NGS Section 4: Volcanic Landforms

How does a shield volcano form?

• From lava flowing out and building a broad, sloping mountain

• From alternating layers of lava and ash

• From explosive eruptions of ash and cinders

• From a volcanic mountain's collapse

Answer explanation

hield volcanoes are formed by eruptions of very fluid, runny lava. This type of lava, called basaltic lava, has a low viscosity, meaning it flows easily like thick honey. When a volcano erupts, the lava flows out and spreads over a large area, forming a broad, gently sloping shape like a warrior's shield lying on the ground. Over time, repeated eruptions of this runny lava build up the shield volcano's distinctive shape.  

Here are some key points about shield volcano formation:

• Fluid lava: Shield volcanoes are formed from basaltic lava, which is very fluid and flows easily. This allows the lava to travel long distances and spread out over a large area.  

• Gentle eruptions: The eruptions that form shield volcanoes are typically effusive, meaning they are not explosive. The lava flows out in a steady stream, rather than being violently ejected into the air.  

• Gradual buildup: Shield volcanoes are built up over long periods of time by repeated eruptions of lava. Each eruption adds a new layer of lava to the volcano, gradually increasing its size.  

• Broad shape: The runny lava that forms shield volcanoes creates a broad, gently sloping shape.

  This is in contrast to stratovolcanoes, which are formed from thicker lava and have steeper sides.  

• Large size: Shield volcanoes can be very large. Mauna Loa in Hawaii is the largest active volcano on Earth, and it is a shield volcano.  

What is a caldera?

• A steep, cone-shaped hill

• A tall, cone-shaped mountain with layers of lava and ash

• A high, level area built from lava

• A huge hole left by a volcanic mountain’s collapse

Answer explanation

A caldera is a large, bowl-shaped depression formed when a volcano collapses after a massive eruption. It's like a giant crater, much bigger than the typical cone-shaped volcanoes you might picture.  

Here's a breakdown of how they form:

• Massive Eruption: Calderas are created by some of the most violent volcanic eruptions on Earth. These eruptions empty a volcano's magma chamber (the underground reservoir of molten rock) so quickly and forcefully that the ground above it can no longer support itself.  

• Collapse: With the magma chamber partially emptied, the summit of the volcano collapses inward, forming a huge depression – the caldera.  

• Size: Calderas can be enormous, sometimes spanning tens of kilometers in diameter. Yellowstone Caldera in the United States, for example, is about 45 by 30 miles (72 by 48 km) in size!  

• Features: Calderas can contain various features, such as lakes (like Crater Lake in Oregon), smaller volcanic cones that form after the collapse, and even resurgent domes (areas where the ground is pushed upward again by rising magma).  

Calderas are a testament to the immense power of nature and the potential for catastrophic volcanic events. They are closely studied by scientists to understand volcanic processes and assess potential hazards.

Which of the following is an example of geothermal activity?

• Volcanic neck

• Hot spring

• Dome mountain

• Cinder cone volcano

What forms when magma hardens in a volcano’s pipe?

• Batholith

• Volcanic neck

• Sill

• Dike

Answer explanation

Imagine a volcano like a giant pimple on the Earth. The "pipe" is the pathway that the magma (molten rock) travels through to reach the surface. After a volcano erupts a bunch of times and then becomes inactive, the magma in that pipe cools and hardens into solid rock. This hardened magma is much tougher than the softer rock that makes up the rest of the volcano.

Over long periods of time, wind, rain, and other forces of erosion wear away the softer rock surrounding the old volcano. Think of it like the softer parts of the pimple slowly disappearing. What's left behind? The hard, solidified magma in the neck, sticking up like a tower! That's what we see as a volcanic neck. It's like the plumbing of the old volcano, exposed after everything else has been washed away.

What is a geyser?

• A tall, cone-shaped mountain with layers of lava and ash

• A huge hole left by a volcanic mountain’s collapse

• A fountain of water and steam that erupts from the ground

• A high, level area built from lava

Answer explanation

A fountain of water and steam that erupts from the ground.

Imagine a giant, natural pressure cooker underground. That's kind of what a geyser is!

Here's how it works:

1. Hot Rocks: Deep down in the Earth, there are hot rocks.

2. Underground Water: Water seeps into the ground and comes into contact with these hot rocks. It gets superheated, much hotter than boiling water on your stove.

