Essential Science for Teachers: Earth and Space Science
When Continents Collide When Continents Collide | A Closer Look
A Closer Look
Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.
A Closer Look: Metamorphic Rocks
What are metamorphic rocks?
How are these high pressures and temperatures generated?
The answer lies in the processes of plate tectonics. Plates that move against each other produce huge forces that create high pressures and temperatures that can deform rock by bending or breaking it. Rock can also be buried and metamorphosed when plates collide. Temperatures and pressure within the Earth increase with depth, so that rock deep in the crust will experience extreme heat and pressure. Rock can also be subjected to high temperatures in regions of volcanism as well as in places beneath the Earth where magma intrudes into the rock above it.
What types of metamorphism exist?
Regional Metamorphism: Many metamorphic rocks form by regional metamorphism, named for the large areas of the crust that are affected. Regional metamorphism usually results from mountain building processes, which are caused by the collision of tectonic plates. These collisions compress and thicken the crust and cause considerable rock deformation.
High-Pressure Metamorphism: Some metamorphic rock forms at high pressures but at temperatures that are relatively low. This type of metamorphism occurs at subduction zones. Here, high pressures result when one plate is submerged under the mantle. Temperatures remain relatively low because the crust that forms the upper part of the subducting plate is cool, having been close to the Earth’s surface. As the plate subducts, it actually cools the mantle. The subducting plate reaches high pressures faster than it heats to high temperatures, and this pressure is enough to cause metamorphism.
High-Temperature Metamorphism: Some metamorphic rock forms at high temperatures but without high pressures. This occurs near hot intrusions of magma from the mantle into the crust. Rock that is in contact with these intrusions undergoes contact, or thermal, metamorphism. This heat causes minerals to react with each other, which produces new minerals.
Hydrothermal Metamorphism: This process is associated with contact metamorphism. When very large masses of magma — called plutons — intrude from the mantle into the crust, a great amount of heat is generated. This huge body of hot magma creates a heat source that can cause fluids in the crust to circulate. Chemical reactions occur as a result of this circulation. This type of metamorphism is common near mid-oceanic ridges and around large plutonic intrusions in the crust.
A Closer Look: Mountains
What generalizations can be made about mountain building?
Mountain Building Related To Subduction Zones
Subduction zones are places where the edge of one plate is forced under the edge of another plate. At subduction zones, mountains can be formed in two ways:
1) Pieces of buoyant lithosphere (the crust fused to the upper part of the mantle) riding on top of the downgoing plate may eventually be brought to the convergent boundary. Examples of buoyant lithosphere with continental crust include small continental fragments and island arcs (an arc-shaped formation of volcanoes built up from the sea floor). Buoyant lithosphere with oceanic crust includes oceanic plateaus (broad regions of thick oceanic crust). Regardless of type, buoyant lithosphere cannot be subducted when it is of the same density or less dense than the material with which it is colliding. As it merges with the overriding slab, the buoyant lithosphere attaches, or “accretes,” itself to the slab’s edge. Over time, a type of fold-thrust mountain belt can be created in which folded mountains appear as rock is pushed upward. The Coast Range of California and the Sierra Nevadas are two parallel mountain ranges formed at least partly in this way.
2) When an oceanic plate subducts, it brings with it materials, like water, that can induce melting in the mantle. This melting can lead to volcanism and the creation of mountain ranges. Examples of mountain ranges created by volcanism at subduction zones include the Andes Mountains in South America and the Cascade Mountains in the western United States.
Mountain Building Related To Continental Collision
In Session 5, we focused on mountain building that occurs where the once oceanic lithosphere between two continents completely subducts and the continental crust riding atop each plate collides. Continental collision is a special case of convergence. One continent may slide a short distance under the other, but continental crust never subducts. The two continents essentially weld together. Intense compression gradually squeezes rock upwards (and downwards) deforming and thickening the crust to create mountains. This is how the Appalachian Mountains formed and how the Himalaya Mountains are forming today. In the Appalachians, geologists can trace back hundreds of millions of years to identify three distinct collisions that each helped to create the mountains we see today. As a result, the Appalachians are an ideal tectonic setting to study metamorphic rocks.
Session 1 Earth’s Solid Membrane: Soil
How does soil appear on a newly born, barren volcanic island? In this session, participants explore how soil is formed, its role in certain Earth processes, its composition and structure, and its place in the structure of the Earth.
Session 2 Every Rock Tells A Story
How can we use rocks to understand events in the Earth's past? In this session, participants explore the processes that form sedimentary rocks, learn how fossils are preserved, and are introduced to the theory of plate tectonics.
Session 3 Journey to the Earth’s Interior
How do we know what the interior of the Earth is like if we've never been there? In this session, participants examine the internal structure of the Earth and learn how it is possible for entire continents to move across its surface.
Session 4 The Engine That Drives the Earth
What drives the movement of tectonic plates? In this session, participants learn how plates interact at plate margins, how volcanoes work, and the story of Hawaii's formation.
Session 5 When Continents Collide
How is it possible that marine fossils are found on Mount Everest, the world's highest continental mountain? In this session, participants learn what happens when continents collide and how this process shapes the surface of the Earth.
Session 6 Restless Landscapes
If almost all mountains are formed the same way, why do they look so different? In this session, participants learn about the forces continually at work on the surface of the Earth that sculpt the ever-changing landscape.
Session 7 Our Nearest Neighbor: The Moon
Why is the Moon, our nearest neighbor in the solar system, so different from the Earth? In this session, participants explore the complex connections between the Earth and Moon, the origin of the Moon, and the roles played by gravity and collisions in the Earth-Moon system.
Session 8 Order out of Chaos: Our Solar System
Why do all the planets orbit the Sun in the same direction and why are the planets closest to the Sun so different from the gas giants farther out? In this session, participants gain a better understanding of the nature of the solar system by examining its formation.