Earth's Interior, Rocks and Landforms

What to remember

• Earth has three main layers. Crust (outer, thin), mantle (middle, thickest) and core (inner, iron and nickel). Our knowledge comes mostly from seismic waves.
• Rocks are of three types. Igneous (cooled magma), sedimentary (layers of deposits) and metamorphic (changed by heat and pressure). The rock cycle links them.
• Landforms come from internal and external forces. Plate movement, volcanoes and earthquakes build the surface; weathering, rivers, wind, glaciers and waves wear it down.

Structure of the Earth

Direct observation goes only a few kilometres down, so scientists use seismic waves, meteorites, gravity and magnetic data.

The boundary between the crust and mantle is the Mohorovicic (Moho) discontinuity. The mantle-core boundary is the Gutenberg discontinuity. The outer-inner core boundary is the Lehmann discontinuity.

Temperature and pressure rise with depth. The magnetic field of Earth is related to the movement of the liquid outer core.

Rocks

A rock is a natural mass of one or more minerals.

Igneous rocks form when molten magma or lava cools and solidifies. They are called primary rocks.

• *Intrusive (plutonic)*: cool slowly inside the crust; large crystals. Example: granite.
• *Extrusive (volcanic)*: cool quickly on the surface; fine grains. Example: basalt. The Deccan Plateau has black cotton soil formed from basalt.

Sedimentary rocks form from the compaction of sediments carried by water, wind or ice. They have layers (strata) and may contain fossils. Examples: sandstone, limestone, shale and conglomerate. Coal is an organic sedimentary rock.

Metamorphic rocks form when existing rocks change under heat and pressure without melting.

Rock cycle: Igneous rocks are weathered into sediments, which form sedimentary rocks; heat and pressure change them into metamorphic rocks; deep melting makes magma, which cools into igneous rocks again.

In Andhra Pradesh, the Eastern Ghats contain very old metamorphic rocks (such as khondalite and charnockite), the Cuddapah basin has sedimentary rocks, and the Deccan Trap type basalt appears in parts of the west.

Geomorphic processes

Endogenic (internal) forces: folding, faulting, volcanism, earthquakes. They build the relief.

Exogenic (external) forces: weathering, erosion, transport, deposition. They wear it down (gradation).

Weathering is the breaking of rock in place: physical (temperature changes, frost), chemical (oxidation, carbonation, solution) and biological (roots, burrowing).

Mass movement includes landslides, mudflows and soil creep. Gravity moves material down slope.

Landforms made by agents:

Deltas form where a river meets the sea and slows. The Godavari and Krishna deltas of Andhra are fertile deltas.

Mountains and plate tectonics

Fold mountains (Himalayas, Alps, Andes) are young and formed by compression. Block mountains (such as the Vosges and the Black Forest) are formed by faulting; the Aravalli is a very old, worn-down fold mountain. Volcanic mountains (Mount Fuji) form from eruptions. Residual mountains are what remain after erosion. The Eastern Ghats are old and discontinuous.

Plate tectonics: The lithosphere is broken into large plates (Pacific, Eurasian, African, Indian, North American and others) that move slowly over the asthenosphere. Boundary types:

• Divergent: plates move apart; new crust forms (Mid-Atlantic Ridge).
• Convergent: plates collide; one plate may sink (subduction), forming trenches and fold mountains.
• Transform: plates slide past; earthquakes occur (San Andreas Fault).

India's plate moves north and collides with the Eurasian plate, which created the Himalayas. Wegener's continental drift idea (Pangaea) preceded plate tectonics.

Earthquakes

An earthquake is a sudden shaking of the ground. The place of origin underground is the focus (hypocentre); the point on the surface directly above is the epicentre.

Seismic waves:

• P-waves (primary): fastest; travel through solids and liquids; compressional.
• S-waves (secondary): slower; travel only through solids; not through the liquid outer core.
• L-waves (surface): slowest; most damaging.

Instrument: seismograph. Scales: Richter scale (magnitude, logarithmic) and the Mercalli scale (intensity). Each whole number step in Richter magnitude means ten times the amplitude. The "shadow zones" for P and S waves led to the discovery of the liquid outer core.

Most earthquakes occur on plate boundaries, especially around the Pacific. Peninsular India is relatively stable but not free of earthquakes. Himalayan region is highly risky.

Volcanoes

A volcano is an opening in the crust through which lava, ash and gases erupt. Types: shield (Mauna Loa), composite or strato (Mount Fuji), cinder cone, and caldera. Active, dormant and extinct volcanoes are classified by activity. The Ring of Fire circles the Pacific Ocean and holds most volcanoes.

