Volcanoes—Windows Into Earth’s Past

Few natural catastrophes today compare to the awesome spectacle of a volcanic eruption. The Mount St. Helens eruption of May 18, 1980, the most destructive in recorded U.S. history, unleashed the same energy as 400 million tons of TNT, or approximately 20,000 Hiroshima-size atomic bombs.1 Yet its explosive power is miniscule compared to past eruptions.

The Mount St. Helens eruption produced an impressive 0.25 cubic miles (1 km³) of volcanic ash. But that is nothing compared to the eruption of Taupo (New Zealand) about 1,800 years ago, which produced 8 cubic miles (35 km³) of ash. Even this is dwarfed by an earlier Yellowstone eruption, soon after the Flood, which produced at least 480 cubic miles (2000 km³) of ash.2

Such was the magnitude of these explosions that they blasted away huge holes in the earth, called calderas. The Taupo caldera is now filled by a huge lake, and the Yellowstone “hole” is so big you can only discern its boundaries with the help of satellites.

It’s hard to imagine the scale of a volcanic eruption that could deposit the Deccan Traps, stacks of volcanic rocks one mile deep, spread over 200,000 square miles.

Yet these eruptions are tiny compared to a different type of volcano that deposited gargantuan stacks of thick layers known as “continental flood basalts.”3 For example, the Deccan Traps of India are over a mile (2000 m) thick and spread over nearly 200,000 square miles of the Indian subcontinent (500,000 km²)—about the same area as modern France! The Siberian Traps in Russia are even thicker (more than 480,000 cubic miles [2 million km³] in volume), though they cover a slightly smaller area (130,000 square miles [340,000 km²]).

It’s hard to imagine the scale of an event that would produce these flood basalts. Many large cracks, or fissures, had to open in the earth all at once, for so much lava to pour out over such a wide area. No eruptions today are that large, though a small fissure did open in Iceland back in 1783–84, belching out around 3.4 cubic miles (14 km³) of basalt lava (the same Laki area that created so much international concern in April 2010).

Before we can understand the unique forces necessary to fuel such immense eruptions in the past, we must first look at clues about volcanoes in the present.

Types of Volcanoes

Volcanoes occur in various sizes and forms. Many of the best-known volcanoes, especially the picturesque ones, are large, steep-sided cones. These include Mount St. Helens, Mount Fujiyama (Japan), Mount Pinatubo (Philippines), Mount Ngauruhoe (New Zealand), and many others. They form as successive layers of lava and ash pile up. Technically they are called explosive composite volcanoes, or stratovolcanoes.4^,5 It is likely that Yellowstone was originally a stratovolcano, but much bigger than any volcano we see today.

Another type of volcano is found in Hawaii, such as Kilauea. These have gently sloping sides and are called shield volcanoes. Though spectacular, their eruptions are not explosive. Actually, these Hawaiian volcanoes are taller than Mount Everest, if their heights are measured from their bases on the deep ocean floor.

They produce oozing lava similar to the material we find in the Deccan Traps, but not on the same scale. The lava is usually only a few feet deep over a square mile or less.

Types of Lavas and Eruptions

The different types of volcanoes vary depending on the content of the molten rock (magma) that formed them.

Volcanoes are fed from deep inside the earth. Various geologic events, such as friction of moving plates, cause the melting of rock deep beneath the earth’s surface. This molten rock then rises to the surface as magma. If the melting rock includes lots of a chemical component called silica, it will be very thick (viscous) and resist moving. But if the magma is low in silica, it will flow very easily.

Silica is very common in the rocks of the earth’s “outer skin,” or crust. The region below the crust, called the mantle, does not have so much silica. If the magma comes from melting at the top of the mantle, then it will be low in silica and flow easily. This rock is called basalt, and it’s what we find in the traps and in Hawaii’s shield volcanoes.

If, however, the magma gets contaminated as it rises through fractures in the crust, then the proportion of silica increases and the magma gets thicker. (Rocks of this kind are called andesite and dacite.) On the other hand, if just crustal rocks melt, they form another, thicker kind of magma—granite—which has the highest silica content. When these granite magmas reach the surface, they are called rhyolite.

