Course 3: Earth, a Dynamic Planet

Radhitya Perdhana, S.Si., M.Sc.

MPG-1101 INTRODUCTORY GEOPHYSICS

GEOPHYSICS STUDY PROGRAM

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Historical introduction

• The Earth is a dynamic planet, perpetually changing both externally and internally
• Its surface is constantly being altered by endogenic processes resulting in volcanism and tectonism, as well as by exogenic processes such as erosion and deposition
• The Earth’s interior is also in motion. The mantle appears hard and solid to seismic waves, but is believed to exhibit a softer, plastic behavior over long geological time intervals, flowing (or “creeping”) at rates of several cm yr^-1
• Deeper inside the Earth, the liquid core probably flows at a geologically rapid rate of a few tenths of a millimeter per second

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Continental drift

• In 1912 Wegener suggested that all of the continents were together in the Late Paleozoic, so that the land area of the Earth formed a single landmass
• He coined the name Pangaea (Greek for “all Earth”) for this super continent, which he envisioned was surrounded by a single ocean (Panthalassa)
• Wegener referred to the large-scale horizontal displacement of crustal blocks having continen tal dimensions as Kontinentalverschiebung (continental drift)
• Wegener was a meteorologist, the best evidence he could present for the continental drift hypothesis was by geological indicators of paleoclimates
• Wegener’s continental drift hypothesis was bolstered in 1937 by the studies Alexander du Toit, who noted the geological similarities between western Africa and eastern South America

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Paleomagnetism and continental drift

• In the late nineteenth century geologists discovered that rocks can carry a stable record of the geomagnetic field direction at the time of their formation
• From the magneti zation direction it is possible to calculate the position of the magnetic pole at that time; this is called the virtual geo magnetic pole (VGP) position
• The consistency of paleomagnetic results leaves little room for doubt that the continents have changed position relative to each other throughout geological time
• This lends justification to the concept of continental drift, but it does not account for the mechanism by which it has taken place

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Earth structure

• Early in the twentieth century it became evident from the study of seismic waves that the interior of the Earth has a radially layered structure, like that of an onion
• The four main layers are the crust, mantle and the outer and inner cores
• The boundaries between the layers are marked by abrupt changes in seismic velocity or velocity gradient
• In 1909, A. Mohoroviçiç discovered the sharp discontinuity of seismic velocities at depths of a few tens of kilometers under continents and less than ten kilometers
• This seismic discontinuity represents the boundary between the crust and mantle, which then known as “The Mohoroviçiç discontinuity”

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Lithospheric plates

• The radially layered model of the Earth’s interior which assumes spherical symmetry is not valid for the crust and upper mantle 🡺 they varied laterally
• The crust and uppermost mantle down to a depth of about 70–100km under deep ocean basins and 100–150km under continents are rigid, forming a hard outer shell called the lithosphere
• Beneath the lithosphere lies the asthenosphere, a layer in which seismic velocities often decrease, suggesting lower rigidity
• This weaker layer is thought to be partially molten; it may be able to flow over long periods of time like a viscous liquid or plastic solid
• The asthenosphere plays an important role in plate tectonics, because it makes possible the relative motions of the overlying lithospheric plates

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Lithospheric plates (cont.)

• The brittle condition of the lithosphere causes it to fracture when strongly stressed
• The rupture produces an earthquake, which is the violent release of elastic energy due to sudden displacement on a fault plane
• Earthquakes are not distributed evenly over the surface of the globe, but occur predominantly in well-defined narrow seismic zones that are often associated with volcanic activity
• The seismic zones subdivide the lithosphere laterally into tectonic plates

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Illustration of how the global seismic observations helps scientist reveal the earth’s interior

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Illustration on how the Moho discontinuity was discovered

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Types of plate margin

• An important factor in the evolution of modern plate tectonic theory was the development of oceanography in the years following World War II
• The bathymetry oft he oceans was charted extensively by echo-sounding and within a few years several striking features became evident
• Deep trenches, more than twice the depth of the ocean basins, were discovered close to island arcs and some continental margins
• A prominent submarine mountain chain called an oceanic ridge – was found in each ocean
• The ridge system is offset at inter vals by long horizontal faults forming fracture zones
• These three features – trenches, ridges and fracture zones originate from different plate tectonic processes

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“Map of the ocean floor”

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The Vine–Matthews–Morley hypothesis

• Paleomagnetic studies in the late 1950s and early 1960s of radiometrically dated continental lavas showed that the geomagnetic field has changed polarity at irregular time interval
• In the late 1950s magnetic surveys over the oceans revealed remarkable striped patterns of alternately positive and negative magnetic anomalies over large areas of oceanic crust
• A magnetic anomaly is a departure from the theoretical magnetic field at a given location
• If the field is stronger than expected, the anomaly is positive; if it is weaker than expected, the anomaly is negative.
• In 1963 the F. J. Vine and D. H. Matthews and, independently, L. W. Morley, formulated a landmark hypothesis that explains the origin of the anomaly patterns

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Illustration of how the earth’s paleomagnetic field can be “recorded” in rocks

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The keyword is the “Curie temperature”

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Vertical cross-section through a lithospheric plate

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Triple junctions

• Inspecting the map of the plate boundaries closely, one would notice that there are several places where three plates come together
• The meeting points of three plate boundaries are called triple junctions
• Consider the plate velocities at an RTF junction shown on the figure
• The relative velocities of the plates involved can be represented and related by

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Hotspots

• In terms of tectonics and geodynamics, a “hotspot” refer to a long-lasting center of surface volcanism and locally high heat flow
• The hotspots may occur on the continents (e.g., Yellowstone), but are more common in the ocean basins

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Polar wandering

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