1. Chapter 20 Mountain building

2. Objectives:
1. Describe the elevation distribution of Earth’s surface.
2. Explain isostasy and how it pertains to Earth’s mountains
3. Describe how Earth’s crust responds to the addition and removal of mass

3. Section 20.1 Crust-Mantle Relationships (pg 523-525)
Earth’s topography
Oceans cover about 71% of the Earth’s surface.
Most of Earth’s elevations are between 2 ranges (modes).
0 to 1 Km above sea level and 4 to 5 Km below sea level.
Why are there these differences in elevation?
1. Since ocean crust (basalt) is more dense than continental crust (granite), ocean crust will displace more of the mantle.
2. The continental crust is also thicker than the ocean crust. This allows it to extend deeper into the mantle and rises above the oceans.

4. Think of a big piece of ice compared to a small piece of ice.
If you place them in water, which one sticks out more?
It’s true that 10% of each will be above the water but 10% of
a 10,000 pound iceberg is a lot more than 10% of an ice cube!

5. Section 20.1 Crust-Mantle Relationships pg 525
Isostasy: page 946 glossary
Condition of equilibrium that describes the displacement of Earth’s mantle by Earth’s continental and oceanic crust.
The equilibrium mentioned here refers to the balance between the buoyant force of the mantle pushing the crust up and the force of gravity pulling the crust down.
Hint: think of a person getting in or out of a canoe.
What happens to the boat as a person gets in or out?

6. Section 20.1 Crust-Mantle Relationships pg 525-526
Mountains have a “root” that extends into the mantle which provides support to balance the buoyant force.

7. Section 20.1 Crust-Mantle Relationships pg 525-526
The force of gravity pulls the mountain down but
the buoyant force pushes it back up!

8. Section 20.1 Crust-Mantle Relationships pg 526-527
The rates of erosion are as such that there should be no mountains left! So why are they still here???
As the peaks are weathered and eroded away, the mass of the mountain decreases and the roots become smaller. It will take millions of years to flatten out these mountains.
Isostatic rebound: Slow process of which Earth’s crust is rising as the result of the removal of mass from the crust. Page 946

9. Section 20.1 Crust-Mantle Relationships pg 527
Key points:
A) Elevation of earth’s crust depends on two things
1. Thickness of crust
2. Density of crust
B) Mountain peaks are countered by roots
1. Roots provide support and buoyancy
2. Roots can be many times larger than peaks

10. Section 20.2 Convergent-Boundary Mountains pg 528
Orogeny: The process that forms all mountain ranges.
Orogenic belts: the broad, linear regions of deformation resulting from orogeny.
Most of these belts occur at Plate boundaries.
The tallest were created at convergent plate boundaries.
Look at page 9 in your ESRT’s to see how and when the mountains of NY were formed.

11. Section 20.2 Convergent-Boundary Mountains pg 529
Remember the three types of convergence zones.
1. Ocean-Ocean
2. Ocean-Continental
3. Continental-Continental

12. Section 20.2 Convergent-Boundary Mountains pg 529 A) Ocean -Ocean:
1. Subduction zone and trench is created
2. As plate melts magma rises -> Island arc forms
3. Island arcs made from basaltic and andesitic magma
4. Island arcs may then hit a coast and form a range

13. Section 20.2 Convergent-Boundary Mountains pg 530
B) Ocean -Continental:
1. Subduction zone and trench is created
2. Ocean crust uplifts continental crust
3. Compression causes crust to fold and thicken
4. Magma intrudes -> granitic intrusions + volcanoes
5. Sediments fill low areas-> folded, faulted + meta’ed

14. Section 20.2 Convergent-Boundary Mountains pg 531
C) Continental -Continental:
1. Tallest mountain ranges
2. Less dense crust therefore no subduction zone
3. Compression causes crust to fold and thicken
4. Magma that forms as a result creates granite batholiths
5. Presents of marine sediments at peak!

15. Section 20.3 Other types of Mountains pg 535
There are two other classes of how mountains form.
1. Divergent -Boundary Mountains
2. Nonboundary Mountains

16. Section 20.3 Other types of Mountains pg 535
A) Divergent -Boundary Mountains
1. Magma comes up at the boundary -> heats lithosphere
2. Lithosphere then bulges upward along ridge
3. As crust moves away from ridge -> cools + contracts
4. Ridges can be thousands of Km wide
5. These mountains are igneous

18. Section 20.3 Other types of Mountains pg 537
B) Non Boundary Mountains
1. Uplifted Mountains
a) Large regions of Earth are forced upward
b) Less deformation here than at boundaries
c) Not fully understood as to why this occurs
d) ex: The Adirondacks
2. Fault Block Mountains
a) Crust at a fault are tilted, uplifted, or dropped
b) Range in south western US and northern Mexico
3. Volcanic peaks
a) Shield volcanoes that form over hot spots
b) Kilauea in Hawaii