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Summary Chapter 4 The way the Earth works: Plate Tectonics
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Summary Chapter 4 The way the Earth works: Plate Tectonics Marshak: Earth portrait of a Planet Systeem Aarde 1
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Chapter 4: The way the Earth works: plate tectonics4.1 Introduction
Scientific revoluttion: abrupt changes in thinking.
Plate tectonics: which states that the outer layer of the Earth, the lithosphere, consists of separate
pieces, or plates, that move with respect to one another. No fied contnents led to a complete
restructuring of how geologists think about Earth history.
4.2 what do we mean by plate tectonics?
The concept of a lithosphere plate
So the lithosphere consists of crust and upper mantle is very rigid, would faster break than bend. The
asthenosphere is plastc, composed of mantle that can fow when acted on by force. The
asthenosphere and lithosphere mantle consist of the same very dense ultramafc rock. Only the
temperature in the asthenosphere is higher which causes it to fow. Above 1280 -> plastc, lower is
rigid.
Contnental lithosphere is thicker 150-200 than the oceanic 110 km beneath mid-ocean ridges to
about 100 km beneath abyssal plains.
The crust of contnental is thicker and less dense (25-70 km, low-felsic and intermediate rock) than
that of oceanic crust (7-10 km and consists of dense mafc rock).
We can picture the rigid lithosphere as foatng on the plastc asthenosphere. Because contnental
lithosphere is thicker and has a less dense crust, its surface sits higher than does the surface of the
seafoor that’ss why contnents protrude above sea level and the ocean foor lies beneath sea level.
Lithosphere is discontnuous – 20 plates – lithosphere plates/ plates contacts between them are
plate boutndaries. 12 major plates, rest are macro plates. Contnental margin: contnent and ocean
boundary..
Active mariins: plate boundaries and passive mariins: not plate boundaries. Along a passive margin,
older contnental crust is relatvely thin and lies buried beneath an accumulaton of sediment 10-15
km thick. -> passive-margin basin. The surface of this sediment pile is a broad area of shallow
seafoor called the contnental shelf.
The basic principles of plate tectonics restated
The Earth’s lithosphere is divided into plates that move relatve to one another. As a plate moves,
it’s internal area remains mostly, but not perfectly, rigid and intact; the moton of one plate
relatve to its neighbor taees place by slip along plate boundaries. Contnents are parts of some
plates, and as these move, the contnents move with them.
Identifying plate boundaries
The moton of one plate relatve to its neighbor is accommodated by sliding along plate boundaries.
Seismic belts, therefore, defne plate boundaries.
Plate interiors: regions away from the plate boundaries, remain relatvely earthquake-free because
very litle movement takes place within them.
4.3 divergent-plate boundaries and seafoor spreading.
Diverient boutndary: 2 oceanic plates move apart by the process of seafoor spreading. As the plate
moves, new ocean foor forms. This process takes place at mid-oceanic ridges.
, Ridge aiis: the centerline of the mid-ocean ridge. Typically a 1-km-deep elongate trough the midian
valley follows the trace of the ridge aiis. Formaton of new oceanic crust takes place on or beneath
the foor of the aiial valley. A series of steep scarps form the walls of the trough. Outside the trough,
seafoor slopes from the ridge toward the abyssal plain reaching the abyssal plain depth at a distance
of about 500-800 km from the ridge aiis.
How does oceanic crust form at the mid-ocean ridge
The oceanic crust is not just the frozen top of the mantle. Rather the crust is shaped from melt that
forms in the rising asthenosphere when it reaches shallower depths. At a depth between 30 and 60
km, about 15% of it melts and turns into magma. Not all the minerals in the ultramafc rock melt
equally. So magma has another compositon than the source rock. Mafc compositon, more silica.
Density of magma is less than surrounding material and therefore moves upward. Accumulates in a
region called maima chamber lies at depth 2 to 7 km. contains mush of solid mineral crystals and
liquid melt. Some solidifes to become coarse-grained, mafc igneous rock -> gabbro. Some move
higher into vertcal cracks were it solidifes and form wall-like sheets of basalt -> basalt dikes. Magma
that reaches the surface, spills out of small submarine volcanoes as lava. Cools to form pillow basalt,
piles of meter-wide basalt blobs.
- Pillow basalt layer
- Layer of sheeted dikes
- Layer of gabbro
Black smokers: chimneys spewing hot, mineralized water. The water they emit looks like a cloud of
dark smoke. Comes from a suspension of tny mineral grains that precipitate in cold seawater the
instant that the rising mineralized water cools. Form because seawater percolates down into the
oceanic crust through a network of cracks. Heat from rising magma warms up water, which starts to
rise. The hot water dissolves minerals and carries them along, in soluton, as it spews back into the
sea.
When new ocean foor is made it moves away from the ridge aiis. All seafoor is made like this. So
closer to the ridge aiis the seafoor is younger. The tension applied to newly formed solid crust as
spreading takes place breaks this new crust, resultng in the formaton of faults. Movements on these
faults generates earthquakes and produces scarps that border the median valley of the mid-ocean
ridge.
How does the lithospheric mantle form at a mid-ocean ridge
Cooler part mantle less than 1280ᵒC. By the ridge it is above this temperature even in the crust, so
here is no real lithospheric mantle. As it moves away from the ridge aiis, they cool. When the
temperature is less than 1280ᵒC it is part of the lithosphere, because it behaves rigidly. How farther
away how thicker the lithospheric mantle becomes. The seafoor thickness doesn’st change. The
whole oceanic lithosphere does become thicker. When it cools, it gets denser and sinks down into
the asthenosphere. So the ocean is deeper over older ocean foor than over younger ocean foor.
4.4 Convergent-plate boundaries and subduction
Convergent-plate boundaries: 2 plates move toward each other (1 oceanic).
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