Barren Island in the Andaman Sea is India's only confirmed active volcano. Volcanic landforms: lava plateau (Deccan), crater, caldera, geysers and hot springs.

Landslides

Landslides are the downward movement of rock and soil on slopes. Causes: heavy rain, earthquakes, deforestation, steep slopes, road cutting and mining. They are common in the Himalayas and the Western Ghats. Reduction measures: afforestation, retaining walls, drainage, and avoiding construction on unstable slopes.

Minerals, soils and key terms

Minerals are natural, inorganic substances with a definite chemical make-up and crystal form. The most common rock-forming minerals are feldspar, quartz, pyroxene, amphibole, mica and olivine. Feldspar is the most abundant in the crust. Metallic minerals (iron, copper) and non-metallic minerals (mica, limestone) are mined for use.

Soil forms by weathering of rocks with the action of climate, organisms and time. Black soil of the Deccan comes from basalt; red soil develops on old crystalline rocks such as granite and gneiss, which cover much of Rayalaseema and the Andhra interior.

Other terms to know:

• Folds and faults: a fold is a bend in layers (anticline = arch, syncline = trough); a fault is a break with movement. A rift valley forms when a block sinks between two faults; the Narmada and Tapi valleys are rift valleys.
• Isostasy: the balance between the crust and the denser mantle beneath.
• Epeirogenic and orogenic movements: the first raise or lower large areas slowly (continents); the second build mountains by folding.
• Tsunami: a series of large sea waves caused mainly by undersea earthquakes or landslides. The Indian Ocean tsunami struck the Indian coast, including Andhra, in December 2004.
• Plateau: an elevated, flat-topped land. The Deccan is a plateau of volcanic and old crystalline rocks.

Exam traps

• 1. Sial vs sima: continents are sial; ocean floor is sima.
• 2. Moho separates the crust and mantle; Gutenberg the mantle and core.
• 3. P-waves pass through liquids; S-waves do not.
• 4. Focus vs epicentre: focus is underground; epicentre is on the surface.
• 5. Richter measures magnitude; Mercalli measures intensity.
• 6. Granite is igneous; gneiss is its metamorphic form.
• 7. U-shaped valley is glacial; V-shaped valley is made by rivers.
• 8. Intrusive rocks have large crystals; extrusive rocks have fine grains.

One-liners

• 1. Basalt is an extrusive igneous rock.
• 2. Marble is formed from limestone.
• 3. Fossils are found in sedimentary rocks.
• 4. The Himalayas are young fold mountains.
• 5. The Aravalli range is among the oldest fold mountains.
• 6. The Ring of Fire surrounds the Pacific Ocean.
• 7. Barren Island is India's active volcano.
• 8. Seismographs record earthquake waves.
• 9. L-waves are the most destructive.
• 10. Slate is formed from shale.
• 11. The Godavari delta lies on the Andhra coast.
• 12. The outer core is liquid.

Practice questions

1. The outermost layer of the Earth is the

   1. Crust
   2. Inner core
   3. Mantle
   4. Outer core
   Answer

   A. Crust

   The crust is the thin outer shell.

2. The boundary between the crust and the mantle is called the

   1. Gutenberg discontinuity
   2. Conrad line
   3. Lehmann discontinuity
   4. Mohorovicic discontinuity
   Answer

   D. Mohorovicic discontinuity

   Moho separates crust from mantle.

3. The Earth's outer core is

   1. Solid
   2. Plastic rock
   3. Liquid
   4. Gaseous
   Answer

   C. Liquid

   S-waves do not pass through it.

4. The main elements of the core are

   1. Iron and nickel
   2. Silica and aluminium
   3. Silica and magnesium
   4. Carbon and calcium
   Answer

   A. Iron and nickel

   The core is called nife.

5. Continental crust is mainly made of

   1. Peridotite
   2. Sial
   3. Sima
   4. Nife
   Answer

   B. Sial

   Sial means silica and aluminium.

6. Granite is an example of an

   1. Sedimentary rock
   2. Extrusive igneous rock
   3. Metamorphic rock
   4. Intrusive igneous rock
   Answer

   D. Intrusive igneous rock

   It cools slowly below the surface.

7. Basalt is an

   1. Sedimentary rock
   2. Metamorphic rock
   3. Extrusive igneous rock
   4. Intrusive igneous rock
   Answer

   C. Extrusive igneous rock

   It forms when lava cools quickly.