3. Pressure Builds: This hot water is trapped underground, so it can't expand like it normally would. This makes the pressure build up a lot, like in a pressure cooker.

4. Eruption! Eventually, the pressure gets so high that the hot water and steam force their way up through cracks in the ground. This causes a spectacular eruption of water and steam, shooting high into the air!

5. Repeat: After the eruption, the pressure is released. Then, the whole process starts again: water seeps in, heats up, pressure builds, and another eruption happens. This is why geysers erupt periodically, like Old Faithful, which erupts about every 90 minutes. It's like the Earth taking a hot shower!

What is a composite volcano also known as?

• Dome mountain

• Cinder cone volcano

• Stratovolcano

• Shield volcano

Answer explanation

A composite volcano is also known as a stratovolcano. They are the same thing!

Think of it like this: "composite" and "strato" both refer to layers. Composite volcanoes are built up of layers of hardened lava flows, volcanic ash, cinders, and other volcanic material. "Strato" comes from the word "strata," which also means layers. So, the name "stratovolcano" perfectly describes their layered structure.

What forms when magma forces itself across rock layers and hardens?

• Sill

• Dike

• Batholith

• Volcanic neck

Answer explanation

Imagine layers of rock like a layered cake.

• A sill is like a layer of frosting between two cake layers. It forms when magma pushes its way between existing rock layers and then hardens.  

• A dike is like stabbing a knife straight down through your cake. It forms when magma forces its way across existing rock layers, cutting through them vertically (or at a steep angle) and then hardens.

• A batholith is a huge, irregular mass of igneous rock that forms deep within the Earth's crust. It's much, much larger than a dike or sill and forms over long periods of time.  

• A volcanic neck is the hardened magma that remains in the "throat" of a volcano after the softer surrounding rock has eroded away.

  It's not formed by magma forcing its way across layers of rock.

What is a lava plateau?

• A huge hole left by a volcanic mountain’s collapse

• A tall, cone-shaped mountain with layers of lava and ash

• A high, level area built from lava seeping out of cracks

• A steep, cone-shaped hill

Answer explanation

Imagine squeezing toothpaste out of a tube, but instead of toothpaste, it's hot, liquid rock called lava. Now, imagine the tube splitting open along the side instead of just at the end. That's kind of how a lava plateau forms.

Instead of erupting from one central opening like a volcano, the lava oozes out of long cracks in the ground. It's runny lava, so it spreads out over a large, flat area. Over time, as more and more lava flows out of these cracks and cools, it builds up layer upon layer, creating a large, elevated, and mostly flat area called a lava plateau. Think of it like a giant, layered pancake made of rock!

What forms when magma squeezes between horizontal layers of rock?

• Batholith

• Volcanic neck

• Sill

• Dike

Answer explanation

Imagine a layered cake. The layers of cake are like the layers of rock in the ground. A sill is like a layer of frosting that spreads between two layers of cake. It's what happens when magma pushes its way between existing layers of rock and then cools and hardens. It lies flat, parallel to the existing rock layers.

Here's why the other options aren't the best fit:

• Batholith: A batholith is a huge, irregularly shaped mass of intrusive igneous rock that forms deep within the Earth's crust. It's much, much larger than a sill and forms over a long period of time.

• Volcanic neck: A volcanic neck is the hardened magma that remains in the "throat" of a volcano after the softer surrounding rock has eroded away. It's not formed by magma squeezing between layers.

• Dike: A dike forms when magma forces its way across existing rock layers, not between them. Imagine stabbing a knife through your layered cake – that's like a dike.

What is geothermal energy?

• Energy from water heated by magma

• Energy from burning fossil fuels

• Energy from wind

• Energy from the sun

Answer explanation

Imagine the Earth as a giant, hot potato. The inside is super hot, and that heat is what we call geothermal energy. "Geo" means Earth, and "thermal" means heat. So, geothermal energy is heat from the Earth.

One way we can get this heat is by using water. Think of it like this:

1. Hot Rocks: Deep underground, there are hot rocks.

2. Water Gets Heated: Rainwater seeps into the ground and comes into contact with these hot rocks. The water gets heated up, sometimes super hot!

3. We Use the Heat: This hot water can then be used to generate electricity or to heat buildings. It's like a giant, natural hot water heater!

So, geothermal energy is basically using the Earth's own heat to power things. It's a clean and renewable energy source because the Earth's heat is constantly being replenished.