Since basalt lavas flow easily, they erupt non-explosively. Consequently, they tend to spread out to build gently sloping shield volcanoes. However, dacite magma is thicker, so it squeezes out like tar, forming steep-sided volcanoes. Also, any gas and steam that is trapped in the rising dacite magma can’t easily escape. So the pressure increases, like that in a corked bottle which has been shaken. Eventually the volcano erupts violently, breaking up the magma into frothy blocks (pumice) or fine-grained particles of ash (pyroclastics).6

Windows into Earth’s Interior

The lavas that flow out of today’s volcanoes are very small in volume compared to lavas from the monster volcanoes of the past. How can we explain the physical forces that could produce so much magma?

Conventional geologists face a quandary. Today’s lava flows are small because the magma chambers below the volcanoes contain only small amounts of magma, and the continental plates are moving so slowly that they can’t facilitate the melting of much new magma.

In contrast, the basalts of the Deccan and Siberian Traps are massive. The eruptions must have been enormous, with huge volumes of lava constantly pouring out rapidly from many large fissure volcanoes. Only some unique catastrophe could have formed all this magma.

Where Do Volcanoes Occur and Why?

Volcanoes give us another clue about the forces at work inside the earth. Most active volcanoes are located near the margins of the earth’s crustal tectonic plates7 (Figure 1). This is especially true where one plate appears to be sinking or is being pushed under the adjoining plate, such as where the Pacific plate is sinking under the North American plate in the U.S. Northwest (see d in Figure 2), or where the Philippine plate is sinking under the Eurasian plate near Japan (see a in Figure 2). The sinking, or “subducting,” slab causes mantle and crustal rock to melt, which produces magmas that rise to erupt through volcanoes.

Other active volcanoes occur where the plates are splitting apart, such as in the East African Rift Valley and along ridges in the middle of the ocean basins (see c in Figure 2).8

In a few exceptional places, active volcanoes are located over “hot spots” under the plates, where plumes of hot mantle rocks are rising towards the earth’s surface (see b in Figure 2).9 The best known examples of these are the Hawaiian volcanoes. The continental flood basalts are found where such hot spots occurred in the past.

Earth’s Catastrophic Past

Present volcanoes and eruptions are not the key to understanding the earth’s past. The volume of some past lava deposits is much too large to be explained by today’s volcanic activity.

Contrary to conventional (slow-and-gradual) thinking in geology, present volcanoes are not the key to understanding the earth’s past. The volume of lava and deposits was much too large and catastrophic to be explained by today’s volcanic activity.10^,11

For old-earth scientists, the location of continental flood basalts above former mantle plumes is explained by slow-and-gradual plate tectonics theory, but Flood geologists point out that the volume is not explained. The conventional idea of a slow rate of mantle flow and plate movements cannot explain such huge volumes of basalt lavas. Indeed, they had to be generated and erupt catastrophically. Even using conventional long-ages dating, these flood basalts were produced in a veritable geologic “instant.”

The Biblical Flood

This is another powerful example of evidence that can be explained by catastrophic plate tectonics during the biblical Flood.12 The breakup of the fountains of the great deep at its onset and continuing for 150 days would have involved not only the bursting out of water from inside the earth, but also steam and prodigious volumes of lavas. Then after the fountains were closed and plate movements slowed, volcanic activity decreased at the end of the Flood. This is also reflected in the documented declining power of post-Flood volcanoes to their relative quiescence today.13

Only the record of the cataclysmic Flood in God’s Word makes sense of the evidence we see in the geologic record of God’s world.

When God descended onto Mount Sinai in fire to give the Ten Commandments (Exodus 19:16, 18), there were thunderings and lightnings, and the mountain quaked greatly and was covered in smoke like a furnace. Such a description is reminiscent of a violent volcanic eruption. No wonder the children of Israel were fearful.

We can only imagine the fiery holocaust that engulfed huge regions of the earth at the time of God’s judgment of the biblical Flood. The evidence of His past judgments and His power should remind us to fear and trust Him always, so that we can be saved from His final judgment to come.