hence statement 1 is incorrect.
  • Geologists often refer to the rocks of the continental crust as “Sial.”
  • Sial stands for silicate and aluminum, the most abundant minerals in the continental crust.
  • Sial can be much thicker than sima (as thick as 70 kilometers (44 miles)).
  • It is slightly less dense (about 2.7 grams per cubic centimeter (1.6 ounces per cubic inch)).
  • Cratons are the oldest and most stable part of the continental lithosphere.
  • These parts of the continental crust are usually found deep in the interior of most continents.
  • Cratons are divided into two categories:
    • Shields are cratons in which the ancient basement rock crops out into the atmosphere.
    • Platforms are cratons in which the basement rock is buried beneath overlying sediment.
  • Since Sial is lighter than Sima, continents are said to floating on the sea of denser Sima. Hence statement 3 is correct.​