Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
header
geology

all about geology
•  How Old is the Earth?
•  Deep Time
•  The Rock Public Library
•  Sedimentary Rocks
•  The Fundamental Principles of Geology
•  The Physical Properties of Rocks and Minerals
•  Lava, Magma, and Other Hot Stuff
•  Lava Flows
•  Volcanoes
•  Nuèe ardente
Hello, you! You don't mind if we call you "you," do you? Good. Today, you are going to learn all about rocks. Yeah, rocks.

The first thing you should know about rocks is that the people who study them are known as geologists. Geologists don't study just rocks, they study the Earth—our home. And, just like doctors, geologists have specialties. For example, some focus on the oceans, others on the interior of the Earth; some even study other planets. Geology's many disciplines are separated into two broad categories: historical geology and physical geology. Historical geologists look at the formation and evolution of the Earth and life on Earth. Physical geologists study what materials the Earth is made of, and what's happening on and beneath the Earth's surface.

How Old is the Earth?

Now, the Earth was here long before you and I got here. It was here even before your great-grandparents were born. In fact, the earth is older than the first humans. Makes sense, doesn't it? If there was no Earth when the first humans were born, then they would've just fallen through space.

Anyway, the Earth is really old. We can think of the Earth's age in terms of geologic time. Think of it this way: if the 4.6 billion years of the Earth's history were compacted into one year, the first forms of life would appear in May; the dinosaurs wouldn't be around until mid-December; humans wouldn't appear until the last few hours of December the 31st! So, we humans have only been around for only a few tenths of one percent of the Earth's existence.

By studying the rocks on Earth, geologists have been able to construct a complete record that outlines the periods of time when rocks were formed. There are two types of dating that geologists can use to construct time lines: relative dating and absolute dating. Relative dating doesn't assign an age in years to rock formations or geologic events. Instead, relative dating puts these events in sequential order. The oldest comes first, and subsequent events follow on the relative dating line.

What about absolute dating? Well, as the techniques and technology that geologists have used have become more sophisticated, they've been able to pinpoint dates more exactly. In fact, absolute dating assigns specific dates to rock formations and geologic events.

Deep Time

Hey, guess what? You live in the Phanerozoic Eon. Me, too. It's something geologists made up. Well, they didn't just make it up—they did it by studying and classifying something called deep time.

Deep time is the billions of years that cover the Earth's history. Now, that's a lot of years, so geologists have broken these huge slabs of time into more manageable chunks: eons, eras, periods, and epochs. Eons are the biggest. Think of them as years. Think of eras as being roughly like months, periods, like weeks; and epochs, like days. As mentioned, we live in the Phanerozoic Eon, which started 570 million years ago. So, we can divide the Phanerozoic Eon into eras, periods, and epochs.

Here's some bonus info: In addition to the Phanerozoic Eon, there are three eons that account for most of the Earth's existence. They are the Pre-Archean, the Archean, and the Proterozoic.

The Rock Public Library

We know that geologists spend a lot of time studying rocks. Why do they, though? Do they not have anything better to do?

Actually, geologists study rocks because rocks are like history books. As the Earth has changed over the past 4,600 million years, it has written its story in the rocks. So, when geologists "read" rocks, they're really "reading" the history of the Earth! Pretty cool!

Geologists call this whole library of rock history books the geologic record. In this respect, the Grand Canyon is like one whole wing of the Library of Congress! There's a vast amount of information and history written into the cliffs and crags of the Grand Canyon, so, naturally, geologists have a field day there.

The Canyon's layered beds of rock contain information about the Earth and its life forms from way back before we humans were around. For instance, even though the Grand Canyon sits on a plateau 7,500 feet above sea level, there are marine fossils in its rock beds. Pretty freaky, huh?

In addition to being readers of rocks, geologists are Earth detectives. They collect clues from their observations of the geologic record to figure out what happened in the Earth's past.

Sedimentary Rocks

Let's start our own investigation by looking at sedimentary rocks. Sedimentary rocks are formed from sediments—things like little pieces of gravel, sand, silt, and clay, as well as the remains of animals and plants. All this sediment gets carried along and then left by things like water, wind, and glaciers.

