Extrusive Igneous Rocks Igneous rocks are formed by the cooling and crystalization of hot, molten rock called magma. The word igneous means “formed by fire.” Igneous rocks make up about 95% of the Earth’s crust. Basalt and granite are common igneous rocks Conceptual Physical Science Hewitt, Suchocki, Hewitt Extrusive Igneous rocks are created from magma that cools on the surface of the Earth, such as this basalt being formed from a lava flow in Hawaii.
Sunset Crater, Flagstaff AZ This field of basalt, and extrusive igneous rock, is about 1000 years old.
Intrusive Igneous Rocks The Navajo word for Shiprock is Tse'Bit'Ai, and means "rock with wings." Shiprock was formed by a volcanic vent 27 million years ago. At that time the surrounding land was 2000 feet higher. When the column of lava cooled, the softer earth eroded and left a 1700 foot stone pillar atop a 5500 plain. The wings are three lava walls 150 feet high and three feet wide. Shiprock is the world's finest example of an exposed volcanic throat and can be seen for 100 miles.
Intrusive Igneous: Granite Granite is a common type of intrusive igneous rock. It is formed far below the surface of the Earth at high pressure and temperature and as a consequence is more dense than basalt.
Classification of Igneous Rocks
Sedimentary Rocks: Sandstone Sandstone is a sedimentary rock formed from sand in ancient beaches, deserts, or sea beds. Sandstone is composed primarily of quartz. If you examine sandstone carefully you can see the individual sand grains from ancient terrains. A wall of Coconino sandstone in the Grand Canyon
Sedimentary Rocks: Limestone Limestone is composed mostly of calcium carbonate (CaCO 3 ). It is created from skeletons and shells that accumulated on the floor of ancient sea beds or from ancient corral reefs. Limestone deposits commonly contain fossils of ancient sea life.
Metamorphic Rocks Marble is a metamorphic rock that originally began as limestone. Continuing sedimentary processes buried the limestone under layers of shale, sand, and other limestone. Movements on the Earth’s crust caused great pressures and temperatures that converted the limestone to marble. Eventually, the marble was returned to the Earth’s surface, where it is mined by humans. A marble outcrop in Vermont is shown to the left. Other types of metamorphic rocks include schists, gneisses, and quartzsite.
The Surface of the Earth: Erosion & sedimentary rocks What more can we require? Nothing but time. James Hutton, defining uniformitarianism (vs catastrophism)
Review of 3 Rock Types The surface of the Earth is made of mostly sedimentary rock while the crust (surface and layers below) is made of mostly igneous rock.
Erosion of Igneous Rocks This photo shows a thunder storm over Yosemite valley in Sierra Nevada mountain range. The Sierra Nevada are made mostly of granite. Rain accompanies the thunder and lightning and has a more lasting impact on the mountains.
The water flows downstream As this waterfall in Yosemite shows, water from rain and snow melt flows to lower altitudes taking with it some dissolved minerals and small pieces of rocks and debris.
The Sediment is Deposited The Colorado River is the main drainage for the southwestern United States. The material carried along by the flowing waters in eventually deposited in the ocean and the Colorado river delta in Baja, Mexico. Today, essentially all of the Colorado river is used for irrigation in support of cities in the southwestern USA. Very little water makes it south of the border to Mexico and the delta is dry compared with its former state.
Herodotus & the Nile Herodotus (~450 BC) in the Histories mentions that fossils of sea life can be found hundreds of stade inland along the Nile river in Egypt. He reasoned correctly that the silt deposited by the river must be extending the reach of land and inferred that Egypt was created by this process. It was 2500 years before his suggestion was taken seriously. A satellite view of the Nile delta. The brownish/green areas are sediment.
Old and Young Mountains The Rocky Mountains (right) are 65 million years old while the Smoky Mountains (above) are 400 million years old. Because of erosion the Smokies are rounded with gentle slopes, while the Rockies have sharp edges and steep slopes.
Lifetime of a Mountain All mountains have streams. Let’s assume that the average stream carries a cubic volume of 0.1 m 3 of sediment off the mountain each day. Let’s also assume that the average mountain has 4 streams. The sediment removed each year is then Erosion Rate=4 streams x 0.1 m 3 /stream-day x 365 days/year = 146 m 3 /year How big is the average mountain? There is no average mountain, but typical dimensions might be 2 km high by 4 km wide and 4 km long (1.2 mi x 2.5 mi x 2.5 mi). The volume is Volume = 2 km x 4 km x 4 km = 32 km 3 = 3.2x10 10 m 3 The lifetime of the mountain is the volume divided by the erosion rate or, Lifetime = 3.2x10 10 m 3 / 146 m 3 /year = 2.13x10 8 years = 213 million years Although this seems like a long time, it is short compared with the lifetime of the Earth (4.5 billion years). Thus, mountain ranges formed when the Earth was young must be long gone. It follows that mountains must be continually formed.
The Age of Earth James Ussher, Archbishop of Ireland ( ): Earth was created on Saturday evening, October 22, 4004 BC James Hutton: The Earth is much older than that as witnessed by presence of thick layers of sedimentary rock.
Erosion in the Ice Age 20,000 years ago the Earth experienced a relatively cool period known as an ice age. During this time the polar ice sheets extended to much lower latitudes than they do now and glaciers cover the northern parts of the American, European, and Asian continents. A flowing glacier is a particularly powerful agent for erosion.
Rivers Make V-Shaped Valleys This photograph shows how the Yellowstone river carves out the V-shaped Yellowstone valley. The valley comes to a point at the bottom that is as wide as the river itself.
U-Shaped Valley Some valleys are U-shaped with a wide flat bottom, much broader than the width of the river. The U-shaped valleys have been carved by glaciers.
Hanging Valleys Bridal Veil falls (left) marks the location of a hanging valley, a common feature in glacially carved valleys.
Glacial Moraines When a glacier retreats (melts) it leaves behind a pile of debris formerly held in the ice. These debris piles are called moraines.
Some Famous Glacial Moraines Cape Cod Long Island Cape Cod and Long Island mark the southern most extent of the ice cap during the ice age.
Erosion of Sedimentary Rocks Sedimentary rocks tend to be soft and are quickly eroded. The effects of flowing water can be clearly seen in the photograph of Zion national park.
Stratigraphy Principle of original horizontally. Sediments are deposited as horizontal beds. Principle of superposition. Younger layers lie on top of older layers.
Principle of Uniformity physical and chemical laws are invariant with time, geomorphic processes are of the same kind and rate as in the past, and thus the history of the earth can be explained from current observations and relationships.
The History of the Earth can be read in Strata Thousands of years