Plate Tectonics

Ag Earth Science – Chapter 9.1 9.1 Vocabulary

continental drift A hypothesis that originally proposed that the continents had once been joined to form a single supercontinent

Pangaea The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses

An Idea Before Its Time German scientist names Alfred Wegener proposed the hypothesis of continental drift. Continental Drift – the continents had once been joined to form a single supercontinent Pangaea – The name of the “supercontinent” meaning all land.

Evidence for Continental Drift Hypothesis The continental puzzle – continents fit together like a puzzle Matching fossils – fossil evidence for continental drift includes several fossil organisms found on different continents. Rock type and structures – rock evidence for continental drift exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean. Ancient climates – ancient climate evidence supported continental drift

A New Theory Emerges During the years that followed Wegener’s hypothesis, major strides in technology enabled scientists to map the ocean floor. Extensive data on earthquakes and the Earth’s magnetic field led to a new theory known as plate tectonics.

Ag Earth Science – Chapter 9.2 9.2 Vocabulary

plate tectonics The theory that proposes that the earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains and the crust itself

plate One of numerous rigid sections of the lithosphere that moves as a unit over the material of the asthenosphere

divergent boundary A region where the rigid plates are moving apart, typified by the oceanic ridges.

convergent boundary A boundary in which two plates move together

transform fault boundary A boundary in which two plates slide past each other without creating or destroying lithosphere

Earth’s Major Plates According to the plate tectonic theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere. The lithosphere is divided into plates.

Types of Plate Boundaries The three main types of boundaries are convergent, divergent, and transform fault boundaries.   Divergent Boundaries – two plates move APART

Types of Plate Boundaries The three main types of boundaries are convergent, divergent, and transform fault boundaries.   Convergent Boundaries – two plates move TOGETHER

Types of Plate Boundaries The three main types of boundaries are convergent, divergent, and transform fault boundaries.   Transform Fault Boundaries – two plates grind past each other side-by-side

Ag Earth Science – Chapter 9.3 9.3 Vocabulary

oceanic ridge A continuous elevated zone on the floor of all major ocean basins and varying in width from 600 miles to 2400 miles. The crests of ridges represent divergent plate boundaries.

rift valley Deep faulted structure found along the axes of divergent plate boundaries. Can be found on seafloor or land.

seafloor spreading The process by which plate tectonics produces new oceanic lithosphere at ocean ridges

subduction zone A destructive plate margin where oceanic crust is being pushed down into the mantle beneath a second plate

trench A surface feature in the seafloor produced by the descending plate during subduction

continental volcanic arc Mountains formed in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent

volcanic island arc A chain of volcanic islands generally located a few hundred miles from a trench where subduction of one oceanic slab beneath another is occurring

Divergent Boundaries Most divergent boundaries are located along the crests of oceanic ridges.   Along well-developed divergent plate boundaries, the seafloor is elevated, forming the oceanic ridge. Deep faulted structures called rift valleys are found along the axes of some segments.

Divergent Boundaries Seafloor spreading is the process by which plate tectonics produces new oceanic lithosphere.

Divergent Boundaries Continental Rifts - When spreading centers develop within a continent, the landmass may split into two or more smaller segments.

Convergent Boundaries As two plates slowly converge (come together head on), the leading edge of one plate is bent downward, allowing it to slide beneath the other. Destructive plate margins where oceanic crust is being pushed down into the mantle are called SUBDUCTION ZONES. A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate.

Convergent Boundaries Oceanic – Continental When the leading edge of a continental plate converges with an oceanic plate, the less dense continental plate remains floating. Continental volcanic arc / volcanic activity is caused by the subduction of the oceanic crust melting below when it sinks into the asthenosphere.

Convergent Boundaries Oceanic – Oceanic When two oceanic slabs converge, one descends beneath the other.  This causes similar volcanic activity to what occurs with oceanic-continental boundary, however the volcanoes form on the ocean floor.   The newly formed land consisting of an arc shaped chain of small volcanic islands is called a volcanic island arc. (example – Aleutian Islands near Alaska)

Convergent Boundaries Continental – Continental   When an oceanic plate is subducted beneath continental lithosphere, a continental volcanic arc is created. However, if the subducting plate also has continental lithosphere, the subduction eventually brings the two continents together and the collision forms complex mountains.

Transform Boundaries At a transform fault boundary, plates grind past each other without destroying the lithosphere. Example – San Andreas Fault