Sedimentary rocks can give geologists a lot of clues as to what was happening on Earth when they were formed, because they contain all those little pieces of stuff that were around at that time!

Most sedimentary rocks come from sediments deposited on the margins of the ocean. As more and more sediment is deposited on top, the older sediment gets buried. This burial results in compaction and cementation. This overall process is called lithification, which is basically the process of turning a sediment into a sedimentary rock.

Often, plant and animal remains that were present in the sediment are preserved in the sedimentary rock. Scientists get pretty excited when they come across the preserved remains of some wacky prehistoric thing, because the remains are big clues to the Earth's past. But rocks hold more clues than just these, my friend.

The Fundamental Principles of Geology

For instance, the way rock beds are situated in relation to each other and to the Earth tells geologists a lot about how and when the rocks were formed.

There are several fundamental principles that these stalwart Earth detectives use as they gather clues about the geologic history of an area. These clues help them establish the relative ages of sedimentary rocks. (Remember, relative age refers to the age of rocks in relation to other rocks. Relative age shows the sequence of formation of a group of rocks, indicating which rock is the oldest, which one formed next, and which is the youngest, but it doesn't assign a precise age in years.)

What's the point of these fundamental principles? They give you a general understanding of how layers of sedimentary rocks were initially deposited. In other words, they can help you determine when and where each layer formed. Once you know these general principles, you can apply what you know to the rocks and formations around you and impress your friends with your apparently vast knowledge of geology. Ready? Good.

We'll start with the principle of original horizontality. What's the principle of original horizontality? Geologists know that sediments are deposited in horizontal beds. This makes sense, right? As the sediments settle and accumulate on the sea floor, they are lying horizontally.

The area that's now the Grand Canyon was once under lots of water, and the sediments that settled out of the water formed the layers that we can now see on the canyon walls. As each layer was buried underneath the next and lithified (formed into rock), the layering was preserved. So, here's the kicker of the principle of original horizontality: if beds of lithified sediments aren't lying horizontally, then something caused them to move.

A vertical sedimentary rock is a clue that something happened that was powerful enough to move the rock from its original position.

For example, say you find a sedimentary rock that is turned up on its side. Well, you have to dig around for clues to figure out what moved the rock. The principle of original horizontality gives us the first clue to the geologic history of the area where the rock was formed.

The next fundamental principle is the principle of superposition. Here's how it goes: if a rock bed hasn't been disturbed since it was formed, you know that it is younger than the layer of rock below it. The top layer is the last one to form.

So, if you want to study the most recent layer of sedimentary rock in a particular area, study the top layer. That is, Sherlock, unless the rock layers have been disturbed or overturned! If you find a layer of sedimentary rock that you know is super-super old (through absolute dating), then you have a clue that something disturbed the rock layers and flipped them over.

Our next fundamental principle of geology is the principle of lateral continuity. According to this principle, sediments are deposited initially in a layer that extends horizontally in all directions. The layers thin out and end eventually. Not too tough.

Next, we have the law of cross-cutting relationships. A cross-cutting relationship occurs when an igneous rock cuts across another rock. What's an igneous rock? Well, an igneous rock is a rock that forms when molten rock, called magma, cools down and crystallizes. Take a deep breath and we'll explain.

Here's how a cross-cutting relationship forms. Say there are existing beds of sedimentary rock under the ground, just hanging out. The next thing you know, wham! Here comes magma, pushing its way through the existing sedimentary rock bed. The magma moves quickly, fills existing cracks, forms new cracks, and even melts some of the sedimentary rock. Eventually, the magma cools and becomes igneous rock. We say that the igneous rock "cross-cuts" the old sedimentary rocks. What does this tell us geologists? Well, we know that the rock that was cut through is older than the igneous rock.

Okay, lets move on to another fundamental principle of geology, the principle of faunal and floral succession (also known as fossil succession). Let's break that down a bit, shall we? "Fauna" is basically animal life, and "flora" is plant life. "Succession" just deals with how things come after or follow each other. So, we're talking about how, over vast geologic times, different groups plant and animal life have followed each other. Since life forms have changed so much over the years, groups of fossils from different time periods will be different from one another.

