Inside the Earth
There are three major Crustal divisions
§ Oceanic
§ Transition and
§ Continental
Out of these only oceanic and continental divisions are of major importance.
Ø Oceanic crust ranges from 5 to 15 km thick
Ø Islands, island arcs and continental margins are examples of transitional crust that exhibit thicknesses of 15-30 km
Ø Continental crust generally ranges from 30-50 km thick, with thickness up to 80 km reported in some area (under large mountain ranges, such as the Alps or the Sierra Nevada)
Ø Below the crust is the mantle, a dense, hot layer of semi‐solid rock approximately 2900 km thick.
Ø The mantle, which contains more iron, magnesium, and calcium than the crust , is hotter and denser because temperature and pressure inside the Earth increase with depth.
Ø At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron‐nickel alloy) rather than stony.
Earth's core is actually made up of two distinct parts:
Ø 2,200 km‐thick liquid outer core and Ø 1,250 km‐thick solid inner core.
Ø As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field.
Layer Relative position Density(in gm/cu
cm) Composition
Crust Outer most layer: thinnest under the Ocean and thickest under the continents.
Crust and top of the Mantle called lithosphere
Oceanic crust (3‐3.3)
Continental crust (2.7‐3.0)
Solid rocks‐ mostly silicon and oxygen Oceanic crust: basalt Continental crust:
granite
Mantle Middle Layer: thickest layer;
top portion called the asthenosphere
Mantle
3.3‐5.7) Hot soften rock;
contains iron and magnesium
Core Inner layer, consists of two parts
‐ outer core and inner core Outer core (9.9‐12.2) Inner core (12.6‐13.0)
Mostly iron and nickle;
Outer core slow flowing liquid, Inner core solid
Magmatism: mechanism and causes
Firstly, understand, what is magma
What is Magma
Ø Magma is a slushy mix of molten rock, gases and mineral crystals within the Earth.
Ø Magma is called lava when it reaches the surface.
Ø The elements found in magma are the same major elements found in magma are the same major elements found in Earth's crust: Oxygen (O), magnesium (Mg), Calcium (Ca), potassium (K), and sodium (Na). Of all the compounds found in magma, silica (Sio2) is the most abundant and has the greatest effect on magma characteristics.
Ø Magma are classified as basaltic, andesitic, and rhyolitic, based on the amount of Sio2 they contain.
Ø Silica content affects melting temperature and also impacts how quickly magma flows.
Group Sio2
Rhyolitic 70 %
Andesitic 60 %
Basaltic 50%
Viscosity: a measure of how easily a fluid flows. Water has a low viscosity, molasses has a much higher viscosity.
Viscosity, in turn, controls the amount of gas that can be trapped in the magma.
The greater the viscosity the more gas in the magma.
There are three basic types of magma:
Basaltic Magma Andesitic Magma Rhyolitic Magma
The names are based on the rock type that forms when the magma crystallizes.
Magma Type
Chemical Composition
Temperature (degrees C)
Viscosity Gas
Content Basaltic 45-55% SiO2;
High in Fe, Mg, Ca; Low in K, Na.
1000 - 1200 Low Low
Andesitic 55-65% SiO2; Intermediate Fe, Mg, Ca, Na, K
800-1000 Intermediate Intermediate
Rhyolitic 65-75% SiO2; Low in Fe, Mg, Ca; High in K, Na
650-800 High High
Magmatism occurs primarily at
1. Convergent plate boundaries
2. Divergent plate boundaries, and 3. Hotspots
Magmatism
Magmatism: The formation of igneous rock from magma.
Ø Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks.
Ø Volcanism is the surface expression of magmatism.
Ø Magmatism is one of the main processes responsible for mountain formation.
Ø The nature of magmatism depends on the tectonic settings.
For example,
1. Convergent plate margin : Andesitic magmatism associated with the formation of island arc.
2. Divergent plate margin: Basaltic magmatism at Mid Oceanic Ridges during sea floor spreading.
Spreading centers (mid‐ocean ridges) and subduction zones (trenches) and island arcs associated with the theory of seafloor spreading.
Ø Convergent margin magmatism may occur for thousands of kilometers parallel to the trench, and up to 500 km perpendicular to the trench in the direction of subduction.
Ø Plutonic rocks at convergent margins include diorite, granodiorite, quartz diorite, granite, gabbro, tonalite and rocks referred to as trondhjemite.
Ø Convergent margin volcanoes may be quite explosive due to a high magma viscosity and high volatile content.
Ø Convergent plate boundaries are characterized by the development of composite volcanoes (also called stratovolcanoes). Andesite to rhyolite compositions are the most common crystalline volcanic rocks at composite volcanoes.
1. Convergent plate margin:
Ø Magmatism at convergent margins is largely due to the partial melting of the lithospheric wedge overlying the subduction zone.
Ø Divergent plate boundaries are dominated by basaltic lava flows. Divergent plate margins occur as a continuous belt of mountain ridges along the ocean floor. Ocean ridges are the site of plate extension, wherein rift valleys develop.
Ø Magmatism at divergent margins is due to the partial melting of the upper mantle due to mantle uplift and decompression.
Ø Low viscosity basaltic lava tends to erupt in a quiescent fashion such that violent explosions are rare.
