Mountain ranges are built by continent-continent convergence, subduction of an oceanic plate, and continental rifting.
Explain how converging or diverging plates can create mountain ranges.
How do plate motions create mountains?
Plate tectonic processes create some of the world’s most beautiful places. The North Cascades Mountains in Washington State are a continental volcanic arc. The mountains currently host some glaciers and there are many features left by the more abundant ice age glaciers. Changes in altitude make the range a habitable place for many living organisms.
Converging plates create the world’s largest mountain ranges. Each combination of plate types — continent-continent, continent-ocean, and ocean-ocean — creates mountains.
Converging Continental Plates
Two converging continental plates smash upwards to create gigantic mountain ranges (Figure below). Stresses from this uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards. As was stated previously there is currently no mountain range of this type in the western U.S., but we can find one where India is pushing into Eurasia.
The Himalaya Mountains rise as India rams into Eurasia
(a) The world’s highest mountain range, the Himalayas, is growing from the collision between the Indian and the Eurasian plates. (b) The crumpling of the Indian and Eurasian plates of continental crust creates the Himalayas.
Subducting Oceanic Plates
Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. This happens on continental crust, as in the Andes Mountains (Figure below), or on oceanic crust, as with the Aleutian Islands, which we visited earlier. The Cascades Mountains of the western U.S. are also created this way.
The Andes Mountains formed due to oceanic plate subduction
The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca Plate subducts beneath the South American Plate.
Amazingly, even divergence can create mountain ranges. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys, known as a basin-and-range (Figure below). In basin-and-range, some blocks are uplifted to form ranges, known as horsts, and some are down-dropped to form basins, known as grabens.
Transform Plate Boundaries
Plates sliding past each other create transform faults.
Describe transform plate boundaries and transform faults.
What does a transform plate boundary look like?
In the dry part of central California, it looks like this.
On one side is the Pacific Plate. On the other side is the North American Plate. Before plate tectonics theory, people thought the San Andreas was just a fault. Now it’s known to be a plate boundary!
Transform Plate Boundaries
Two plates may slide past each other in opposite directions. This is called a transform plate boundary. The plates meet at a transform fault. As you might imagine, plates do not slide past each other easily. These plate boundaries experience massive earthquakes. The world’s best known transform fault is the San Andreas Fault in California (Figure below). At this fault, the Pacific and North American plates grind past each other. Transform plate boundaries are common as offsets along mid-ocean ridges. They are very small compared to transform faults on land.
Transform plate boundaries are different from the other two types of plate boundaries. At divergent plate boundaries, new oceanic crust is formed. At convergent boundaries, old oceanic crust is destroyed. But at transform plate boundaries, crust is neither created nor destroyed.
Map of the San Andreas Fault
The red line is the San Andreas Fault. On the left is the Pacific Plate, which is moving northeast. On the right is the North American Plate, which is moving southwest. The movement of the plates is relative to each other.
At transform plate boundaries, two plates move in opposite direction.
Transform faults are the site of massive earthquakes.
The San Andreas Fault is the boundary between the Pacific and North American plates. It is the site of massive earthquakes.
The Geologic Time Scale
Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history.
Dividing Geologic Time
Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example.
Putting Events in Order
To create the geologic time scale, geologists correlated rock layers. Steno’s laws were used to determine the relative ages of rocks. Older rocks are at the bottom, and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.
Divisions of the Geologic Time Scale
The largest blocks of time on the geologic time scale are called “eons.” Eons are split into “eras.” Each era is divided into “periods.” Periods may be further divided into “epochs.” Geologists may just use “early” or “late.” An example is “late Jurassic,” or “early Cretaceous.” Pictured below is the geologic time scale (Figure below).
The geologic time scale divides Earth history into named units. Naming time periods makes it easier to talk about them.
The units of the time scale are separated by major events in Earth or life history.
In the geologic time scale, time units are divided and subdivided into smaller pieces.
Geologic Time Condensed to One Year
It’s always fun to think about geologic time in a framework that we can more readily understand. Here are when some major events in Earth history would have occurred if all of earth history was condensed down to one calendar year.
January 1 12 am: Earth forms from the planetary nebula – 4600 million years ago
February 25, 12:30 pm: The origin of life; the first cells – 3900 million years ago
March 4, 3:39 pm: Oldest dated rocks – 3800 million years ago
March 20, 1:33 pm: First stromatolite fossils – 3600 million years ago
July 17, 9:54 pm: first fossil evidence of cells with nuclei – 2100 million years ago
November 18, 5:11 pm: Cambrian Explosion – 544 million years ago
December 1, 8:49 am: first insects – 385 million years ago
December 2, 3:54 am: first land animals, amphibians – 375 million years ago
December 5, 5:50 pm: first reptiles – 330 million years ago
December 12, 12:09 pm: Permo-Triassic Extinction – 245 million years ago
December 13, 8:37 pm: first dinosaurs – 228 million years ago
December 14, 9:59 am: first mammals — 220 million years ago
December 22, 8:24 pm: first flowering plants – 115 million years ago
December 26, 7:52 pm: Cretaceous-Tertiary Extinction – 66 million years ago
December 26, 9:47 pm: first ancestors of dogs – 64 million years ago
December 27, 5:25 am: widespread grasses – 60 million years ago
December 27, 11:09 am: first ancestors of pigs and deer – 57 million years ago
December 28, 9:31 pm: first monkeys – 39 million years ago
December 31, 5:18 pm: oldest hominid – 4 million years ago
December 31, 11:02 pm: oldest direct human ancestor – 1 million years ago
December 31, 11:48 pm: first modern human – 200,000 years ago
December 31, 11:59 pm: Revolutionary War – 235 years ago
Fossils are the traces of ancient animals and plants found buried in rock. In order for an organism to be fossilized, the remains need to be covered by sediment, frozen, desiccated, or come to rest in an oxygen free environment.
Work should be done in their science notebook. If a child decides to type their work, it must be printed and glued inside their science notebook. Students may take their notebooks home to complete their work.
During our study of the rock cycle some students discovered an unknown specimen. They believe it to be amber. This week in class we are going to study the specimens and students will determine if their specimen is indeed amber.