Fossils are often found in sedimentary rocks. So it makes sense that older beds of rocks will have older fossils in them. Remember the principle of superposition? It says that if the rocks are undisturbed, the oldest layers of rock should be on the bottom. Following this logic, the oldest fossils should be on the bottom, too. If we know the age of the rocks, we can figure out how old the fossils in the rocks are. On the other hand, if we know the age of the fossils in the rocks, we can figure out how old the rocks are. And that's the principle of faunal and floral succession.

Okay, one final principle to go. It's the principle of uniformitarianism. It basically states that we can understand geologic events of the past by looking at geologic events of the present. What are "geologic events"? Things like volcanoes and earthquakes—that kind of stuff. According to uniformitarianism, a volcano eruption that happens today is pretty much like a volcano eruption was a million years ago.

But there's a catch. Nobody was around a million years ago to see if that volcano eruption actually was like the ones we have today. We can't really be sure that the processes we see today happened exactly the same way or at exactly the same rate as they did in the past. Something to think about.

The Physical Properties of Rocks and Minerals

Now, more about rocks! Hey, it's geology, isn't it? We're gonna see what these babies are made of.

Rocks are made of minerals, so to understand rocks, we have to understand minerals. A mineral is an inorganic solid with a unique chemical composition and crystalline structure. Don't sweat about that definition to much. Just know that rocks are made of minerals. Let's discuss some of the physical properties of minerals.

First, there's hardness. A mineral's hardness is its ability to resist abrasion. A while back, this smart guy named Friedrich Mohs came up with a scale to measure and compare the hardness of minerals. He called it the Mohs hardness scale. Hey, he invented it, why can't he name it after himself?

Anyway, The Mohs hardness scale rates ten minerals from diamond, the hardest mineral, to talc, a very soft mineral. Here's the scale:

mineral hardness
talc 1
gypsum 2
calcite 3
fluorite 4
apatite 5
orthoclase 6
quartz 7
topaz 8
corundum 9
diamond 10

Things get harder as you increase the number. So, quartz is harder than calcite. Also, quartz will scratch calcite, but calcite won't scratch quartz. Calcite is harder than gypsum, however, because it has a higher number. See how that works?

The next physical property we use to identify and classify minerals is called cleavage. Cleavage is the way crystalline minerals split or break along planar (on an even plane) surfaces. Where and how minerals split, or cleave, depends on the strength and arrangement of the bonds in their crystal structure.

Fracture, another physical property, is a lot like cleavage—but it's a break along an uneven (non-planar) surface.

The next physical property of minerals is luster. Luster refers to the way that minerals reflect light. There are two types of luster: metallic and nonmetallic.

A mineral with metallic luster looks like a metal when light reflects off it. Galena, for example, is composed of lead sulfide, and it has a metallic luster. Minerals with a nonmetallic luster, on the other hand, like orthoclase, may look glassy, greasy, waxy, brilliant, or dull and earthy.

Color is the final physical property that we'll cover. Color is color. Pretty basic. Now, a lot of minerals always appear to be the same color. Halite, for one, is always white. Not so with fluorite—it can be purple, green, blue, or yellow!

Lava, Magma, and Other Hot Stuff

Magma is hot, melted rock that's below the Earth's surface. Really, it's just liquid Earth crust. Magma stays warm and cozy under the earth—waiting, biding its time, until it shoots out of a volcano! Or just kind of ooooooozzzzes out of a crack in the crust. When that happens we call this hot molten rock by another name—lava.

Well, eventually things cool down, and both magma and lava form rocks—igneous rocks. Igneous is a general term that describes any rocks that form from magma or the accumulation of the stuff (like lava or ash) that oozes or shoots out of a volcano.

Now, there are lots of different kinds of igneous rocks, but there are two broad categories that we're concerned with: the rock that forms from lava on the surface of the Earth, and the rock that forms from magma below the surface of the Earth.