2. Divergent plate margin
3. Hotspots
§ An area of abnormally intense active volcanism thought to be underlain by a mantle plume. Many hot spots, for example Hawaii are located in the middle of a lithospheric plate. Where as others such as Iceland are located on divergent (constructive) plate margins.
§ Hotspots (Wilson, 1963 ) are long ‐ lived areas in the mantle where anomalously large volumes of magma are generated.
§ They occur beneath both
§ Oceanic lithosphere (e.g., Hawaii)
§ Continental lithosphere (e.g., Yellowstone National Park, Wyoming), and
§ along divergent plate boundaries (e.g., Iceland).
Ø linear shape of the Hawaiian Island‐Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present‐day position of the Island of Hawaii.
Ø Wilson suggested that continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases.
Ø As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor.
World map showing the locations of selected prominent hotspots
Earthquakes at Plate Margins
Needs prior understanding of plate tectonics and seafloor topography
Ø A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock.
Ø Earth's lithosphere is divided into a series of major and minor mobile plates.
Ø These plates are mobile , moving in constant, slow motion measured in rates of centimeters per year.
Ø The movements of plates over millions of years resulted in the opening and closure of oceans and the formation and disassembly of continents.
Plate Tectonics
South American Plate Eurasian Plate
Indian Plate
Australian Plate
Major tectonic plates
Pacific plate
North American plate Cocos plate
Nazca plate Antarctic Plate
The Pacific and Antarctic Plates are among the largest
Ø Plate size can vary greatly, from a few hundred to thousands of kilometers.
Ø Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere.
Ø The destruction of oceanic lithosphere below oceanic trenches explains the occurrence of earthquakes and volcanoes adjacent to trenches.
Major tectonic plates
Distribution of tectonic plates with type of plate boundary
Ø Plates are typically composed of both continental and oceanic lithosphere. For example, the South American plate contains the continent of South America and the southwestern Atlantic Ocean.
Ø Plate boundaries may occur along continental margins ( active margins) that are characterized by volcanism and earthquakes.
Ø Continental margins that do not mark a plate boundary are known as passive margins and are free of volcanism and earthquakes. The Atlantic coastlines of North and South America are examples of passive margins.
Seafloor Topography
Ø The ocean floor varies considerably in depth and character.
Ø Starting at the edge of the continents we can recognize four principal depth zones.
1. The first depth level is the continental shelf, shallow ocean floor (0-150 meters) immediately adjacent to continental land masses.
2. Beyond the shelf, the ocean floor steps down to the second depth level, the deep ocean basins known as the abyssal plains often over 4 km below sea level.
3. The ocean floor rises to a third level approaching the oceanic ridge system, a submarine mountain chain that can be traced around the world. The ocean floor is relatively shallow, less than 3 km deep along the ridge system.
4. The fourth depth level in the oceans is apparent in narrow, deep (> 7 km) oceanic trenches found along the margins of some continents.
• Geophysicists have long recognized that deep earthquakes are associated with trenches down to depths of 700 to 800 km, far below the ocean floor.
• Shallow earthquakes are mainly located near trenches and oceanic ridges.
Distribution of ocean ridges and trenches on the sea floor. Oceanic ridges (white) form a network of submarine mountains on the seafloor. Trenches (red) are concentrated around the margins of the Pacific Ocean. Numbered trenches are: 1. Aleutian; 2. Kurile‐Japan; 3. Mariana; 4. Philippine; 5.
Bougainville; 6. Tonga‐Kermadec; 7. Central America; 8 Peru‐Chile; 9. Puerto Rico; 10.
SouthSandwich; 11. Java.
Ø During the 20th century, improvements in seismic instrumentation and greater use of earthquake‐
recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges.
Earthquakes at Plate Margins
Ø seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40‐60° from the horizontal and extended several hundred kilometers into the Earth.
Ø These zones later became known as Wadati-Benioff zones , or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States
Map showing the concentration of earthquakes along the zones indicated by dots and cross‐hatched areas.
Inclined zone of earthquake foci adjacent to oceanic trench slopes downward under the overriding plate. The distribution of foci define the Wadati-Benioff zone.
The map below locates earthquakes around the globe
Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin.
i. Ocean-Ocean: Japan
Earthquakes in Japan are caused by ocean-ocean convergence . The Philippine Plate and the Pacific Plate subduct beneath oceanic crust on the North American or Eurasian plates.
In March 2011 an enormous 9.0 earthquake struck off of Sendai in northeastern Japan.
ii. Ocean-Continent: Cascades
The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. The subduction of three small plates beneath North America produces active volcanoes , the Cascades. Surprisingly, large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9. A quake of that magnitude today could produce an incredible amount of destruction and untold fatalities.
iii. Continent-Continent: Asia
Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge. The 2001 Gujarat earthquake in India was responsible for about 20,000 deaths, and many more people became injured or homeless
Divergent Plate Boundary
Earthquakes are located along the normal faults that form the sides of the rift or beneath the floor of the rift. Divergent faults and rift valleys within a continental mass also host shallow‐focus earthquakes.
Shallow‐focus earthquakes occur along transform boundaries where two plates move past each other.
But!!!! what was the significance of the connection between earthquakes and oceanic trenches and ridges?
The recognition of such a connection helped confirm the seafloor‐spreading hypothesis by pin‐pointing the zones where Hess had predicted oceanic crust is being generated (along the ridges) and the zones where oceanic lithosphere sinks back into the mantle (beneath the trenches).