8. Fossils are generally found in

   1. Igneous rocks
   2. Metamorphic rocks
   3. Sedimentary rocks
   4. Volcanic glass
   Answer

   C. Sedimentary rocks

   Layered sediments can bury remains of life.

9. Marble is formed from

   1. Limestone
   2. Sandstone
   3. Shale
   4. Granite
   Answer

   A. Limestone

   Heat and pressure change limestone to marble.

10. Slate is a metamorphic form of

    1. Limestone
    2. Coal
    3. Granite
    4. Shale
    Answer

    D. Shale

    Shale or clay becomes slate.

11. Quartzite is the metamorphic form of

    1. Basalt
    2. Sandstone
    3. Shale
    4. Limestone
    Answer

    B. Sandstone

    Quartzite is hard and compact.

12. The point on the surface directly above the focus of an earthquake is the

    1. Hypocentre
    2. Epicentre
    3. Seismic gap
    4. Fault line
    Answer

    B. Epicentre

    The focus lies below the epicentre.

13. The seismic waves that cannot travel through liquids are

    1. P-waves
    2. Radio waves
    3. S-waves
    4. L-waves
    Answer

    C. S-waves

    This showed the outer core to be liquid.

14. The instrument used to record earthquake waves is the

    1. Anemometer
    2. Barograph
    3. Hygrometer
    4. Seismograph
    Answer

    D. Seismograph

    It records ground motion.

15. The Richter scale measures the

    1. Magnitude of an earthquake
    2. Intensity of damage
    3. Depth of the ocean
    4. Speed of wind
    Answer

    A. Magnitude of an earthquake

    Mercalli measures intensity.

16. The Ring of Fire surrounds the

    1. Atlantic Ocean
    2. Indian Ocean
    3. Pacific Ocean
    4. Arctic Ocean
    Answer

    C. Pacific Ocean

    Many volcanoes and earthquakes occur there.

17. India's only confirmed active volcano is on

    1. Lakshadweep
    2. Barren Island
    3. Narcondam Island
    4. Car Nicobar
    Answer

    B. Barren Island

    It lies in the Andaman Sea.

18. A U-shaped valley is formed by

    1. Glaciers
    2. Rivers
    3. Wind
    4. Sea waves
    Answer

    A. Glaciers

    Glaciers widen and deepen valleys.

19. A delta is formed by the work of a

    1. Glacier
    2. Volcano
    3. Wind
    4. River
    Answer

    D. River

    A river deposits sediment where it meets the sea.

20. Which of these is a depositional landform of wind?

    1. Sand dune
    2. Cirque
    3. Sea stack
    4. Gorge
    Answer

    A. Sand dune

    Wind moves sand to build dunes.

21. Why does a convergent plate boundary often produce deep trenches and volcanoes?

    1. Plates slide past without pressure
    2. One plate sinks beneath another and melts
    3. Rivers cut the crust
    4. Plates move apart and new crust forms
    Answer

    B. One plate sinks beneath another and melts

    Subduction creates trenches and volcanic arcs.

22. The Himalayas were formed by the collision of the Indian Plate with the

    1. Antarctic Plate
    2. African Plate
    3. Eurasian Plate
    4. Pacific Plate
    Answer

    C. Eurasian Plate

    Compression raised the Himalayas.

23. Which type of mountain is the Aravalli range?

    1. Old, worn-down fold mountain
    2. Young fold mountain
    3. Volcanic mountain
    4. Block mountain only
    Answer

    A. Old, worn-down fold mountain

    It is among the oldest fold ranges.

24. Each one-unit increase on the Richter scale represents how many times more amplitude?

    1. Five times
    2. One hundred times
    3. Two times
    4. Ten times
    Answer

    D. Ten times

    The scale is logarithmic.

25. Which is the fastest seismic wave?

    1. Surface wave
    2. P-wave
    3. S-wave
    4. L-wave
    Answer

    B. P-wave

    P-waves arrive first at a station.

26. The soft, weak layer of the upper mantle on which plates move is the

    1. Hydrosphere
    2. Biosphere
    3. Lithosphere
    4. Asthenosphere
    Answer

    D. Asthenosphere

    The lithosphere rides over it.

27. Which of the following is a common cause of landslides?

    1. Calm sea conditions
    2. A gentle plain with thick grass
    3. Heavy rain on a steep, deforested slope
    4. Dry weather on flat land
    Answer

    C. Heavy rain on a steep, deforested slope

    Water and weak slopes lead to failure.

