10 - Plate Tectonics and Regional Features#
10.1 - What is Inside Earth?#
The outermost layer of the Earth is the crust, which consists of oceanic crust and continental crust. Beneath the crust is the mantle, the voluminous layer. The molten outer core and solid inner core are at the Earth’s center.
Continental crust averages 20-25 miles in thickness.
Oceanic crust averages 4 miles in thickness.
Much of the upper mantle is composed of the green mineral olivine.
The lower mantle is similar in composition to the upper mantle, but it contains minerals that form at high pressure.
Most of the mantle is solid.
The top of the mantle is solidly attached to the overlying crust, and is considered part of the lithosphere, which is about 60 miles thick as a whole. This part of the mantle is called the lithospheric mantle.
The mantle directly beneath the lithospheric mantle is called the asthenosphere, which is made up of mostly solid rock that can flow under pressure.
The concept of different thicknesses of crust riding on the mantle is called isostasy, and it explains most of the variations in elevation from one region to another.
10.2 - What Are the Major Features of Earth?#
Most mountains on the seafloor do not reach the surface and are called seamounts.
Much of the seafloor is moderately deep (9800-16000 ft.) and has a fairly smooth surface. These parts are called abyssal plains.
A mid-ocean ridge is a broad, symmetrical ridge that crosses an ocean basin.
Some continents continue out into the ocean for hundreds of miles before they drop off into deep ocean. These areas are called continental shelves.
10.3 - Why Do Some Continents Have Matching Shapes?#
Alfred Wegener, a German physical geographer, observed that the continents fit together and explained this via continental drift.
Continental drift explained why identical plant and animal fossils were found on different continents.
Fossil data suggested that Antarctica used to be further north.
Areas near the equator had evidence of being covered by glaciers in the past. These marks in rock are called striations.
Earth scientists gave the proposed supercontinent made up of the southern continents the name Gondwana.
The crucial weakness to the theory of continental drift is that it did not explain how or why the continents moved.
10.5 - What Causes Tectonic Activity to Occur in Belts?#
Earthquakes, volcanoes, and other processes that deform the crust and mantle are called tectonic activity, or simply tectonics.
10.6 - What Happens at Divergent Boundaries?#
Mid-ocean ridges are divergent plate boundaries where new oceanic lithosphere forms as two oceanic plates move apart. These boundaries are also called spreading centers.
A divergent plate boundary that occurs within a continent is called a continental rift.
A continental egde that lacks tectonic activity is called a passive margin.
10.8 - What Happens Along Transform Boundaries?#
The place where three plate boundaries meet is called a triple junction.
10.9 - Why and How Do Plates Move?#
For a plate to move, it must be subjected to a driving force that exceeds the resisting forces.
A slab pull is when subducting oceanic lithosphere is more dense than the asthenosphere, so gravity pulls the plate down into the asthenosphere. Slab pull is a significant force, and a plate that is being subducted generally moves faster than a plate that is not.
Ridge push occurs when a mid-ocean ridge is higher than the seafloor