4. Plate tectonics Flashcards
What is mantle convection?
What does mantle convection has to do it plate tectonics?
Mantle convection is the slow creeping motion of Earth’s solid silicate mantle caused by convection currents carrying heat from the interior of the Earth to the surface.
It is because the mantle can convect that the tectonic plates are able to move around the Earth’s surface.
Plate movement is driven by mantle convection.
It causes the inclusion of mantle material into the crust at mid-ocean ridges and recycling of crust into the mantle at subduction zones.
How many continental plates do we have?
What are the names of the largest lithospheric plates and their relative movement direction?
7 large and 15-20 rigid lithospheric plates move on the hot, plastic asthenosphere. Eurasian plate, Indian plate, Pacific plate, Antarctic plate, American plate, Nazca plate and African plate.
What is the origin of the theory of plate tectonics?
Name the facts: -The main idea of Wegener and others was that modern continents formed a single landmass in the past.
- This idea was supported by the geographic matching of geologic features and also similarities in rock ages and trends in geologic structures on opposite sides of the Atlantic.
- Fossils in similar sedimentary rocks on the different continents showed similarities in their evolution until the postulated breakup time.
- During the war, the latest radar technology was used to map the seafloor. Rapidly, evidence pointing to seafloor spreading and effective plate motion was accumulated.
- After the war, marine geology was developed, which led to the discovery of the subduction process under the continental margins. Subduction was a perfect way to balance the extension observed at the mid-ocean ridges by recycling oceanic lithosphere in the mantle.
- Plate tectonics theory was then widely accepted.
How oceanic crust is generated?
What is the age of oceanic crust?
New oceanic crust is formed when the seafloor spreads as hot molten rock (magma) wells up into the rifts of the mid ocean ridge.
The oceanic crust is younger than 180 million years
What are the main properties of oceanic crust (incl. its compositional layering)?
On a global basis, the average thickness of oceanic crust is only about 7 km, compared to almost 40 km for the continents. Moreover, rocks in the oceanic crust contain more iron and are therefore denser than continental rocks.
- Some magma rises through the narrow cracks that open where the plates separate and erupts into the ocean, forming the basaltic pillow lavas that cover the seafloor (see Figure 4.13).
- Some magma freezes in the cracks as vertical, sheeted dikes of gabbro.
- The remaining magma freezes as massive gabbros as the underlying magma chamber is pulled apart by seafloor spreading.
These igneous units—pillow lavas, sheeted dikes, and massive gabbros—are the basic layers of the crust that geologists have found throughout the world’s oceans.
Seafloor spreading results in another layer beneath this oceanic crust: the residual peridotite from which the basaltic magma was originally derived.
A thin blanket of deep-sea sediments begins to cover the newly formed ocean crust. As the seafloor spreads, the layers of sediments, lavas, dikes, and gabbros are transported away from the mid-ocean ridge.
Where on Earth different types of plate boundaries exist?
- Divergent:
Ocean spreading center or mid-ocean ridges (Atlantic ocean).
Continental rifting (in Africa) - Convergent:
Subduction zones such as Ocean-ocean convergence forming island arc, and ocean-continent convergence (western coast of South America - Andes).
And collision zones or continent-continent convergence (India and Asia)
Transform faults or tranform plate boundaries (San Andreas Fault)
Why does Iceland exist as an island?
The island of Iceland exposes a segment of the otherwise submerged Mid-Atlantic Ridge, that is the process of plate separation and seafloor spreading.
How can mountains and mountain chains form?
In ocean-continent convergence if one plate has a continental edge, it overrides the oceanic plate, because continental crust is lighter and much less easily subducted than oceanic crust. The continental margin crumples and is uplifted into a mountain chain roughly parallel to the deep-sea trench.
In Continent-Continent Convergence The collision creates a double thickness of crust forming the highest mountain range in the world
The concept of plate movement on a spherical surface:
Plates move relative to each other. To describe their motion on the surface of a spherical Earth, one needs to use Euler’s fixed point theorem, which can be stated as: The most general displacement of a rigid body over the surface of a sphere can be regarded as a rotation about a suitable axis which passes through the center of that sphere.
Thus all plate motions can be described by a rotation axis, which passes through the center of the Earth and cuts the surface at two points, called the poles of rotation. The relative motion of two plates then needs a pole of rotation and an angular velocity to be defined.
Why and where occur volcanism and earthquakes?
Divergent margins: Extensional forces cause shallow earthquakes. There is non-explosive volcanism.
Transform fault margins: Shallow to intermediate earthquakes. Little or no volcanism.
Convergent plate margins: Explosive volcanism (high viscosity). Earthquake activity to depths of 700 km
- Shallow below trenches
- Deep in subduction zone
What are the kinds of convergent plate margins? Describe how each one work:
Oceanic - continental (subduction of oceanic crust)
Oceanic crust is denser than continental crust. That is why there is no uplift in the subduction zones.
The convection currents push the oceanic crust into the continental crust. Since the oceanic crust is denser, it dives underneath the continental crust and melts as it approaches the mantle.
The result of the movement of these convergent plates in the continental crust is the formation of folded mountains. After a while, the continental crust creaks forming volcanoes.
Oceanic - oceanic (subduction of oceanic crust)
When oceanic lithosphere meets oceanic lithosphere, one plate is subducted under the other and a deep-sea trech and a volcanic island arc are formed.
Continental - continental (collision)
Since both margins have the same density and are made of the same materials, when they collide, they have nowhere to go but up.
Why can India be (partly) subducted underneath Asia?
Where plate convergence involves two continents, oceanic-type subduction cannot occur.
The Eurasian Plate overrides the Indian Plate, but India and Asia remain afloat. The collision creates a double thickness of crust forming the highest mountain range in the world, the Himalaya, as well as the vast high plateau of Tibet.
What is a hotspot?
- Heat flows from the lower mantle
- It penetrates the upper mantle and the crust
- The location remains stable, it does not move with the plates
- Causes eruption of basaltic magma
