Chapter 20

MOUNTAIN BUILDING

Mountains are spectacular features that can occur as individual peaks or immense ranges that cover many kilometers of landscape

How these geographic features are supported relies solely on the relationships between the Earth’s crust and the underlying mantle

70% of the Earth’s surface is below sea level and 30% lies above the Earth’s surface

Crust – Mantle Relationships

Most of the Earth’s topography follows a pattern of two modes of elevation: 0 to 1 km above sea level 4 to 5 km below sea level

• These two modes reflect basic differences in density and thickness between continental and oceanic crust Oceanic crust is made of basalt, which is more dense than

continental crust (made of granite) Although oceanic crust is more dense, the continental crust extends

deeper into the mantle under mountain ranges due to the thickness of the mountain ranges

Likewise, the continental crust extends higher above the Earth’s surface due to its lower density reading than oceanic crust

Earth’s Topography

Continental Crust is less dense and thicker than oceanic crust, so it extends higher above Earth’s surface and deeper into the mantle

The displacement of the mantle by Earth’s continental and oceanic crust

The crust and mantle are in equilibrium with one another; meaning, the force of gravity on the mass of crust (mountain range) is balanced by buoyancy A good model for isostasy is the water line of a

boat when someone boards or leaves the boat

ISOSTAsY

As the mountain peaks are eroded, mass decreases and the roots become smaller. The balance between erosion and the decrease in root size will continue for hundreds of millions of years until both mountains and roots disappear. (Ex. The Ozarks in Missouri)

The slow process of the crust’s rising due to removal of overlying material is called isostatic rebound. This is the reason that some mountain ranges, such as the Appalachian Mountains, still exist today!

Isostasy and Erosion

Appalachian MountainsThe Appalachian mountains are the oldest mountain range on Earth and are believed to have once been connected to Europe as evidence shows similar geologic structures and rocks in this region

Mountains form as result of tectonic interactions.

Orogeny is the process that forms all mountain ranges, which results in broad, linear regions of deformation known as orogenic belts.

The greatest variety and tallest belts are found at convergent boundaries.

Processes that build mountains

At the convergent boundaries, two plates come together causing intense deformation of land.

Examples of deformation include folding, faulting, metamorphism, and igneous intrusion.

Each type of convergent boundary makes a different type of mountain range.

Convergent – Boundary Mountains

When oceanic plates collide with one another, one goes down into the mantle to form a subduction zone

As parts of the subducted plate melts, magma is forced upward to form volcanic peaks referred to as an island arc

Oceanic – Oceanic Convergence

This boundary is similar to oceanic-oceanic because the convergence of the two plates creates a subduction zone and trenches

However, major mountain belts can result from this convergent boundary

As the oceanic plate subducts it pushes up the continental plate, resulting in the beginning of orogeny

At these boundaries, compression causes the continental crust to fold and thicken

Oceanic – Continental Convergence

Forms Earth’s tallest mountain rangesDue to low density of the plates, neither

subduct. Instead, both plates receive energy transfer from the collision. This causes the crust to become highly folded and faulted

Famous example = the Himalayas

Continental – Continental Convergence

Divergent Boundary MountainsMagma is less dense than surrounding material

so it is forced upward and warms the overlying lithosphere. The lithosphere then bulges upward and is higher than the surrounding oceanic crust to form a gently sloping mountain range

Ocean ridge mountain ranges can be thousands of kilometers wide and are composed primarily of igneous rocks

Sometimes, magma pushes through plutons and erupts onto the seafloor to form igneous rocks called pillow basalts

Other Types of Mountains

Sometimes, mountains and peaks form in places far removed from tectonic boundaries

Three nonboundary types of mountains include:

1. Uplifted Mountains2. Fault-Block Mountains3. Volcanic Peaks

Nonboundary Mountains

Form when large regions of Earth have been slowly forced upward as a unit

The Adirondacks in New York

Uplifted Mountains

Form when large pieces of crust are tilted, uplifted, or dropped downward between large faults

Grand Tetons in Wyoming

Fault – Block Mountains

Volcanic PeaksVolcanoes that form

along oceanic-continental convergent margins are typically part of long mountain ranges

Volcanoes that form over hot spots are solitary peaks that form far from plate boundaries

Volcanic peaks of Hawaii formed over hot spots, such as Mauna Kea