The first category is extrusive igneous rock. Extrusive igneous rocks form from lava, so they cool and crystallize on the Earth's surface. Extrusive igneous rock is also sometimes known as "volcanic igneous rock."

And the second category? Well, sometimes magma doesn't make it out of the crust, but it cools and forms solid rock anyway. Magma that cools and crystallizes within the Earth's crust is intrusive igneous rock.

Lava Flows

Ah, the lava flow. You know, when this hot stuff is on the Earth's surface, it doesn't wanna just hang out where it is. No, it wants to move around, see what's happening, get into the flow of things!

Pahoehoe and aa are two different categories of lava flow.

Pahoehoe flows get their name from the Hawaiian word for "ropy," because this type of flow has a ropy-looking surface. Pahoehoe forms when congealed surface lava is dragged along over hot-moving lava. The congealed part rolls over the hot part, forming folds that look like ropes.

An aa flow has a lumpier texture. That's because it's thicker and more viscous than a pahoehoe flow. When it cools, the rock it forms can be sharp and treacherous, so don't walk across it barefoot!

Volcanoes

When a volcano erupts explosively, a bunch of junk spews out of it. But you knew that. Anyway, this junk hardens into what we call pyroclastic materials. Pyroclastic materials include ash, pumice, and tuff. Let's discuss each.

Ash is defined more by its size rather than by its composition. Ash is anything that shoots out of a volcano that's two millimeters or smaller in diameter. It's the little stuff.

Pumice is a form of volcanic glass that's filled with holes. These holes form when gases escape from the lava. Some pumice forms as a hardening crust on a lava flow, and other times pumice is ejected directly from an explosive eruption. And that's tuff, actually.

Tuff is a type of igneous rock formed by the consolidation of ash.

Now let's talk about volcanic edifices. A volcanic edifice is the physical structure of the volcano—it's shape and how it's formed.

The big, concave-sided, symmetrical edifices we typically associate with volcanoes are composite volcanoes (also known as stratovolcanoes). These monsters generally erupt thick, lava with explosive force. Mount St. Helen's in Washington state is a composite volcano.

Cinder cones are volcanic edifices that form from the buildup of pyroclastic materials. Ash and little hot stuff that look like cinders shoot out of a vent, or opening in the ground, and then they fall back down around the vent, forming a steep-sided cone. These cones can grow pretty fast during an eruption, but they don't usually get more than 400 meters high. You can often find cinder cones on the sides of bigger volcanoes (like composite volcanoes) where smaller vents on the volcano's flanks spewed lava and formed a buildup of pyroclasts around the vent.

An eruption of hot ash, dust fragments, and gases that proceeds downhill with great speed and devastating effects is called a nuèe ardente. In 1902, Mt. Pelee in Martinique erupted, producing a nuèe ardente. The cloud of gas and debris exploded out of the volcano. It traveled at about 100 miles per hour and wreaked tremendous havoc. The layman's word for this phenomenon is "disaster."

Nuèe ardente—it may sound like some trendy French meal, but it's not. Unless you like death for dessert!

Worldwide, there are two major volcano belts. The Ring of Fire in the Pacific contains 60% of all active volcanoes. Another 20% are in the Mediterranean Belt, and most of the rest are located on mid-oceanic ridges like the Mid-Atlantic Ridge, which, as its name implies, runs right down the center of the Atlantic Ocean.

Geologists have some luck predicting volcanic eruptions, but there's not much chance that humans will ever be able to control volcanoes. So don't try. Here's our advice: if you see a volcano erupting, run! Not only do volcanoes spew hot lava and ash everywhere, they also muck up the atmosphere with dust and noxious gases. So it's not healthy to hang around when there's a volcano erupting. Additionally, all the dust they cough up blocks sunlight and can actually reduce the temperature here on Earth.

There you have it, my friends—geology. Rocks. A little bit about what they are, where they live, how they got there, and what happens when they get angry and turn into white-hot magma and lava. See ya later, rock hounds!

Now that you've read All About Geology, test your knowledge with our Sample Test.

Back to Top
SDTV Home | About SDTV | Episodes & Resources
For the Classroom | Screensavers