28. The Godavari and Krishna deltas in Andhra were built by

    1. River deposition
    2. Glacial moraines
    3. Wind erosion
    4. Volcanic lava
    Answer

    A. River deposition

    Rivers deposit silt at their mouths.

29. Weathering means

    1. Breakdown of rock in place
    2. Movement of rock by rivers
    3. Movement of rock by gravity only
    4. Melting of rock into magma
    Answer

    A. Breakdown of rock in place

    Erosion needs transport; weathering does not.

30. Consider the statements on rocks. 1. Igneous rocks are formed from cooled magma. 2. Sedimentary rocks are formed by heat and pressure.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Sedimentary rocks form from deposits; heat and pressure are for metamorphic rocks.

31. Consider the statements on seismic waves. 1. S-waves travel through liquids. 2. P-waves travel through solids and liquids.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    S-waves move only through solids.

32. Consider the statements on the Earth's layers. 1. The crust is the thickest layer. 2. The mantle lies between the crust and core.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The mantle is the thickest layer; the crust is thin.

33. Consider the statements on landforms. 1. Stalactites are formed by wind. 2. Moraines are formed by glaciers.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Stalactites form by dripping groundwater in caves.

34. Consider the statements on plate boundaries. 1. Divergent boundaries create new crust. 2. Transform boundaries involve plates colliding head-on.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    At transform boundaries plates slide past each other.

35. Consider the statements on earthquakes. 1. The epicentre is at the focus. 2. The focus is below the surface.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The epicentre is on the surface above the focus.

36. Consider the statements. 1. The Mohorovicic discontinuity lies between crust and mantle. 2. The Gutenberg discontinuity lies between mantle and core.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

37. Consider the statements on volcanoes. 1. Mount Fuji is a shield volcano. 2. The Deccan lava plateau was formed by volcanic eruption.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Mount Fuji is a composite volcano.

38. Consider the statements on glaciers. 1. They carve U-shaped valleys. 2. They create alluvial fans.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Alluvial fans are formed by rivers.

39. Match rock with its metamorphic form: (1) Granite (2) Limestone (3) Shale (4) Sandstone

    1. Slate, quartzite, gneiss, marble
    2. Marble, gneiss, quartzite, slate
    3. Quartzite, slate, marble, gneiss
    4. Gneiss, marble, slate, quartzite
    Answer

    D. Gneiss, marble, slate, quartzite

    Standard metamorphic pairs.

40. Match agent with landform: (1) River (2) Glacier (3) Wind (4) Sea waves

    1. Stack, dune, moraine, delta
    2. Moraine, delta, stack, dune
    3. Delta, moraine, dune, stack
    4. Dune, stack, delta, moraine
    Answer

    C. Delta, moraine, dune, stack

    Each agent forms its characteristic landform.

41. Match: (1) P-wave (2) S-wave (3) L-wave (4) Richter

    1. Fastest, solids only, surface waves, magnitude
    2. Solids only, fastest, magnitude, surface waves
    3. Surface waves, magnitude, fastest, solids only
    4. Magnitude, surface waves, solids only, fastest
    Answer

    A. Fastest, solids only, surface waves, magnitude

    Key features of seismic waves and the scale.

42. Match: (1) Moho (2) Gutenberg (3) Lehmann (4) Asthenosphere

    1. Mantle-core, crust-mantle, weak upper mantle, outer-inner core
    2. Outer-inner core, weak upper mantle, mantle-core, crust-mantle
    3. Weak upper mantle, outer-inner core, crust-mantle, mantle-core
    4. Crust-mantle, mantle-core, outer-inner core, weak upper mantle
    Answer

    D. Crust-mantle, mantle-core, outer-inner core, weak upper mantle

    Standard boundaries.

43. Which rock would you expect to find in the Cuddapah basin as a sedimentary type?

    1. Gneiss
    2. Granite
    3. Charnockite
    4. Sandstone
    Answer

    D. Sandstone

    Cuddapah basin has sedimentary sequences such as sandstone and shale.

44. Karst topography is formed mainly by the action of

    1. Glaciers on granite
    2. Wind on sand
    3. Groundwater on limestone
    4. Waves on coral
    Answer

    C. Groundwater on limestone

    Solution of limestone makes caves and sinkholes.

45. A volcano that has erupted in historic time and may erupt again is called

    1. Residual
    2. Dormant forever
    3. Extinct
    4. Active
    Answer

    D. Active

    Active volcanoes can erupt.