Ch. 20 – Mountain Building Topography maps are used to show the change in elevations from one land mass to the next. Just by looking at a globe of the Earth you will notice that 70% of the Earth’s surface is below water. Fig. 20-1 (pg. 524) Isostasy – an equilibrium condition from the displacement of the mantle by Earth’s continental and oceanic crust. Video – Intro.

The crust and mantle are in equilibrium when the force of gravity on the mass of crust involved is balanced by the upward force of buoyancy. A similar sinking and rising that result from the addition and removal of mass occurs with the crust that makes up Earth’s mountains. A mountain range requires large roots of continental material to counter the enormous mass of the range above Earth’s surface. Continents and mountains are said to float on the mantle because they are less dense than the underlying mantle and therefore project into the mantle to provide the necessary buoyant support. Video – Mt. Formation

Isostasy and erosion – this equilibrium continues for millions of years because as a mountain range erodes it roots will also decrease to maintain a balance. Fig. 20-4 (pg. 526) Isostatic rebound is the process of the crust rising as the result of removal of overlying material. The elevation of Earth’s crust depends on the thickness of the crust as well as its density and a mountain peak is countered by a root. Ex. Mt. Everest = 9 km high & a 80 km thick root.

Convergent-Boundary Mountains Most mountain ranges, peaks, volcanoes, earthquakes have formed as a result of tectonic interactions Ex. Cascades & Appalachians (U.S.), Andes (S. America), Himalayas (India), Mt. Kilimanjaro (Africa), Alps (Europe). Orogeny – the process in which mountain ranges form. Most orogeny belts are associated with plate boundaries. Fig. 20-6 (pg. 528). The tallest and greatest variety of these belts are found at convergent boundaries.

The interactions at these convergent boundaries produces various types of mountain ranges by intense deformation. Video – Fault Mt. Video – Fold Mt. Types of Convergence 1. Oceanic-Oceanic Convergence 2 oceanic plates collide. 1 plate descends into the mantle to create a subduction zone, which melts and forces up magma to create volcanic peaks or an island arc complex. Fig. 20-7 (pg.529) 2. Oceanic-Continental Convergence The oceanic plate descends forcing the edge of the continental plate upward. This starts the beginning of orogeny. The crust begins to thicken. Deep roots develop to support the massive crust.

Example of Mt. Formation – Appalachian Mountains 3. Continental-Continental Convergence These include the tallest Mt. ranges (Himalayas) When the 2 continental plates collide neither is subducted. The energy is transferred into a highly folded and faulted crust. The crust can double its thickness due to this. The magma that forms from this hardens beneath Earth’s surface as granite batholiths. Video – Mt. Types Example of Mt. Formation – Appalachian Mountains

Other Types of Mountains Are there oceanic mountains? They snake along Earth’s ocean floor for over 65,000 km or 40,300 miles. Appendix C – pg. 912-913 Divergent-Boundary Mountains Magma is less dense than the surrounding mantle, thus a divergent boundary bulges upward and stands higher than the surrounding ocean crust to form a gently sloping mountain range (fig 20-13). Thousands of kilometers wide.

Ocean-Ridge Rocks Boundary Mountains These are mainly igneous rocks. Tectonic plates separate along an ocean ridge forcing magma upward. The magma flows into the overlying rock to form a series of vertical dikes (fig. 20-14b) Some of the magma pushes through the dikes to form pillow basalts (fig. 20-14a), which resembles sandbags. Boundary Mountains Island arc complexes, intrusive volcanic mountain ranges, highly folded continental mountains, and oceanic ridges – ALL OF THESE HAVE PLATE BOUNDARIES.

Nonboundary Mountains 1. Uplifted Mountains – these are large regions of Earth that have been slowly forced upward as a unit. Adirondack Mountains (Fig. 20-15) Not much structural deformation occurs. 2. Fault-Block Mountains – they form from large pieces of crust that are tilted, uplifted, or dropped downward between large faults. S.W. U.S & Northern Mexico (Fig. 20-16)

3. Volcanic Peaks – these form over hot spots in which plumes of mantle material are forced through the crust to form a volcanic peak. Shield Volcanoes of Hawaii (Fig 20-17). All of these various mountain formations are evidence that Earth is a dynamic planet. Video – U.S. Mts.